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  • richardmitnick 10:07 pm on May 13, 2021 Permalink | Reply
    Tags: "Why precision luminosity measurements matter", , , CERN ATLAS, , , , , , ,   

    From European Organization for Nuclear Research (Organisation européenne pour la recherche nucléaire)(EU) [CERN]: “Why precision luminosity measurements matter” 

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    From European Organization for Nuclear Research (Organisation européenne pour la recherche nucléaire)(EU) [CERN]

    12 May, 2021
    Piotr Traczyk

    Both the CMS and ATLAS experiments have performed luminosity measurements with spectacular precision.

    1
    The interaction region of the CMS experiment right before the installation of the new beam pipe. (Image: CERN)

    The ATLAS and CMS experiments at the Large Hadron Collider (LHC) have performed luminosity measurements with spectacular precision. A recent physics briefing from CMS complements earlier ATLAS results and shows that by combining multiple methods, both experiments have reached a precision better than 2%. For physics analyses – such as searches for new particles, rare processes or measurements of the properties of known particles – it is not only important for accelerators to increase luminosity, but also for physicists to understand it with the best possible precision.

    Luminosity is one of the fundamental parameters to measure an accelerator’s performance. In the LHC, the circulating beams of protons are not continuous beams but are grouped into packets, or “bunches”, of about 100 billion protons. These bunches collide with oncoming bunches 40 million times per second at the interaction points within particle detectors. But when two such bunches pass through each other, only a few protons from each bunch end up interacting with the protons circulating in the opposite direction. Luminosity is a measure of the number of these interactions. Two main aspects of luminosity are instantaneous luminosity, describing the number of collisions happening in a unit of time (for example every second), and integrated luminosity, measuring the total number of collisions produced over a period of time.

    Integrated luminosity is usually expressed in units of “inverse femtobarns” (fb-1). A femtobarn is a unit of cross-section, a measure of the probability for a process to occur in a particle interaction. This is best illustrated with an example: the total cross-section for Higgs boson production in proton–proton collisions at 13 TeV at the LHC is of the order of 6000 fb. This means that every time the LHC delivers 1 fb-1 of integrated luminosity, about 6000 fb x 1 fb-1 = 6000 Higgs bosons are produced.

    Knowing the integrated luminosity allows physicists to compare observations with theoretical predictions and simulations. For example, physicists can look for dark matter particles that escape collisions undetected by looking at energies and momenta of all particles produced in a collision. If there is an imbalance, it could be caused by an undetected, potentially dark matter, particle carrying energy away. This is a powerful method of searching for a large class of new phenomena, but it has to take into account many effects, such as neutrinos produced in the collisions. Neutrinos also escape undetected and leave an energy imbalance, so in principle, they are indistinguishable from the new phenomena. To see if something unexpected has been produced, physicists have to look at the numbers.

    So if 11000 events show an energy imbalance, and the simulations predict 10000 events containing neutrinos, this could be significant. But if physicists only know luminosity with a precision of 10%, they could have easily had 11000 neutrino events, but there were just 10% more collisions than assumed. Clearly, a precise determination of luminosity is critical.

    There are also types of analyses that depend much less on absolute knowledge of the number of collisions. For example, in measurements of ratios of different particle decays, such as the recent LHCb measurement.

    Here, uncertainties in luminosity get cancelled out in the ratio calculations. Other searches for new particles look for peaks in mass distribution and so rely more on the shape of the observed distribution and less on the absolute number of events. But these also need to know the luminosity for any kind of interpretation of the results.

    Ultimately, the greater the precision of the luminosity measurement, the more physicists can understand their observations and delve into hidden corners beyond our current knowledge.

    See the full article here.


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  • richardmitnick 12:54 pm on September 23, 2020 Permalink | Reply
    Tags: "Unraveling Nature's secrets: vector boson scattering at the LHC", , CERN ATLAS, , , ,   

    From CERN ATLAS- “Unraveling Nature’s secrets: vector boson scattering at the LHC” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    22nd September 2020
    Lucia Di Ciaccio
    Simone Pagan Griso

    1
    Figure 1: “Wandering the immeasurable”, a sculpture designed by Gayle Hermick welcomes the CERN visitors. From the Mesopotamians’ cuneiform script to the mathematical formalism behind the discovery of the Higgs boson, the sculpture narrates the story of how knowledge is passed through the generations and illustrates the aesthetic nature of the mathematics behind physics. (Image: J. Guillaume/CERN.)

    In 2017, the ATLAS and CMS Collaborations announced the detection of a process in high-energy proton–proton collisions that had not been observed before: the vector boson scattering. It results in the production of two W particles with the same electric charge as well as two collimated sprays of particles called “jets” (see Figure 2). The observation of vector boson scattering didn’t receive as much attention from the media as the Higgs discovery in 2012, even though it was an important event for the particle physics community. Another missing piece of the big puzzle had been found – the puzzle that is the mathematical description of the microscopic world (see Figure 1).

    The W+ and W– bosons are unstable particles, which decay (transform) into a lepton and an antilepton or a quark and an antiquark with a mean lifetime of only a few 10-25 seconds. They have integer spin (characteristic of bosons) and are carriers of the weak force. Though the weak force is not directly experienced in everyday life, it is nevertheless important as it is responsible for radioactive β decay, which plays a role in the fusion of hydrogen into helium that powers the Sun’s thermonuclear process.

    To appreciate the importance of this discovery, it is instructive to follow the history of how and why the W+ and W– bosons were introduced; it illustrates nicely how the interplay between experimental information, theoretical models and mathematical principles drives progress in physics.

    2
    Figure 2: Simplified view of a proton–proton collision event recorded with the ATLAS detector that was selected as a candidate for vector-boson-scattering production. The insert depicts a schematic view of the candidate physics process. Protons (p) from the LHC beam travel from left to right and from right to left in this view. They collide at approximately the centre of the detector. Within a very short period of time, too short to be resolved, two W bosons are emitted independently by the incoming quarks (q) from each of the LHC proton beams. These W bosons interact and each of the resulting W bosons decays to a muon (μ) and a neutrino (ν), where the neutrinos leave the ATLAS detector undetected. The outgoing quarks undergo a process called hadronization and manifest as a spray of particles called a “jet”. (Image: ATLAS Collaboration/CERN.)

    Enrico Fermi originally formulated a mathematical description of the weak force in 1933 as a “contact interaction” between particles, occurring at a single point without a carrier particle propagating the force. This formulation successfully described the known experimental observations, including the radioactive β decay for which it was developed. However it was soon realised that its predictions at high energy, a regime not yet experimentally accessible at that time, were bound to fail.

    Indeed, Fermi’s theory predicts that the production rate of some processes caused by the weak force – such as the elastic scattering of neutrinos on electrons – increases linearly with the neutrino energy. This continuous growth, however, leads to the violation of a limit derived from the conservation of probability in a scattering process. In other words, predictions become unphysical at a high enough energy. To overcome this problem, physicists modified Fermi’s theory by introducing “by hand” two massive spin-one (“vector”) charged particles propagating the interaction between neutrinos and electrons, dubbed “intermediate vector bosons”. This development came well before the discovery of the W bosons decades later.

    So even if the discovery of the long-awaited W± bosons in 1983 – and, five months later, of a neutral companion, the Z boson – didn’t come as a real surprise to physicists, it was certainly an epochal experimental achievement. Fermi’s theory remains an example of an effective theory valid only at low energy (well below the mass of the force carrier boson) – an approximation of a more general, universally valid theory.

    Along this line, the search for a consistent description of the fundamental forces between the ultimate constituents of matter has led to the Standard Model of particle physics: a mathematical construction based on fundamental principles and experimental observations. The Standard Model provides a coherent, unified picture of three of the four fundamental interactions, namely the electromagnetic, weak and strong force. The fourth force, not included in the Standard Model, is gravity. So far, the Standard Model has been successful at describing a myriad of experimental measurements in the microscopic world. Its success is, by all means, mind blowing.

    Standard Model of Particle Physics, Quantum Diaries.

    We do not know why natural phenomena are so well described by mathematical entities and relations but, experimentally, we know that it works. Just as Galileo said four hundred years ago, the big book of Nature is written in a mathematical language[1] – the Standard Model and Einstein’s theory of gravity, for example, are additional chapters of this book.

    Particle physics makes use of a theoretical tool in which all particles are represented mathematically by quantum fields. These entities encode properties like spin and mass of a particle. In the Standard Model, the existence of the electromagnetic, weak and strong force carriers follows from the invariance of the behaviour of quantum fields under a “local gauge transformation”. This is a transformation from one field configuration to another, which can be imagined as a rotation in an abstract mathematical space. The parameters of the transformation may vary from point to point in space-time, and thus the transformation is defined as “local”. Gauge invariance or gauge symmetry is the lack of changes in measurable quantities under gauge transformations, despite the quantum fields (which represent particles) being transformed.

    Gauge invariance holds if spin-one particles are introduced, which interact in a well-defined manner with the elementary constituents of matter such as electrons and quarks (constituents of the proton and neutron, or, more generally, of hadrons). These spin-one particles are interpreted as “the carriers of the interaction” between the matter particles, with the photon the carrier for the electromagnetic force, the W–, W+ and Z bosons for the weak force, and eight gluons for the strong force. These are the (intermediate) vector bosons introduced above.

    In this way, the Standard Model forces (or interactions) emerge in a very elegant manner from one general principle, namely a fundamental symmetry of Nature. Interestingly enough, in the model, the electromagnetic and weak interactions manifest themselves at high energy as different aspects of a single “electroweak” force, while at low energy the weak interaction remains feebler than the electromagnetic interaction. As a consequence, the photon, Z boson and W± bosons are collectively named “electroweak gauge bosons”.

    In the example above, Fermi’s followed a bottom-up approach: going from an observation to a mathematical description (a contact-interaction theory), which was modified “by hand” with few additions to obey the general principle of probability conservation (known as “unitarity” in physics). Starting from this premise, the work of many physicists consequently led to a more general theory. One in which the description of the fundamental forces follows the opposite path: predictions are obtained from fundamental principles (as gauge invariance) in a mathematically and physically coherent framework.[2] The interplay between these two ways of developing knowledge had been common in physics since before Newton’s time, and still valid today.

    In both cases, a theory is successful not only when it describes the known experimental facts, but also when it has predictive power. The Standard Model possesses both virtues and examples of its predictive power include the discoveries of the Higgs boson and the neutral kind of weak interaction mediated by the Z boson.

    As a matter of fact, the Standard Model tells us (much) more: the quantum fields representing the new spin-one particles will also transform under a local gauge transformation. To ensure that the measurable quantities describing their behaviour do not change (gauge invariance, mentioned above), interactions among the carriers of the weak force must also exist, as well as among the carriers of the strong force. These self-interactions may involve three or four gauge bosons. No self-interaction among photons is possible, except indirectly through virtual processes involving intermediate particles such as electrons, as observed in a dedicated ATLAS measurement.

    The process first observed by the ATLAS and CMS Collaborations in 2017, characterised by the presence of two W bosons with the same electric charge and two jets, is a signature of the occurrence of an electroweak interaction. The dominant part of the process is due to the self-interaction among four weak gauge bosons; another central prediction of the Standard Model finally confirmed by the LHC experiments. This self-interaction manifests as a “vector boson scattering”, where two incoming gauge bosons interact and produce two, potentially different, gauge bosons as final state particles. The production rate of this electroweak process is very low – lower than that of Higgs boson – which is why it was observed only recently. And just like the Higgs boson discovery, the observation of this process didn’t come out of the blue.

    At the Large Electron–Positron (LEP) collider, which operated at CERN between 1989 and 2000 in what is today the LHC tunnel, physicists had already observed the self-interaction among three gauge bosons. They measured the production of a pair of gauge bosons of opposite charge, a W+ and a W– boson, in the collisions of beams of electrons and positrons, the antiparticle of the electron. According to the Standard Model, three main processes contribute to this production. They proceed via the exchange of either a photon, neutrino or Z boson between the electron and positron of the initial state and the W pair of the final state (Figure 3).

    CERN Large Electron–Positron Collider.

    They measured the production of a pair of gauge bosons of opposite charge, a W+ and a W– boson, in the collisions of beams of electrons and positrons, the antiparticle of the electron. According to the Standard Model, three main processes contribute to this production. They proceed via the exchange of either a photon, neutrino or Z boson between the electron and positron of the initial state and the W pair of the final state (Figure 3).

    3
    Figure 3: Diagrams representing three processes contributing to the e+e- → W+W- production. They illustrate the exchange between the initial e+e- and final W+W- of (from left to right), a neutrino (ν), photon (γ), and a Z boson, respectively. The symbol γ* is often used when a photon mediates the interaction. Following the rules of quantum mechanics, the production rate of a process is computed by the square of the sum of all possible diagrams contributing to it. The diagrams in the sum may have different relative signs, so they may cancel (destructive interference), in the same way that waves can cancel each other if they arrive out of synchronization. In the case discussed here, each diagram is necessary to avoid an unphysical continuous increase of the production rate with the collision energy and to ensure the preservation of the gauge invariance of the theory. (Image: S. Pagan Griso, L. Di Ciaccio/ATLAS Collaboration.)

    The exchange of a photon or a Z boson occurs via the self-interaction of three weak gauge bosons: WWγ and WWZ, respectively. The main point here is that without considering all three processes, the calculated production rate would grow continuously with energy, leading to the already encountered unphysical behaviour. The observation of this process at LEP, with a production rate consistent with the Standard Model prediction, therefore confirmed the existence of a self-interaction among three bosons.

    It is striking that the theory predicts the structure of each underlying process such that, even though each of them gives to the calculated production rate a contribution which at high energy becomes unphysical, violating unitarity, the unphysical behaviour cancels out when all of the processes are considered together.

    So far, so good – but there’s a catch. The W± and Z bosons observed and identified by experiments as the carriers of the weak interaction are massive, yet gauge invariance is only preserved if the carriers are massless. Should physicists give up the principle of gauge invariance to reconcile the theory with experimental facts?

    A solution to this puzzle was proposed in 1964, postulating the existence of a new spin-zero (“scalar”) field with a slightly more complex mathematical structure. While the basic laws of the forces remain exactly gauge symmetric, in the sense explained above, Nature has randomly chosen (among many possibilities) a particular lowest-energy state of this field, breaking with this choice the gauge symmetry in a limited way, called “spontaneous”.

    The consequences are dramatic. Out of this new field, a new particle emerges – the scalar Higgs boson – and the W± and Z bosons become massive. Physicists now believe that gauge symmetry was not always spontaneously broken. The universe transitioned from an “unbroken phase” with massless gauge bosons to our current “broken phase” during expansion and cool-down, a fraction of a second after the Big Bang.

    The discovery of the Higgs boson in 2012 by the ATLAS and CMS Collaborations is a great success of the Standard Model theory, especially when considering that it was found to have the mass that indirect clues were pointing to.

    CERN CMS Higgs Event May 27, 2012.

    CERN ATLAS Higgs Event
    June 12, 2012.

    While the Higgs boson mass is not predicted by theory, the existence of the Higgs boson with a given mass leaves a delicate footprint in natural phenomena such that, if measured very precisely (as was done at LEP and at Tevatron, the smaller predecessor of the LHC at Fermilab, nearby Chicago, USA), physicists could derive constraints on its mass. The Higgs boson’s discovery was thus an experimental prowess as well as a consecration of the Standard Model. It emphasized the remarkable role of the precision measurements at LEP, even though the energy of that accelerator was not high enough to directly produce the Higgs boson.

    Obviously, the story doesn’t end here. Solid indications exist that the Standard Model is not complete and that it must be encompassed in a more general theory. This possibility is not surprising. As Fermi’s weak interaction theory exemplifies, history has shown that a theory’s validity is related to the energy range (or, equivalently, size of space) accessible by experiments.

    More generally, classical mechanics is appropriate and predictive for the macroscopic world, when the speed of the objects is small with respect to the speed of light. To describe the microscopic world, however, quantum mechanics must be invoked, and the special theory of relativity must be applied to appropriately describe the behaviour of objects moving close to light speed.

    How can physicists find experimental signs that may help to formulate a more general theory than the Standard Model?

    A valuable approach is to directly search collision events for particles not included in the Standard Model. However this is inherently limited: only particles with a mass at or below the collision energy can be directly produced, due to the fundamental principle of energy conservation and following the equivalence between mass and energy. Alternative avenues, which suffer less from this limitation but are indirect, include performing very precise measurements of fundamental parameters of the Standard Model or measuring rare processes to look for deviations with respect to theoretical predictions. Such measurements are able to explore a higher energy domain, as the LEP Higgs-boson example showed.

    Vector boson scattering is one of these rare processes. It is special because closely related to the Higgs mechanism, and able to shed light on unexplored corners of Nature at the highest energy available in a laboratory. Similar to the LEP vector-boson study described above, vector boson scattering is expected to proceed via several processes, this time including the self-interaction of four gauge bosons as well as the exchange of a Higgs boson (see Figure 4). Without accounting for all of the processes, the calculated scattering rate grows indefinitely with energy, leading to the above-mentioned unphysical behaviour (violation of unitarity).

    It could be argued that this question is already settled, since we know that the Higgs boson exists. The key issue is that the way in which the Higgs boson interacts with the gauge bosons in the Standard Model is exactly what is required to moderate the growth of the scattering rate at high energy; a minimal deviation of the Higgs mechanism from the Standard Model prediction could result in an apparent breakdown of unitarity.

    Vector boson scattering would then occur at a rate different from what is predicted by the Standard Model, and unitarity would have to be recovered by a yet-unknown mechanism. The study of vector boson scattering thus allows physicists to investigate the Higgs mechanism in the highest energy domain accessible, where there may be signs of new physics.

    4
    Figure 4: Diagrams of some of the processes contributing to the W+W+ → W+W+ process. Analogous diagrams contribute to the W-W- → W-W- process. Similarly to the explanation given in Figure 3, in order to compute the production rate each contribution is first added before their sum is squared. The individual contributions may have different relative signs leading to cancellations. In this case each contribution is necessary to avoid an unphysical continuous increase of the production rate with (the square or fourth power of) the scattering energy. (Image: S. Pagan Griso, L. Di Ciaccio/ATLAS Collaboration.)

    The LHC is the perfect place to look for rare processes like vector boson scattering, as it collides protons with the highest energy and rate ever reached. Furthermore, the ATLAS and CMS experiments are designed to select and record these rare events.

    As weak gauge bosons are extremely short-lived particles, experiments search for the scattering of vector bosons by looking for the production of two jets and two lepton–antilepton pairs in proton-proton collisions. Imagine this as two gauge bosons being emitted by the quarks from each of the incoming LHC proton beams. These gauge bosons subsequently scatter off each other and the bosons emerging from this interaction promptly decay (see Figure 2). The quarks are subsequently deflected and appear in the detector as jets of particles, typically emitted at a relatively small angle with respect to the beam direction. This is called an “electroweak” process as it is mediated by electroweak gauge bosons.

    The experimental signature of vector boson scattering is therefore characterised by the presence of the decay particles of the two bosons, accompanied by two jets with large angular separation. The W and Z bosons predominantly decay into a quark and antiquark pair. Nevertheless, the search of these rare events preferentially exploits the decays into a lepton and an anti-lepton because a concurrent process, the multi-jet production, being mediated by the strong interaction has an overwhelming rate and obscures processes with a much smaller rate.

    Still, the search for vector boson scattering is very challenging. This is not only because the rate of the process is low – accounting for only one in hundreds of trillions of proton–proton interactions – but also because, even making use of the leptonic decays, several “background” processes produce the same kinds of particles in the detector, mimicking the process’ signal.

    Due to its high rate, a particularly challenging background process is one in which the jets accompanying the decay products of the gauge bosons arise as a result of the strong-force interaction. The impact of this background with respect to the signal depends on the kind of gauge bosons which scatter. When they are W bosons with the same electric charge, the production rate of the two processes (signal and background) is comparable.

    For this reason, same-charge WW production is considered the golden channel for experimental measurements and was the first target for the ATLAS Collaboration to study vector-boson-scattering processes. ATLAS physicists reported for the first time strong hints of the process in a 2014 paper [Evidence for Electroweak Production of W±W±jj in pp Collisions at s√=8 TeV with the ATLAS Detector] – a milestone in the LHC physics programme. However, it took three more years to arrive at an unambiguous observation, passing the five-sigma threshold that particle physicists use to define a discovery and corresponding to a probability of less than one in 3.5 million that a signal observation could be due to a mere upward statistical fluctuation of the number of background events. In the years between the first hint and discovery, the LHC was upgraded to increase its proton–proton collision energy – from 8 TeV to 13 TeV – as well as its collision rate – yielding about six times more collected data. These improvements made observation of vector boson scattering possible – the era of its study had at last begun.

    However, not all electroweak bosons are equal. While the observation of two same-charge W bosons has allowed physicists to start testing the interaction of four W bosons (WWWW, Figure 2), the quest to test other self-interactions remained. The Standard Model only allows a specific set of combinations of four-gauge-boson self-interactions: WWWW, WWγγ, WWZγ and WWZZ, forbidding interactions among four neutral bosons.

    Not all of these electroweak interactions are predicted to have the same strength and, because of this, probing them requires identifying processes that are less and less frequent. Similarly to the case of two same-charge W bosons, electroweak processes involving two jets and a WZ pair, a Zγ pair, or a ZZ pair are increasingly rare or have significantly larger backgrounds. Hunting for such processes among the billions of proton–proton collisions recorded by ATLAS requires physicists to look for subtle differences in order to distinguish a signal from very similar background processes occurring at much higher rates.

    5
    Table 1. List of processes presented in the text (first column) that are used to study vector boson scattering: WW with same charge, WZ, ZZ or Zɣ production in association with two jets (j), and photon-induced production of two W bosons. For each process a check indicates the four bosons involved in the self-interaction. Other measurements performed at the LHC also play a role to test these self-interactions but have been omitted in this table for simplicity. (Image: ATLAS Collaboration/CERN.)

    While such a task was commonly regarded as requiring a much larger amount of data than collected so far, the LHC experiments used artificial-intelligence algorithms to distinguish between the sought-after signal and the much larger background. Thanks to such innovations, in 2018 and 2019, ATLAS reported the observations of WZ and ZZ electroweak production, and saw a hint of the Zγ process. Suddenly, this brand-new field saw a surge in the number of processes that could be used to probe the self-interaction of gauge bosons.

    The most recent addition is ATLAS’ observation of two W bosons produced by the interaction of two photons, each radiated by the LHC protons. This phenomenon occurs when the accelerated protons skim each other, producing extremely high electromagnetic fields, with photons mediating an electromagnetic interaction between them. Such an interaction is only possible when quantum mechanical effects of electromagnetism are taken into account.

    This is a direct and clean probe of the γγWW gauge bosons interaction. A peculiarity of this process is that the protons participate as a whole and can remain intact after the interaction; this is very different from inelastic interactions where the quarks, the protons’ constituents, are the main actors (see Figure 2).

    Table 1 [above] summarises the processes that are used to study vector boson scattering at the LHC. It also shows the four bosons involved in the self-interaction. The study of each process provides a different test of the Standard Model, as modifications of the theory can differently alter the strength of the self-interactions.

    Now, ten years on from the first high-energy collisions took place in the LHC, the study of the vector boson scattering is a very active field – though still in its adolescence, both from the experimental and theoretical point of view. Experimentally, the size of the available signal sample is limited. The upcoming data-taking period (from 2022 to 2024) and the high-luminosity phase of the LHC (starting in 2027) will increase the amount of collected data by more than a factor two and by an additional factor of ten, respectively. An extensive upgrade of the LHC experiments is also ongoing, which will improve further the detection capabilities for the vector-boson-scattering processes.

    In parallel, physicists will continue to improve their analysis methods, relying on more and more advanced artificial-intelligence algorithms to disentangle the rare signal processes from the abundant backgrounds. Physicists are also employing advanced calculation techniques to improve the precision of Standard Model predictions to match the increased measurement precision.

    Furthermore, a bottom-up approach is being introduced which follows in the footsteps of Enrico Fermi. Physicists have developed a theoretical framework that allows new mathematical terms, respecting basic conservation rules and symmetries, to be added “by hand” to the Standard Model, without relying on a specific new physics model. These terms change the predictions in the high-energy regime where new physics could be expected (Figure 5). The simplest form of this approach is called Standard Model Effective Field Theory.

    6
    Figure 5. Distribution of the photon energy in the search for events resulting from the electroweak production of two vector bosons (a Z and a γ) associated with two jets (Zγjj-EW). The black polymarkers represent the data, the full histograms with different colours represent the Standard Model predicted contributions for the signal (in brown) and the many background processes (in different colours). All expected contributions are stacked. The dotted blue line in the upper panel indicates the calculated signal distribution when a new term is added to the Standard Model theory. (Image: ATLAS Collaboration/CERN.)

    Even though we know that an effective theory cannot work at an arbitrary high energy scale, history has shown that, supplemented by measurements, it can provide useful guidance at lower energy. Different production-rate measurements – including those of the Higgs boson, boson self-interactions and the top quark – can be, separately or simultaneously, compared to predictions in the same effective theoretical framework.

    It would be a sensation if more precise measurements indicated that such new terms are necessary to describe the data. It would be a sign of physics beyond the Standard Model and indication of the direction to take in order to develop a more complete theory, depending on which kinds of terms are needed. The interplay between experimental observations and models in the quest for a complete theory would continue.

    Ultimately, all ongoing experimental collider and non-collider studies in particle physics will contribute to building knowledge – be they direct searches for new particles, precision measurements exploiting the power of quantum fluctuations or studies of rare processes. This experimental work is complemented by ever more precise theoretical calculations. In this task, the next generation of powerful particle accelerators now being planned are indispensable tools to find new phenomena that would help us understand the remaining mysteries of the microscopic world.

    See the full article here.


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  • richardmitnick 9:06 am on August 26, 2020 Permalink | Reply
    Tags: "Z bosons zoom through quark–gluon plasma as jets quench", , ATLAS physicists have measured jet-quenching phenomena in the quark–gluon plasma with help of Z bosons., , CERN ATLAS, , , , , When beams of lead ions collide head-on in the LHC the matter comprising the nuclei melts away and forms a high-temperature quark–gluon plasma (QGP).   

    From CERN ATLAS: “Z bosons zoom through quark–gluon plasma as jets quench” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    17th August 2020 [Just now in social media.]
    ATLAS Collaboration

    1
    Figure 1: ATLAS event display showing the Z+jet process occurring in a lead–lead collision. In this event, the Z boson is identified by its decay to two muons (red lines). The jet can be seen as a small, collimated set of blue towers, surrounded by the transparent green cone in the lower image. (Image: ATLAS Collaboration/CERN.)

    With new data from the LHC, ATLAS physicists have measured jet-quenching phenomena in the quark–gluon plasma with help of Z bosons.

    When beams of lead ions collide head-on in the LHC, the matter comprising the nuclei melts away and forms a high-temperature quark–gluon plasma (QGP) – an extended region of interacting quarks and gluons. As high-momentum jets of particles attempt to traverse this region, their energy and radiative properties change through interactions with the QGP medium. This phenomenon is known as jet quenching. Its study can help physicists understand the properties of the QGP and give new insight into the theory of the strong nuclear force (quantum chromodynamics).

    The ATLAS Collaboration has performed extensive studies of how jets are quenched in the QGP. The emerging picture is that the quarks and gluons lose energy in the medium, fragmenting into fewer high-momentum particles. The remaining energy is redistributed by the QGP and appears as low-momentum, “thermal” particles over a broad area around the jet. For example, studies have shown that the total momentum in a jet is depleted relative to expectations from proton–proton collisions and that the distribution of particles inside the jet is modified.

    However, one challenge in interpreting these measurements is that they are made after the quenching process – so it is impossible to tell whether a given jet has traversed the medium or merely glanced at it. This can be overcome by studying events where the jet is produced as a partner to a high-momentum photon, with the two moving in opposite directions in the detector. Since photons do not interact via the strong nuclear force they pass through the QGP medium without being affected. Thus, photons serve as a “tag” or a control experiment for the jet’s momentum before quenching. ATLAS physicists have previously used this key feature to measure jet quenching and jet structure modification in photon–jet events using lead–lead data recorded in 2015.

    In new results released today based on the large lead–lead collision dataset accumulated in 2018, ATLAS researchers applied this same strategy to measure jet quenching tagged with another particle: the Z boson. Similar to photon-tagged events, a jet can be produced alongside a Z boson which decays into particles (two electrons or two muons) that do not interact with the QGP medium. An example of such a collision event can be seen in Figure 1, where a Z boson decays into two muons (red lines).

    2
    Figure 2: Ratio of the yield of charged particles opposite in angle to a Z boson between lead–lead collisions and proton-proton collisions. The charged particles are the result of jet fragmentation. As a result of the jet-quenching process, the ratio is below one at high transverse momentum (pTch), and above one at low transverse momentum. The data (red) are compared to theoretical predictions (purple, blue, green yellow). (Image: ATLAS Collaboration/CERN.)

    Unlike photons, Z bosons can be measured in a low momentum range, where experiments have difficulty triggering on photons and distinguishing them from various background particles. At these low scales, the jet momentum matches the QGP temperature scale more closely and quenching effects are expected to be large. This region is thus particularly interesting to explore, despite being experimentally difficult to access.

    The new ATLAS result measures the production of charged particles opposite a Z boson. The measurement compares lead–lead collisions and proton–proton collisions, using the leptonic Z boson “tag” to select a similar population of jets arising predominantly from high-momentum quarks. Physicists looked at the charged particle yield for each tagged Z-boson event, and measured the ratio of this quantity between head-on (“central”) lead–lead and proton–proton events (as shown in Figure 2). At large transverse momentum (> 3 GeV), there are significantly fewer charged particles in lead–lead collisions, consistent with the picture of energy loss and softer fragmentation in the QGP. At small transverse momentum (< 3 GeV), there are significantly more charged particles, reflecting the thermalization of the lost energy by the medium.

    Researchers then compared the results to a variety of state-of-the-art theoretical calculations, which describe the jet-quenching process according to different models. These calculations all indicate a suppression of high-momentum particles, with a corresponding enhancement of low-momentum particles, but each prediction differs quantitatively from the rest. These comparisons highlight the value of new experimental data to constrain theory in this particular area. The upcoming Run 3 of the LHC should bring many more Z boson events in lead–lead collisions – opening further avenues for these discriminating measurements.

    Links

    Medium-induced modification of Z-tagged charged particle yields in Pb+Pb collisions at 5.02 TeV with the ATLAS detector (submitted to Phys. Rev. Lett, see figures)
    Measurement of the nuclear modification factor for inclusive jets in lead–lead at 5.02 TeV with the ATLAS detector (Phys. Lett. B 790 (2019) 108, see figures)
    Measurement of jet fragmentation in lead–lead and proton–proton collisions at 5.02 TeV with the ATLAS detector (Phys. Rev. C 98 (2018) 024908, see figures)
    Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from proton–proton and lead–lead Collisions in ATLAS (Phys. Rev. Lett. 123 (2019) 042001, see figures)
    Measurement of photon-jet transverse momentum correlations in 5.02 TeV lead–lead and proton–proton collisions with ATLAS (Phys. Lett. B 789 (2019) 167, see figures)
    Photon-tagged jet quenching in the quark-gluon plasma, Physics Briefing, October 2017
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .


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  • richardmitnick 7:20 am on August 13, 2020 Permalink | Reply
    Tags: "ATLAS observes W-boson pair production from light colliding with light", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS observes W-boson pair production from light colliding with light” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    5th August 2020
    ATLAS Collaboration

    1
    Figure 1: A 2018 ATLAS event display consistent with the production of a pair of W bosons from two photons, where the W bosons decay into a muon and an electron (visible in the detector) and neutrinos (not detected). The muon path (red line) and electron path (yellow line) are shown. The electron deposits its energy in the electromagnetic calorimeter (yellow blocks). The many particles reconstructed in the Inner Detector are shown in orange. Top left corner shows that these particles do not originate from the same interaction and are thus attributed to additional proton–proton interactions. (Image: ATLAS Collaboration/CERN.)

    2
    Figure 2: Feynman diagram depicting the production of a pair of W bosons from two photons in a four force-carrier interaction. The photons are scattered off of two protons which in the process lose energy but remain intact. (Image: ATLAS Collaboration/CERN.)

    The ATLAS Collaboration announces the first observation of two W bosons produced from the scattering of two photons — particles of light — at the International Conference on High-Energy Physics (ICHEP 2020).

    In everyday life, two crossing light beams follow the rules of classical electrodynamics and do not deflect, absorb or disrupt one another. However, at the high energies seen in LHC collisions, effects of quantum electrodynamics become important. For a short moment, photons radiated off the incoming proton beams can scatter and transform into a particle–antiparticle pair which appears as light-by-light interactions in the detector. This process was first observed by the ATLAS Collaboration in 2019. Indeed, the Standard Model describes quantum electrodynamics as part of electroweak theory, which not only predicts that force-carrying particles – the W bosons, Z boson and photon – interact with ordinary matter, but also among themselves.

    The newly observed process proceeds via a very rare type of phenomenon where two photons collide to directly produce two W bosons of opposite electric charge via a four force-carrier interaction, among others (see Figure 2). Although the ATLAS and CMS Collaborations saw first evidence of this process in data recorded during Run 1 of the LHC (2011–2012), its observation required the substantially larger dataset taken during Run 2 (2015–2018).

    This rare process occurs as bunches of high-energy protons skim past each other in “ultra-peripheral collisions”, if only their surrounding electromagnetic fields interact. Quasi-real photons from these fields scatter off one another to produce a pair of W bosons and leave a distinct signature in the ATLAS experiment. As the skimming protons stay intact, the only detectable particles produced in the interaction are the visible decay products of the W bosons – namely, for this measurement, an electron and a muon with opposite electric charge.

    3
    Figure 3: The distribution of the number of particles reconstructed in the ATLAS inner detector, in addition to the electron and the muon. The background process with the largest contribution is the W-boson-pair production from proton constituents; its simulation (blue) describes the observed data (black points) very well. The photon-induced W-boson pair production accumulates at low particle multiplicities (white area). (Image: ATLAS Collaboration/CERN.)

    ATLAS physicists had to overcome several unique challenges to observe this process, starting with separating the signal from background. LHC protons can break up into their constituents and fragment into several detectable particles at very low energy. In particular, W-boson pairs can be produced from the proton’s constituents. This background process is hundreds of times more likely to occur than the photon–photon production of W-boson pairs, and can mimic its signature. To enhance the signal over such a background, physicists only selected collisions where no other charged particles are measured in the vicinity of the electron and the muon, as reconstructed in the ATLAS Inner Detector.

    Further, a typical collision event contains particles from 20 to 60 additional proton–proton interactions occurring simultaneously as bunches of proton cross in ATLAS. These additional particles can prevent the identification of signal events if they are produced in close proximity to the photon–photon interaction (see Figure 1).

    Physicists developed novel experimental techniques to precisely determine the contributions of these effects. Simulated events can be used to estimate the expected backgrounds, but detailed tuning on the data is needed to ensure that they provide a faithful description. Physicists performed auxiliary measurements using data consistent with resonant Z boson production, a well-understood process produced with high frequency and purity at the LHC. This dataset was used to count particles from both the additional proton-proton interactions and the proton fragmentation, and the findings allowed ATLAS physicists to tune the simulation of such events. The accurate description of these background processes made the observation of this rare phenomenon possible (see Figure 3, where the photon–photon signal interactions accumulate at low particle multiplicity).

    A total of 307 events matching the selection requirements were found in the analysed dataset, of which 174 were attributed to be from the photon–photon production of W-boson pairs and the remaining events to various background processes. Such a yield corresponds to a statistical significance of 8.4 standard deviations, which is well above the established 5 standard deviations criterion for the unambiguous observation of a process. The cross section is measured to be 3.13 ± 0.42 fb. This means that only one or two such interactions occurred in the 30 trillion proton–proton interactions of a typical day of data-taking in 2018.

    The four force-carrier interaction is an integral part of electroweak theory and, at the same time, can be sensitive to modifications of the Standard Model by unaccounted-for new physics. The experimental techniques presented by the ATLAS Collaboration will enable future measurements that can probe such modifications and test the electroweak theory in a novel way.

    See the full article for reference literature.

    See the full article here .


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  • richardmitnick 9:12 am on August 4, 2020 Permalink | Reply
    Tags: "New ATLAS result marks milestone in the test of Standard Model properties", , CERN ATLAS, , , ,   

    From CERN ATLAS: “New ATLAS result marks milestone in the test of Standard Model properties” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    3rd August 2020
    ATLAS Collaboration

    1
    Figure 1: Diagrams of a lepton-flavour-violating Z-boson decay (left) and of the two main backgrounds to the search: a lepton-flavour-conserving Z-boson decay into a pair of tau leptons (middle) and a W-boson decay with leptons (right). The green arrows represent electrons or muons (l), the blue triangles are the visible component of hadronically-decaying tau leptons (τ had-vis) or the hadronisation of a quark or a gluon, and the dashed blue lines represent undetected neutrinos. (Image: ATLAS Collaboration/CERN)

    The ATLAS Collaboration has released a new study into a key building block of matter: leptons. This type of particle comes in three different families (flavours) and, according to the Standard Model, should follow strict rules. For instance, except for their mass, leptons of different flavours have identical properties – a feature known as lepton flavour universality. This was recently corroborated by a key measurement of the W-boson decay rates into leptons by the ATLAS Collaboration.

    Yet the Standard Model has known shortcomings. For example, it predicted that leptons could only interact with other leptons of the same flavour. Experiments observed neutrinos (neutral leptons) violate this hypothesis, transforming from one flavour into another in processes known as neutrino oscillation. This led physicists to realise that neutrinos were not, in fact, massless, as originally assumed in the Standard Model. Such discoveries show that the fundamental structure of Nature is more complex than one had thought.

    Could the violation of lepton flavour seen in neutral leptons also occur among charged leptons (with dramatic consequences)? In a new result, the ATLAS Collaboration searched for lepton-flavour-violating decays of the Z boson, where the Z boson decays into two charged leptons of different flavours. Such events are predicted from neutrino oscillation to be so rare – accounting for just one in 1054 Z-boson decays – that they should be undetectable. If they were to be observed at ATLAS, it would be an unequivocal sign of new physics beyond the Standard Model.

    Standard Model of Particle Physics, Quantum Diaries

    2
    Figure 2: Distribution of the neural-network output of some of the Z-boson-decay candidates analysed in the search. In the upper panel, data (black dots) are compared to the stacked expected contributions from background processes, mainly Z-boson decays to tau-lepton pairs (dark blue) and W-boson decays (yellow). The expected contribution from signal events with a decay rate of five in ten thousand is shown by the red dashed line. The lower panel shows the ratio of the data to the background prediction. No significant excess of events is seen in the data. (Image: ATLAS Collaboration/CERN)

    Physicists examined ATLAS data collected over two runs of the Large Hadron Collider (LHC, 2012–2018) to set strong constraints on lepton-flavour-violating decays involving a tau lepton (τ) and an electron (e) or a muon (μ). While there have been several low-energy experiments that specialise in lepton-flavour-violating searches, they cannot easily probe transitions involving Z bosons. This is an area where high-energy accelerators, such as CERN’s previous Large Electron Positron (LEP) collider (1989–2000) and now the LHC, play a special role.

    CERN Large Electron Positron Collider

    The ATLAS result marks a new milestone in a legacy of precision measurements established by LEP. Searching for rare Z-boson decays is a great challenge for LHC experiments. Whereas LEP produced an abundance of Z bosons in a relatively clean environment, only a small fraction of LHC collisions produce a Z boson – and always along with several background collisions. However, the LHC has one major advantage: it can produce Z bosons at a much faster rate! More than 20 years on, ATLAS’ result now supersedes those of the LEP experiments (OPAL, DELPHI).

    To analyse the enormous LHC Run 2 dataset – with its 8 billion Z bosons – ATLAS physicists developed a state-of-the-art machine-learning method using deep neural networks. The neural networks were trained to identify the kinematic properties of the signal process, where a Z boson decays into an electron or a muon and a tau lepton, which itself is unstable and decays (Figure 1). They also had to differentiate the signal process from mainly two others that produce the same particles: the lepton-flavour-conserving Z-boson decay into a pair of tau leptons, where one tau lepton decays into an electron or a muon (plus undetected neutrinos), and the W-boson decay into leptons, produced together with an additional jet of particles.

    The output distributions of the neural networks for the selected candidate event were studied to determine the presence of lepton-flavour-violating Z-boson decays (Figure 2 for the tau lepton and muon case). The classification power of the neural networks – combined with the unprecedented number of Z-boson decays studied – allowed ATLAS to set constraints on the maximum rate at which lepton-flavour-violating Z-boson decays involving a tau lepton can occur. The result excludes, at 95% confidence level, Z-boson decay rates greater than 8.1×10-6 (Z→τe) and 9.5×10-6 (Z→τμ).

    The new ATLAS result provides experimental guidance towards new theories that could explain the shortcomings of the Standard Model. The precision of the result is largely limited by the number of analysed Z-boson decays. Therefore, ATLAS is looking forward to improved sensitivity that is to be expected from the additional data of future LHC runs.

    ________________________________________________

    Links

    Lepton Flavour Violation at the LHC: a search for Z→eτ and Z→μτ decays with the ATLAS detector (ATLAS-CONF-2020-035)
    ICHEP2020 presentation by Karl Jakobs: ATLAS Highlights
    OPAL Collaboration, A search for lepton flavor violating Z0 decays , Z. Phys. C67 (1995) 555
    DELPHI Collaboration, Search for lepton flavor number violating Z0 decays, Z. Phys. C73 (1997) 243
    New ATLAS result addresses long-standing tension in the Standard Model, Physics Briefing, 28 May 2020
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .


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  • richardmitnick 3:27 pm on August 3, 2020 Permalink | Reply
    Tags: "CERN experiments announce first indications of a rare Higgs boson process", , CERN ATLAS, , , , , , , The ATLAS and CMS experiments at CERN have announced new results which show that the Higgs boson decays into two muons.   

    From CERN: “CERN experiments announce first indications of a rare Higgs boson process” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    3 August, 2020

    The ATLAS [below] and CMS [below] experiments at CERN have announced new results which show that the Higgs boson decays into two muons.

    1
    Candidate event displays of a Higgs boson decaying into two muons as recorded by CMS (left) and ATLAS (right). (Image: CERN)

    Geneva. At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

    Physicists at CERN have been studying the Higgs boson since its discovery in 2012 in order to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analysing how it decays into the various fundamental particles and the rate of disintegration.

    CMS achieved evidence of this decay with 3σ, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s 2σ result means the chances are one in 40 [strange, lower statistical signifance but greater probability, never saw that before] . The combination of both results would increase the significance well above 3σ and provides strong evidence for the Higgs boson decay to two muons.

    “CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.

    The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.

    Standard Model of Particle Physics, Quantum Diaries

    “This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics programme. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.

    What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimised to categorise physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.

    The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.

    Scientific materials

    Papers:
    CMS physics analysis summary: https://cds.cern.ch/record/2725423
    ATLAS paper on arXiv: https://arxiv.org/abs/2007.07830

    Physics briefings:
    CMS: https://cmsexperiment.web.cern.ch/news/cms-sees-evidence-higgs-boson-decaying-muons
    ATLAS: https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons

    Event displays and plots:
    CMS: https://cds.cern.ch/record/2720665?ln=en
    http://cds.cern.ch/record/2725728
    ATLAS: https://cds.cern.ch/record/2725717?ln=en
    https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-14

    Images:
    CMS muon system:

    ATLAS muon spectrometer:

    See the full article here.


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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

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  • richardmitnick 11:42 am on August 1, 2020 Permalink | Reply
    Tags: "New measurements of the Higgs boson find strength in unity", , , CERN ATLAS, , , , , The Standard Model remains unperturbed.   

    From CERN ATLAS: “New measurements of the Higgs boson find strength in unity” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    31st July 2020
    ATLAS Collaboration

    ATLAS reports an important boost in the precision of combined measurements of Higgs-boson couplings, as analyses of the full Run-2 dataset proceed.

    1
    Figure 1: Event display of a Higgs-boson candidate produced in association with a Z boson (ZH production), with the Higgs boson decaying to four leptons (H → ZZ*→ 2e2μ), and the Z boson to a pair of muons (Z→μμ). The Higgs boson is reconstructed from the two electrons (two green tracks and bars representing energy deposited in the calorimeter) and two muons (two red tracks on the left passing through the blue muon chambers). The associated Z boson recoils against the Higgs boson to produce two additional muons (two red tracks on the right). (Image: ATLAS Collaboration/CERN)

    The Higgs boson, first predicted in the 1960s and discovered by the ATLAS [above] and CMS experiments in 2012, is a unique elementary particle arising from the mass-generating Higgs mechanism of the Standard Model.

    CERN CMS Higgs Event May 27, 2012

    Standard Model of Particle Physics, Quantum Diaries

    It thus has a peculiar affinity to mass: the larger the mass of an elementary particle, the stronger its interaction (or coupling) with the Higgs boson. Any deviation from this pattern would reveal new physics.

    Physicists can study Higgs-boson couplings in several ways: by measuring the rates of different Higgs boson production mechanisms and decays, and also by studying the particle’s kinematic properties. The ATLAS Collaboration has just presented precise new measurements of these key quantities. Several of these measurements were updated to use the full LHC Run 2 dataset (2015–2018), to provide the best precision to date.

    When combined, ATLAS’ new measurements give detailed insight into this one-of-a-kind particle. They significantly outperform previous measurements, with the overall production rate of the Higgs boson found to be in good agreement with the Standard Model, within a measurement precision of 5% and about 4% uncertainty in the Standard Model prediction.

    2
    Figure 2: Cross sections for ggF, VBF, WH, ZH and ttH+tH normalized to their Standard Model predictions, measured assuming Standard Model values for the decay branching fractions. The black error bars, blue boxes and yellow boxes show the total, systematic, and statistical uncertainties in the measurements, respectively. The gray bands indicate the theory uncertainties in the Standard Model cross-section predictions. The compatibility level between the measurement and the Standard Model prediction is 86%. (Image: ATLAS/CERN)

    Channel surfing with Higgs boson decays

    ATLAS physicists began by measuring all of the main decay “channels” of the Higgs boson: into a pair of photons, W or Z bosons, tau leptons, bottom quarks – and even muons. Though the coupling to muons is difficult to probe, ATLAS physicists recently reported a first hint of the Higgs boson decay to muons. ATLAS researchers also searched for Higgs bosons decaying to “invisible” particles, leaving only missing transverse energy in the detector – a possible portent of dark matter, for example. Their new result sets the strongest limits yet on this process, establishing that less than 13% of Higgs boson decays could be into “invisible” particles.

    These measurements could then be broken down into the major production modes of the Higgs boson: gluon fusion (ggF), vector-boson fusion (VBF), the associated production with a W or Z boson (WH, ZH), and the associated production with top quarks (ttH, tH), as shown in Figure 2. All of these are now observed and precisely measured, with the experimental sensitivity of some modes nearing the precision of state-of-the-art theory predictions. ATLAS has furthermore established for the first time the separate observation of the associated production of the Higgs boson with, respectively, a W boson and a Z boson.

    Further, the kinematic properties of the Higgs boson were assessed with unprecedented precision. Physicists introduced finer partitions of the various production modes – studying, for example, the Higgs boson transverse momentum or the number of jets in an event – to uncover potential hints of new physics. For the first time, ATLAS has also measured the differential distribution of the Higgs boson transverse momentum in ttH production, shedding new light on the boson’s interaction with the top quark.

    With these measurements in hand, physicists were able to decipher the Higgs-boson couplings to other elementary particles. As shown in Figure 3, the strength of the coupling increases with the mass of the elementary particle, in good agreement with the Standard Model. This holds true across a wide range in masses, from the top quark (the heaviest particle in the Standard Model) down to the muon (1600 times lighter than the top quark).

    3
    Figure 3: The coupling-strength for fermions (t, b, τ, μ) and weak gauge bosons (W, Z) on the y-axis vs their mass on the x-axis. The Standard Model prediction is also shown (dotted line). The lower inset shows the ratios of the values to their Standard Model predictions. The level of compatibility between the combined measurement and the Standard Model prediction is 84%. (Image: ATLAS Collaboration/CERN)

    4
    Figure 4: The measured coupling to photons on the y-axis vs the coupling to gluons on the x-axis. The best-fit value for the two measurements is shown by a cross and the Standard Model hypothesis by a star. Ellipses show the 68% and 95% confidence-level contours from a combined fit. The compatibility level between the combined measurement and the Standard Model prediction is 51%, well within the one standard deviation (68%) level. (Image: ATLAS Collaboration/CERN)

    Exploration through combination

    ATLAS physicists paid particular attention to processes such as gluon-fusion production of the Higgs boson and Higgs-boson decays to a pair of photons. Both the gluon and photon are massless, and thus cannot directly interact with the Higgs boson. These processes are therefore mediated by other massive particles via loop interactions, which could be hideouts for new particles.

    Though experiments cannot directly see these loop interactions, there are still ways to infer their content. The presence of new particles would change the rate for ggF production or the Higgs boson decaying into photons. In Figure 4, the measured gluon and the photon couplings are compared to theoretical predictions. A deviation of the measured values from unity, if established, would be a smoking gun for new physics lurking in loop interactions. Instead, ATLAS physicists observed a good agreement with the Standard Model, with measured uncertainties on the measured gluon and photon couplings as low as 5%, and an overall agreement with expectations at the 51% confidence level.

    Finally, by combining together the various Higgs-boson decay measurements and including these loop interactions, ATLAS physicists set another limit on new physics. Showing the value of combined studies, this result sets a new limit of 9% for Higgs boson decays to “invisible” particles – an improvement from the 13% of the measurement quoted above.

    The Standard Model remains unperturbed

    Thanks to the excellent performance of the LHC and the ATLAS detector during Run 2, several ATLAS results have been combined to probe the couplings of the Higgs boson at unprecedented levels. Though the Standard Model remains unperturbed, the exploration is just beginning! Some important but difficult analysis channels are still to use the full Run-2 dataset – offering additional insight into the Higgs boson’s secrets.

    See the full article here .


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  • richardmitnick 7:28 am on July 31, 2020 Permalink | Reply
    Tags: "Looking forward: ATLAS measures proton scattering when light turns into matter", , ATLAS Forward Proton (AFP) spectrometer (fig No.1 in post), , CERN ATLAS, , , , , Physicists studied data recorded by the AFP spectrometer throughout 2017 to establish direct evidence of these scattered protons when matter – electron–positron or muon–antimuon pairs – are cr,   

    From CERN ATLAS: “Looking forward: ATLAS measures proton scattering when light turns into matter” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    30th July 2020
    ATLAS Collaboration

    Today, at the International Conference for High Energy Physics (ICHEP 2020), the ATLAS Collaboration announced first results using the ATLAS Forward Proton (AFP) spectrometer (Figure 1). With this instrument, physicists directly observed and measured the long sought-after prediction of proton scattering when particles of light turn into matter.

    1
    Figure 1: Schematic diagram of ATLAS Forward Proton (AFP) spectrometer relative to the main ATLAS detector (not to scale). After the incident proton beams intersect, the leptons are detected by the main ATLAS detector and the scattered proton is detected by AFP. (Image: ATLAS Collaboration/CERN)

    In 1928, theoretical physicist Paul Dirac predicted the existence of the positron, the positively-charged antimatter partner to the electron. When brought together, this matter–antimatter pair annihilates into two particles of light (photons). Remarkably, quantum mechanics predicts that the reverse can also occur. Two photons with sufficient energy can turn into a matter–antimatter pair, as shown in Figure 2.

    2
    Figure 2: Diagram of a pair of photons (γ) turning into a pair of leptons (electrons or muons) (ℓ). The scattered protons (p) can remain intact in such interactions, but deflected from their paths along the beam so that they can be measured in a proton spectrometer. (Image: ATLAS Collaboration/CERN)

    To observe this phenomenon, physicists can use the LHC, a proton–proton collider, as a photon–photon collider. Usually, particles are created by protons colliding head-on which break apart. However, if two protons pass very close to each other, they can scatter via the electromagnetic force to produce photons that turn into a matter–antimatter pair. The two protons remain intact, continuing their path in the LHC beam pipe, which the AFP spectrometer can detect. Observing these intact scattered protons is a hallmark of a photon–photon collision.

    The AFP spectrometer is unique in many ways. Installed in 2017, it is one of the newest additions to the ATLAS experiment. It sits either side of the main ATLAS cavern, just over 200 metres downstream from the collision point as shown in Figure 1. Its detectors are based on silicon technology, which reach directly into the LHC beam pipe to only two millimetres from the proton beam itself. If a scattered proton emits a photon and loses a few percentage points of energy, the LHC magnets deflect the proton into the AFP spectrometer. These scattered protons are among the highest-energy particles measured at the LHC.

    Physicists studied data recorded by the AFP spectrometer throughout 2017 to establish direct evidence of these scattered protons when matter – electron–positron or muon–antimuon pairs – are created from the interaction of two photons. This was achieved by comparing the proton energy loss measured by the AFP spectrometer from the proton deflection angle to the produced matter–antimatter pair recorded in the central ATLAS experiment, as shown in Figure 3. If the scattered proton arose while photons turned into matter, the measurements from both locations are predicted to be equal (within the measurement precision).

    The ATLAS Collaboration has observed this striking phenomenon, recording 180 events that have an intact proton detected by the AFP spectrometer and a matching electron–positron or muon–antimuon pair measured in the main ATLAS detector. The expected background from accidentally matching forward protons amounts to about 20 events. The statistical significance of this result thus exceeds 9 standard deviations for each electron and muon channels.

    This landmark measurement using the AFP spectrometer provides valuable information about how often the protons stay intact, which is challenging to calculate from theory. These measurements are important tests of how light interacts with matter at the highest laboratory energies. Certain theories predict such interactions are modified by new particles that could explain the mysterious dark matter in our universe. With more data, physicists can use the AFP to search for these phenomena in new ways.

    3
    Figure 3: The fractional proton energy loss measured by the AFP spectrometer (ξAFP) is compared to that measured from the electron or muon pairs in the central ATLAS detector (ξll). A signal peak is observed when these two quantities are approximately equal, indicating that the scattered proton emitted a photon that produced the lepton pair. The labels A and C denote opposite sides of the collision point along the beam line. (Image: ATLAS Collaboration/CERN)

    See the full article here .


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  • richardmitnick 8:39 am on July 30, 2020 Permalink | Reply
    Tags: "Jetting into the dark side: a precision search for dark matter", , , CERN ATLAS, , , Momentum conservation in the transverse detector plane – that is perpendicular to the beam direction ., , ,   

    From CERN ATLAS: “Jetting into the dark side: a precision search for dark matter” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    27th July 2020
    ATLAS Collaboration

    1
    Figure 1: A monojet event recorded by the ATLAS experiment in 2017, with a single jet of 1.9 TeV transverse momentum recoiling against corresponding missing transverse momentum (MET). The green and yellow bars show the energy deposits in the electromagnetic and hadronic calorimeters, respectively. The MET is shown as the red dashed line on the opposite side of the detector. (Image: ATLAS Collaboration/CERN)

    The nature of Dark Matter remains one of the great unsolved puzzles of fundamental physics. Unexplained by the Standard Model, dark matter has led scientists to probe new physics models to understand its existence.

    Standard Model of Particle Physics, Quantum Diaries

    Many such theoretical scenarios postulate that dark matter particles could be produced in the intense high-energy proton–proton collisions of the LHC. While the dark matter would escape the ATLAS detector unseen, it could occasionally be accompanied by a visible jet of particles radiated from the interaction point, thus providing a detectable signal.

    The ATLAS Collaboration set out to find just that. Today, at the International Conference in High-Energy Physics (ICHEP 2020), ATLAS presented a new search for novel phenomena in collision events with jets and high missing transverse momentum (MET). The search was designed to uncover events that could indicate the existence of physics processes that lie outside the Standard Model and, in doing so, open a window to the cosmos.

    To identify such events, physicists exploited the principle of momentum conservation in the transverse detector plane – that is, perpendicular to the beam direction – looking for visible jets recoiling from something invisible. As events with jets are common at the LHC, physicists further refined their parameters: the events had to have at least one highly energetic jet and significant MET, generated by the momentum imbalance of the “invisible” particles. This is known as a monojet event – a spectacular example of which can be seen in Figure 1, a 2017 event display featuring the highest-momentum (1.9 TeV) monojet recorded so far by ATLAS.

    A plethora of exotic phenomena, not directly detectable by collider experiments, could also have yielded this characteristic monojet signature. ATLAS physicists thus set out to make their study inclusive of several new physics models, including those featuring supersymmetry, dark energy, large extra spatial dimensions, or axion-like particles.

    2
    Figure 2: Missing transverse momentum distribution after the monojet selection in data and in the Standard Model predictions. The different background processes are shown with colours. The expected distributions of dark energy, supersymmetric and weakly-interacting massive particle scenarios are illustrated with dashed lines. (Image: ATLAS Collaboration/CERN).

    Evidence of new phenomena would be seen in an excess of collision events with large MET when compared to the Standard Model expectation. Accurately predicting the different background contributions was a key challenge, as several abundant Standard Model processes could exactly mimic the signal topology – such as the production of a jet plus a Z boson, which then decays to two neutrinos that also leave ATLAS without being directly detected.

    Physicists used a combination of data-driven techniques and high-precision theoretical calculations to estimate the Standard Model background. The total background uncertainty in the signal region ranges from about 1% to 4% in the range of MET between 200 GeV and 1.2 TeV. The shape of the MET spectrum was used to enhance the discrimination power between signals and backgrounds, thus increasing the discovery potential. Figure 2 shows a comparison of the MET spectrum observed in the entire dataset collected from the ATLAS experiment during Run 2 (2015–2018), and the Standard Model expectation.

    As no significant excess was observed, physicists used the level of agreement between data and the prediction to set limits on the parameters of new physics models. In the context of weakly-interacting massive particles (a popular dark matter candidate), ATLAS physicists were able to exclude dark matter particle masses up to about 500 GeV and interaction axial-vector mediators up to 2 TeV, both at the 95% confidence level. These results provide the most stringent dark matter limits in collider experiments so far, and a milestone of the ATLAS search programme.

    See the full article here .

    _________________________________________________

    Dark Matter Background
    Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

    Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

    Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova


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  • richardmitnick 6:39 pm on July 29, 2020 Permalink | Reply
    Tags: "ATLAS result addresses long-standing tension in the Standard Model", , , CERN ATLAS, Each lepton flavour is equally likely to interact with a W boson., , Lepton flavour universality, , ,   

    From CERN ATLAS: “ATLAS result addresses long-standing tension in the Standard Model” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    From CERN ATLAS

    29 July, 2020

    A new ATLAS measurement of a key feature of the Standard Model known as lepton flavour universality suggests that a previous discrepancy measured by the LEP collider in W boson decays may be due to a fluctuation.


    Researchers from the ATLAS collaboration explain their new measurement of “lepton flavour universality” – a unique property of the Standard Model of particle physics. (Video: CERN)

    The best-known particle in the lepton family is the electron, a key building block of matter and central to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, the muon and the tau lepton, and together they are known as the three lepton flavours. According to the Standard Model of particle physics, the only difference between the siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lepton is about 17 times heavier than the muon. It is a remarkable feature of the Standard Model that each flavour is equally likely to interact with a W boson, which results from the so-called lepton flavour universality. Lepton flavour universality has been probed in different processes and energy regimes to high precision.

    Standard Model of Particle Physics, Quantum Diaries

    In a new study, described in a paper posted today on the arXiv [ “Test of the universality of τ and μ lepton couplings in W-boson decays from tt¯ events with the ATLAS detector” ( https://arxiv.org/abs/2007.14040 )] and first presented at the LHCP 2020 conference, the ATLAS collaboration presents a precise measurement of lepton flavour universality using a brand-new technique.

    ATLAS physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. “The LHC is a top-quark factory, and produced 100 million top-quark pairs during Run 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a large unbiased sample of W bosons decaying to muons and tau leptons, which was essential for this high-precision measurement.”

    They then measured the relative probability that the lepton resulting from a W-boson decay is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity, as the strength of the interaction with a W boson should be the same for a tau-lepton and a muon. But there has been tension about this ever since the 1990s when experiments at the Large Electron-Positron (LEP) collider measured R(τ/μ) to be 1.070 ± 0.026, deviating from the Standard Model expectation by 2.7 standard deviations.

    CERN Large Electron Positron Collider

    The new ATLAS measurement gives a value of R(τ/μ) = 0.992 ± 0.013. This is the most precise measurement of the ratio to date, with an uncertainty half the size of that from the combination of LEP results. The ATLAS measurement is in agreement with the Standard Model expectation and suggests that the previous LEP discrepancy may be due to a fluctuation.

    “The LHC was designed as a discovery machine for the Higgs boson and heavy new physics,” says ATLAS Spokesperson Karl Jakobs. “But this result further demonstrates that the ATLAS experiment is also capable of measurements at the precision frontier. Our capacity for these types of precision measurements will only improve as we take more data in Run 3 and beyond.”

    Although it has survived this latest test, the principle of lepton flavour universality will not be completely out of the woods until the anomalies in B-meson decays recorded by the LHCb experiment have also been definitively probed.

    See the full article here .


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