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  • richardmitnick 11:32 am on April 6, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , Improving our understanding of photon pairs, ,   

    From CERN ATLAS: “Improving our understanding of photon pairs” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    5th April 2017
    ATLAS Collaboration

    1
    Figure 1: The measured differential cross section as a function of the invariant mass of the photon pair is compared to predictions from four theoretical computations. The invariant mass is often the most scrutinized distribution when searching for new physics. (Image: ATLAS Collaboration/CERN)

    High-energy photon pairs at the LHC are famous for two things. First, as a clean decay channel of the Higgs boson. Second, for triggering some lively discussions in the scientific community in late 2015, when a modest excess above Standard Model predictions was observed by the ATLAS and CMS collaborations. When the much larger 2016 dataset was analysed, however, no excess was observed.

    Yet most photon pairs produced at the LHC do not originate from the decay of a Higgs boson (or a new, undiscovered particle). Instead, more than 99% are from rather simple interactions between the proton constituents, such as quark-antiquark annihilation. ATLAS physicists have put significant effort into improving our understanding of these Standard Model processes.

    ATLAS has released a new measurement of the inclusive di-photon cross section based on the full 2012 proton-proton collision dataset recorded at a centre-of-mass energy of 8 TeV. The precision is increased by a factor of two compared to the previous ATLAS measurement (based on the smaller 2011 data sample recorded at 7 TeV), such that the total experimental uncertainty is now typically 5%.

    According to the theory of strong interactions, the production rate of such Standard Model processes is sensitive to both high-order perturbative terms (more complex particle interactions involving quantum fluctuations) and the dynamics of additional low-energy particles emitted during the scattering process. Theoretical predictions are thus currently precise only at the 10% level. Calculations based on a fixed number of perturbative terms in the series expansion (next-to-leading order and next-to-next to leading order in the strong coupling strength) underestimate the data beyond the projected theoretical uncertainties.

    2
    Figure 2: The measured differential cross section as a function of the φ* variable is compared to predictions from four theoretical computations. The low φ* region is most sensitive to the dynamics of additional low-energy particles emitted during the scattering process. (Image: ATLAS Collaboration/CERN)

    In the new ATLAS result, the distortion in the photon pair production rate originating from the emission of low-energy particles has been probed very precisely thanks to the study of two new observables. By accurately modelling the additional emission, the predictions are found to agree with the data in the sensitive regions.

    These results provide crucial information for both experimentalists and theorists on the dynamics of the strong interaction at the LHC, and should lead to improved Standard Model predictions of di-photon processes.

    Links:

    Measurements of integrated and differential cross sections for isolated photon pair production in pp collisions at 8TeV with the ATLAS detector.

    See the full article here .

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

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  • richardmitnick 10:25 am on April 2, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , ,   

    From CERN ATLAS: “ATLAS highlights from Moriond” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    1
    The highest-mass dijet event measured by ATLAS (mass = 8.12TeV). (Image: ATLAS Collaboration/CERN)

    At this year’s Rencontres de Moriond, the ATLAS collaboration presented the first results examining the combined 2015/2016 LHC data at 13 TeV proton–proton collision energy. Thanks to outstanding performance of the CERN accelerator complex last year, this new dataset is almost three times larger than that available at ICHEP, the last major particle physics conference held in August 2016.

    The significant increase in data volume has greatly improved ATLAS’ sensitivity to possible new particles predicted by theories beyond the Standard Model. At the same time, it has also allowed ATLAS physicists to perform precise measurements of the properties of known Standard Model particles.

    A selection of Moriond 2017 highlights are explored below; find the full list of ATLAS public results here, with recent Run 2 results here.

    The search for supersymmetry

    Supersymmetry (SUSY) has long been considered a front-runner for solving a number of mysteries left unexplained by the Standard Model, including the magnitude of the mass of the Higgs boson and the nature of the dark matter. Among the key new results presented at Moriond were the first searches for SUSY particles using the new dataset. These new ATLAS results, along with those from the CMS experiment, provide the most challenging tests of the SUSY theory carried out so far.

    Searches for “squark” and “gluino” particles decaying to Standard Model particles revealed no evidence for their existence, and have set limits on the masses of these particles which extend, for the first time, as high as 2 TeV. Searches for “top squark” particles, the existence of which is crucial if SUSY is to explain the mass of the Higgs boson, also found no deviations from expected Standard Model processes.

    A new search for long-lived “chargino” particles was also presented. This search utilizes the Insertable B-Layer (IBL) detector installed during the 2014 LHC shutdown. The IBL is a new piece of ATLAS charged particle detection hardware as close as 3.3 cm to the LHC beam pipe. The new search looks for ‘disappearing’ tracks created by charginos traversing the IBL before decaying into invisible dark matter. No evidence for such tracks was found, significantly constraining a large class of SUSY models. An alternative search for new long-lived particles decaying to charged particles via the signature of displaced decay vertices also found the data to be consistent with Standard Model expectations.

    Exotic explorations

    In addition to searches for SUSY particles, ATLAS reported a number of new results in the search for “exotic” forms of beyond the Standard Model physics. Searches for new heavy particles that decay into pairs of jets (thus sensitive to a possible quark substructure) or to a Higgs boson and a W or Z boson set constraints on the masses of these exotic new particles as high as 6 TeV.

    Searches for the production of dark matter particles were also reported. These look at events in which Standard Model particles, such as photons or Higgs bosons, recoil against the invisible dark matter particles to generate an eve­­nt property called missing transverse energy. Again, the data were consistent with expectations from Standard Model processes.

    In addition, a search for a heavy partner of the W boson (a W’ boson), predicted by many Standard Model extensions, was carried out with the new dataset. In the absence of evidence of a signal, the search has set new limits on the W’ mass up to 5.1 TeV.

    Rare Higgs decays

    Following the discovery of the Higgs boson in 2012, a major component of the ATLAS physics programme has been devoted to measuring its properties and searching for rare processes by which it may decay. These analyses are crucial to establish whether the Higgs boson observed by ATLAS is that predicted by the Standard Model, or if it is instead the first evidence of new physics.

    The ATLAS collaboration presented a new search for a rare process where the Higgs boson decays to muon pairs. Observation of this process above the rate predicted by the Standard Model could provide evidence for new physics. No evidence was seen however, allowing limits to be set on the decay probability of 2.7 times the Standard Model expectation. That limit probes (and proves) the fundamental Standard Model prediction of different Higgs boson-to-lepton couplings for different lepton generations.

    Standard Model measurements

    Analysing data taken in 2012, the ATLAS Collaboration presented a number of measurements of the production and properties of known Standard Model particles. Among these was a major milestone result for the LHC programme: the first measurement of the W boson mass by the ATLAS experiment. Measured with a precision of 19 MeV, the result rivals the best previous result from a single experiment. The measurement provides an excellent test of the Standard Model via so-called virtual corrections through the interplay between the W boson, top-quark and Higgs boson masses, all precisely measured by ATLAS.

    Another key new result was a measurement of the decay properties of Bd mesons decaying to a K* meson and two muons. The LHCb and Belle collaborations had previously reported evidence of an excess above Standard Model expectations in one particular decay parameter, P5’. The new ATLAS measurement also provides evidence of a modest excess, albeit with significant statistical uncertainties. Analysis of the new dataset should enable a clearer picture of this process to be obtained.

    In addition, ATLAS presented precise new measurements of the production and properties of photon pairs in 8 TeV collisions. This result represents an important addition to our understanding of quantum chromodynamics (QCD), the Standard Model theory of the strong force.

    The search continues

    While no evidence for new physics has yet been found, these new results have provided crucial input to our theoretical models and has greatly improved our understanding of the Standard Model. We can look forward more results using the new dataset in the coming months. What is more, with the LHC set to continue its excellent performance in 2017, ATLAS can expect even greater sensitivity in results to come.

    See the full article here .

    CERN LHC Map
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    CERN LHC particles
    LHC at CERN

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  • richardmitnick 11:59 am on March 22, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , , Quest for the lost arc   

    From ATLAS: “Quest for the lost arc” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    21st March 2017
    ATLAS Collaboration

    1
    Figure 1: ATLAS simulation showing a hypothetical new charged particle (χ1+) traversing the four layers of the pixel system and decaying to an invisible neutral particle (χ10) and an un-detected pion (π+). The red squares represent the particle interactions with the detector. (Image: ATLAS Collaboration/CERN)

    Nature has surprised physicists many times in history and certainly will do so again. Therefore, physicists have to keep an open mind when searching for phenomena beyond the Standard Model.

    ..
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Some theories predict the existence of new particles that live for a very short time. These particles would decay to known particles that interact with the sophisticated “eyes” of the ATLAS detector. However, this may not be the case. An increasingly popular alternative is that some of these new particles may have masses very close to each other, and would thus travel some distance before decaying. This allows for the intriguing possibility of directly observing a new type of particle with the ATLAS experiment, rather than reconstructing it via its decay products as physicists do for example for the Higgs boson.

    2
    Figure 2: The number of reconstructed short tracks (tracklets) as a function of their transverse momentum (pT). ATLAS data (black points) are compared with the expected contribution from background sources (gray solid line shows the total) . A new particle would appear as an additional contribution at large pT, as shown for example by the dashed red line. The bottom panel shows the ratio of the data and the background predictions. The error band shows the uncertainty of the background expectation including both statistical and systematic uncertainties. (Image: ATLAS Collaboration/CERN)

    An attractive scenario predicts the existence of a new electrically charged particle, a chargino (χ1±), that may live long enough to travel a few tens of centimetres before decaying to an invisible neutral weakly interacting particle, a neutralino (χ10). A charged pion would also be produced in the decay but, due to the very similar mass of the chargino and the neutralino, its energy would not be enough for it to be detected. As shown in Figure 1, simulations predict a quite spectacular signature of a charged particle “disappearing” due to the undetected decay products.

    ATLAS physicists have developed dedicated algorithms to directly observe charged particles travelling as little as 12 centimetres from their origin. Thanks to the new Insertable B-Layer, these algorithms show improved performance reconstructing such charged particles that do not live long enough to interact with other ATLAS detector systems. So far, the abundance and properties of the observed particles are in agreement with what is expected from known background processes.

    New results presented at the Moriond Electroweak conference set very stringent limits on what mass such particles may have, if they exist. These limits severely constrain one important type of Supersymmetry dark matter. Although no new particle has been observed, ATLAS physicists continue the search for this “lost arc”. Stay tuned!

    See the full article here .

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

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  • richardmitnick 1:08 pm on March 21, 2017 Permalink | Reply
    Tags: 30 million collision events, , CERN ATLAS, , ,   

    From ATLAS: “Particle-hunting at the energy frontier” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    20th March 2017
    ATLAS Collaboration

    1
    Fig. 1: The highest-mass dijet event measured by ATLAS (mass = 8.12TeV). Green lines indicate tracks of charged particles. Green and yellow blocks show the energy of the two back-to-back jets deposited in the calorimeters. (Image: ATLAS Collaboration/CERN)

    There are many mysteries the Standard Model of particle physics cannot answer. Why is there an imbalance between matter and anti-matter in our Universe? What is the nature of dark matter or dark energy? And many more. The existence of physics beyond the Standard Model can solve some of these fundamental questions. By studying the head-on collisions of protons at a centre-of-mass energy of 13 TeV provided by the LHC, the ATLAS Collaboration is on the hunt for signs of new physics.

    2
    Fig. 2: Dijet resonance search results. (Image: ATLAS Collaboration/CERN)

    A newly released ATLAS search studies approximately 30 million collision events that produce two high-energy sprays of particles in the final state. These sprays are known as “jets” or, when seen in pairs as in this case, “dijets” (Figure 1). Jets with extraordinarily high energies – copiously produced due to the strong interactions of quarks and gluons – probe the highest energy scales of all processes at the LHC. These jets can provide a window into new physics phenomena, and allow ATLAS physicists to search for mediators between Standard Model and dark matter particles or other hypothetical objects such as non-elementary quarks, heavy “partners” of known Standard Model particles or miniature quantum black-holes (a phenomenon of strong gravity predicted in models with additional spatial dimensions). They can even be used to search for very heavy particles with masses beyond the LHC collision energies, through models known as contact interactions (similar to the Fermi model for weak interactions).

    The dijet search described here consists of two complementary analyses: the resonance analysis and the angular analysis. The resonance analysis looks for a localized excess in the dijet mass spectrum. In the absence of a heavy resonance, the mass distribution is well described by a smooth, monotonically falling function. A statistically significant bump would signify a new particle with mass near the measured bump. The histogram in Figure 2 displays the results of the resonance analysis. The x-axis represents the dijet mass (mjj) and the y-axis (shown with a logarithmic scale) represents the number of observed events. The solid black dots show the data, the red curve represents the fit of a smooth function to the data, and the open green dots show how two non-elementary (“excited”) quark signals might look like. The second panel shows how significant the deviations in the data are as compared to the smooth background fit. The vertical blue lines show the region with the largest significance. A statistical analysis results in a probablility value of 0.63 which means that there is no significant deviation from the Standard Model. The third panel compares the data to the dijet mass prediction; again, no significant deviation from the Standard Model expectation is seen.

    See the full article here .

    CERN LHC Map
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    LHC at CERN

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  • richardmitnick 8:51 pm on March 10, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , , , , , , The strong force (strong interaction)   

    From Symmetry: “A strength test for the strong force [strong interaction]” 

    Symmetry Mag

    Symmetry

    03/10/17
    Sarah Charley

    1
    Science Saturday

    New research could tell us about particle interactions in the early universe and even hint at new physics.

    Much of the matter in the universe is made up of tiny particles called quarks. Normally it’s impossible to see a quark on its own because they are always bound tightly together in groups. Quarks only separate in extreme conditions, such as immediately after the Big Bang or in the center of stars or during high-energy particle collisions generated in particle colliders.

    Scientists at Louisiana Tech University are working on a study of quarks and the force that binds them by analyzing data from the ATLAS experiment at the LHC. Their measurements could tell us more about the conditions of the early universe and could even hint at new, undiscovered principles of physics.


    ATLAS at the LHC

    The particles that stick quarks together are aptly named “gluons.” Gluons carry the strong force, one of four fundamental forces in the universe that govern how particles interact and behave. The strong force binds quarks into particles such as protons, neutrons and atomic nuclei.

    As its name suggests, the strong force [strong interaction] is the strongest—it’s 100 times stronger than the electromagnetic force (which binds electrons into atoms), 10,000 times stronger than the weak force (which governs radioactive decay), and a hundred million million million million million million (1039) times stronger than gravity (which attracts you to the Earth and the Earth to the sun).

    But this ratio shifts when the particles are pumped full of energy. Just as real glue loses its stickiness when overheated, the strong force carried by gluons becomes weaker at higher energies.

    “Particles play by an evolving set of rules,” says Markus Wobisch from Louisiana Tech University. “The strength of the forces and their influence within the subatomic world changes as the particles’ energies increase. This is a fundamental parameter in our understanding of matter, yet has not been fully investigated by scientists at high energies.”

    Characterizing the cohesiveness of the strong force is one of the key ingredients to understanding the formation of particles after the Big Bang and could even provide hints of new physics, such as hidden extra dimensions.

    “Extra dimensions could help explain why the fundamental forces vary dramatically in strength,” says Lee Sawyer, a professor at Louisiana Tech University. “For instance, some of the fundamental forces could only appear weak because they live in hidden extra dimensions and we can’t measure their full strength. If the strong force is weaker or stronger than expected at high energies, this tells us that there’s something missing from our basic model of the universe.”

    By studying the high-energy collisions produced by the LHC, the research team at Louisiana Tech University is characterizing how the strong force pulls energetic quarks into encumbered particles. The challenge they face is that quarks are rambunctious and caper around inside the particle detectors. This subatomic soirée involves hundreds of particles, often arising from about 20 proton-proton collisions happening simultaneously. It leaves a messy signal, which scientists must then reconstruct and categorize.

    Wobisch and his colleagues innovated a new method to study these rowdy groups of quarks called jets. By measuring the angles and orientations of the jets, he and his colleagues are learning important new information about what transpired during the collisions—more than what they can deduce by simple counting the jets.

    The average number of jets produced by proton-proton collisions directly corresponds to the strength of the strong force in the LHC’s energetic environment.

    “If the strong force is stronger than predicted, then we should see an increase in the number of proton-protons collisions that generate three jets. But if the strong force is actually weaker than predicted, then we’d expect to see relatively more collisions that produce only two jets. The ratio between these two possible outcomes is the key to understanding the strong force.”

    After turning on the LHC, scientists doubled their energy reach and have now determined the strength of the strong force up to 1.5 trillion electronvolts, which is roughly the average energy of every particle in the universe just after the Big Bang. Wobisch and his team are hoping to double this number again with more data.

    “So far, all our measurements confirm our predictions,” Wobisch says. “More data will help us look at the strong force at even higher energies, giving us a glimpse as to how the first particles formed and the microscopic structure of space-time.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:04 am on March 10, 2017 Permalink | Reply
    Tags: , , , , CERN ATLAS, , , , , , Xiaofeng Guo   

    From Brookhaven: Women in STEM – “Secrets to Scientific Success: Planning and Coordination” Xiaofeng Guo 

    Brookhaven Lab

    March 8, 2017
    Lida Tunesi

    1
    Xiaofeng Guo

    Very often there are people behind the scenes of scientific advances, quietly organizing the project’s logistics. New facilities and big collaborations require people to create schedules, manage resources, and communicate among teams. The U.S. Department of Energy’s Brookhaven National Laboratory is lucky to have Xiaofeng Guo in its ranks—a skilled project manager who coordinates projects reaching across the U.S. and around the world.

    Guo, who has a Ph.D. in theoretical physics from Iowa State University, is currently deputy manager for the U.S. role in two upgrades to the ATLAS detector, one of two detectors at CERN’s Large Hadron Collider that found the Higgs boson in 2012.


    CERN ATLAS Higgs Event


    CERN/ATLAS detector

    Brookhaven is the host laboratory for both U.S. ATLAS Phase I and High Luminosity LHC (HL-LHC) upgrade projects, which involve hundreds of millions of dollars and 46 institutions across the nation. The upgrades are complex international endeavors that will allow the detector to make use of the LHC’s ramped up particle collision rates. Guo keeps both the capital and the teams on track.

    “I’m in charge of all business processes, project finance, contracts with institutions, baseline plan reports, progress reports—all aspects of business functions in the U.S. project team. It keeps me very busy,” she laughed. “In the beginning I was thinking ‘in my spare time I can still read physics papers, do my own calculations’… And now I have no spare time!”

    Guo’s dual interest in physics and management developed early in her career.

    “When I was an undergraduate there was a period when I actually signed up for a double major, with classes in finance and economics in addition to physics,” Guo recalled. “I’m happy to explore different things!”

    Later, while teaching physics part-time at Iowa State University, Guo desired career flexibility and studied to be a Chartered Financial Analyst. She passed all required exams in just two years but decided to continue her research after receiving a grant from the National Science Foundation.

    Guo joined Brookhaven Lab in 2010 to fill a need for project management in Nuclear and Particle Physics (NPP). The position offered her a way to learn new skills while staying up-to-date on the physics world.

    Early in her time at Brookhaven, Guo participated in the management of the Heavy Flavor Tracker (HFT) upgrade to the STAR particle detector at the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science User Facility for nuclear physics research. The project was successfully completed $600,000 under budget and a whole year ahead of schedule.


    BNL/RHIC Star Detector

    “This was a very good learning experience for me. I participated in all the manager meeting discussions, updated the review documents, and helped them handle some contracts. Through this process I learned all the DOE project rules,” Guo said.

    While working on the HFT upgrade, Guo also helped develop successful, large group proposals for increased computational resources in high-energy physics and other fields of science. She joined the ATLAS Upgrade projects after receiving her Project Management Certification, and her physics and finance background as well as experience with large collaborations have enabled her to orchestrate complex planning efforts.

    For the two phases of the U.S. ATLAS upgrade, Guo directly coordinates more than 140 scientists, engineers, and finance personnel, and oversees all business processes, including finance, contracts, and reports. And taking her job one step further, she’s developed entirely new management tools and reporting procedures to keep the multi-institutional effort synchronized.

    “Dr. Guo is one of our brightest stars,” said Berndt Mueller, Associate Lab Director of NPP. “We are fortunate to have her to assist us with many challenging aspects of project development and execution in NPP. In the process of guiding the work of scores of scientists and engineers, she has single-handedly created a unique and essential role in the development of complex projects with an international context, demonstrating skills of unusual depth and breadth and the ability to apply them across a wide array of disciplines.”

    Guo’s management of Phase I won great respect for the project from the high-energy physics community and the Office of Project Assessment (OPA) at the DOE’s Office of Science. The OPA invited her to participate in a panel discussion to share her expertise and help develop project management guidelines that can be used in other Office of Science projects. Guo also worked with BNL’s Project Management Center to help the lab update its own project management system description to meet DOE standards and lay down valuable groundwork for future large projects.

    As the ATLAS Phase I upgrade proceeds through the final construction stage, Guo is simultaneously managing the planning stages of HL-LHC.

    “We haven’t completely defined the project timeline yet, but it’s projected to go all the way to the end of 2025,” Guo said.

    Like Phase I, HL-LHC will ensure ATLAS can perform well while the LHC operates at much higher collision rates so that physicists can further explore the Higgs as well as search for signs of dark matter and extra dimensions.

    Although she admits to missing doing research herself, Guo is not disheartened.

    “I’m still in the physics world; I’m still working with physicists,” she said. “I enjoy working and interacting with people. So I’m happy.”

    Brookhaven’s work on RHIC and ATLAS is funded by the DOE Office of Science.

    See the full article here .

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    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
    i1

     
  • richardmitnick 3:56 pm on February 17, 2017 Permalink | Reply
    Tags: , Bjorken x variable, CERN ATLAS, , HERA collider, How strange is the proton?, , , , , , Strong interactions   

    From CERN ATLAS: “How strange is the proton?” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    25th January 2017
    ATLAS Collaboration

    1
    Figure 1: The data ellipses illustrate the 68% CL coverage for the total uncertainties (full green) and total excluding the luminosity uncertainty (open black). Theoretical predictions based on various PDF sets are shown with open symbols of different colours. (Image: ATLAS Collaboration/CERN)

    The protons collided by the LHC are not elementary particles, but are instead made up of quarks, antiquarks and gluons. The theory of the strong interactions – quantum chromodynamics (QCD) – does not allow physicists to calculate the composition of protons from first principles. However, QCD can connect measurements made in different processes and at different energy scales such that universal “parton density functions” (PDFs) can be extracted. These determine the dynamic substructure of the proton.

    The discovery of quarks as the elements of the partonic structure of the proton dates about 50 years. Soon after QCD was born and the existence of gluons inside the proton was established. Much has since been learned through a combination of new experimental data and theoretical advances. At the LHC, reactions involve quarks or gluons that carry a certain fraction x of the proton’s momentum, expressed through the Bjorken x variable. Below x of 0.01, the proton constituents are mainly gluons and a sea of quark-antiquark pairs.

    Electron-proton scattering data from the HERA collider has constrained the gluon and the sum of all quarks weighted by the square of their electric charge.

    3
    Data from the HERA collider live on (Image: DESY Hamburg)

    4
    DESY map

    But the low x sea-quark composition – expressed in terms of the lighter quarks named up, down and strange quarks – is still not well understood. New data from the ATLAS experiment shows, with unprecedented precision, the production of W and Z bosons through the weak interaction. This sheds new light on the question: how “strange” the proton is at small x?

    The W production is detected through its decay into a charged lepton (electron or muon) and a neutrino, while the Z boson produces an electron-positron (or muon-antimuon) pair. The experimental detection of electrons and muons poses different challenges and thus the simultaneous measurement in both channels provides an important cross-check of the results, thus improving the final precision achieved. The integrated cross sections for Z boson and W boson production are measured with a precision of 0.3% and 0.6%, respectively, and with an additional common normalisation uncertainty from the luminosity determination of 1.8%. Differential cross sections are also measured in a variety of kinematic regions and about half of the measurement points have a precision of 1% or better.

    2
    Figure 2: Determination of the relative strange-to-light quark fraction R_s. Bands: Present result and its uncertainty contributions from experimental data, QCD fit, and theoretical uncertainties. Closed symbols with horizontal error bars: predictions from different NNLO PDF sets. Open square: previous ATLAS result. (Image: ATLAS Collaboration/CERN)

    The measurements are then compared to state-of-the-art QCD expectations using different PDF sets. Because the production of W and Z bosons through the weak interaction has a different dependence on the specific quark flavours compared to the electromagnetic interaction seen in electron-positron scattering at HERA, analysing both data sets gives new access to the strange quark content of the proton.

    As is shown in Figure 1, the measured production rate of W bosons is very similar for all PDF sets, in good agreement with the data. In contrast, the rate for Z boson production is underestimated significantly for most PDF sets. A dedicated analysis enables this deficit to be attributed to a too small strange quark contribution in most PDF sets. The new PDF set, which this paper presents, requires the strange quark sea to be of a similar size as the up and down quark sea. This is summarised by the quantity RS, which is the ratio of the strange quark sea to the up and down quark sea, and which is found to be close to one, as shown in Figure 2. This result is a striking confirmation of the hypothesis of a light-flavour symmetry of proton structure at low x. This result will generate many further studies, because hitherto there had been indications from low energy neutrino-scattering data that favoured a suppressed strange-quark contribution with respect to the up and down quark parts, leading to an RS close to 0.5.

    The analysis also shows that the potential to which precise W and Z cross sections can provide useful constraints on PDFs is not limited by the now very high experimental precision, but rather by the uncertainty of the currently available theory calculations. The salient results of this paper are thus of fundamental importance for forthcoming high-precision measurements, such as the mass of the W boson, and also represent a strong incentive for further improving the theory of Drell-Yan scattering in proton-proton collisions.

    Links:

    Precision measurement and interpretation of inclusive W+, W− and Z/γ∗ production cross sections with the ATLAS detector, https://arxiv.org/abs/1612.03016

    See the full article here .

    CERN LHC Map
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  • richardmitnick 2:30 pm on January 25, 2017 Permalink | Reply
    Tags: , CERN ATLAS, HERA at DESY, Parton density functions (PDFs), ,   

    From CERN ATLAS: “How strange is the proton?” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    25th January 2017
    ATLAS Collaboration

    1
    Figure 1: The data ellipses illustrate the 68% CL coverage for the total uncertainties (full green) and total excluding the luminosity uncertainty (open black). Theoretical predictions based on various PDF sets are shown with open symbols of different colours. (Image: ATLAS Collaboration/CERN)

    The protons collided by the LHC are not elementary particles, but are instead made up of quarks, antiquarks and gluons. The theory of the strong interactions – quantum chromodynamics (QCD) – does not allow physicists to calculate the composition of protons from first principles. However, QCD can connect measurements made in different processes and at different energy scales such that universal “parton density functions” (PDFs) can be extracted. These determine the dynamic substructure of the proton.

    The discovery of quarks as the elements of the partonic structure of the proton dates about 50 years. Soon after QCD was born and the existence of gluons inside the proton was established. Much has since been learned through a combination of new experimental data and theoretical advances. At the LHC, reactions involve quarks or gluons that carry a certain fraction x of the proton’s momentum, expressed through the Bjorken x variable. Below x of 0.01, the proton constituents are mainly gluons and a sea of quark-antiquark pairs.

    Electron-proton scattering data from the HERA collider has constrained the gluon and the sum of all quarks weighted by the square of their electric charge.

    3
    The electron(positron)-proton collider HERA was shut down at the end of June 2007. HERA was a unique instrument which made a major contribution to high energy particle physics and, in particular, to confirming aspects of quantum chromodynamics (QCD). The knowledge obtained with HERA will be essential for discovering the meaning of data obtained from the large hadron Collider (LHC) at CERN in Geneva. http://www.desy.de/~mpybar/endofHERA.html

    But the low x sea-quark composition – expressed in terms of the lighter quarks named up, down and strange quarks – is still not well understood. New data from the ATLAS experiment shows, with unprecedented precision, the production of W and Z bosons through the weak interaction. This sheds new light on the question: how “strange” the proton is at small x?

    The W production is detected through its decay into a charged lepton (electron or muon) and a neutrino, while the Z boson produces an electron-positron (or muon-antimuon) pair. The experimental detection of electrons and muons poses different challenges and thus the simultaneous measurement in both channels provides an important cross-check of the results, thus improving the final precision achieved. The integrated cross sections for Z boson and W boson production are measured with a precision of 0.3% and 0.6%, respectively, and with an additional common normalisation uncertainty from the luminosity determination of 1.8%. Differential cross sections are also measured in a variety of kinematic regions and about half of the measurement points have a precision of 1% or better.

    2
    Figure 2: Determination of the relative strange-to-down sea quark fractions r_s (left) and R_s (right). Bands: Present result and its uncertainty contributions from experimental data, QCD fit, and theoretical uncertainties. Closed symbols with horizontal error bars: predictions from different NNLO PDF sets. Open square: previous ATLAS result. (Image: ATLAS Collaboration/CERN)

    The measurements are then compared to state-of-the-art QCD expectations using different PDF sets. Because the production of W and Z bosons through the weak interaction has a different dependence on the specific quark flavours compared to the electromagnetic interaction seen in electron-positron scattering at HERA, analysing both data sets gives new access to the strange quark content of the proton.

    As is shown in Figure 1, the measured production rate of W bosons is very similar for all PDF sets, in good agreement with the data. In contrast, the rate for Z boson production is underestimated significantly for most PDF sets. A dedicated analysis enables this deficit to be attributed to a too small strange quark contribution in most PDF sets. The new PDF set, which this paper presents, requires the strange quark sea to be of a similar size as the up and down quark sea. This is summarised by the quantity RS, which is the ratio of the strange quark sea to the the up and down quark sea, and which is found to be close to one, as shown in Figure 2. This result is a striking confirmation of the hypothesis of a light-flavour symmetry of proton structure at low x. This result will generate many further studies, because hitherto there had been indications from low energy neutrino-scattering data that favoured a suppressed strange-quark contribution with respect to the up and down quark parts, leading to an RS close to 0.5.

    The analysis also shows that the potential to which precise W and Z cross sections can provide useful constraints on PDFs is not limited by the now very high experimental precision, but rather by the uncertainty of the currently available theory calculations. The salient results of this paper are thus of fundamental importance for forthcoming high-precision measurements, such as the mass of the W boson, and also represent a strong incentive for further improving the theory of Drell-Yan scattering in proton-proton collisions.

    Links:

    Precision measurement and interpretation of inclusive W+, W− and Z/γ∗ production cross sections with the ATLAS detector, https://arxiv.org/abs/1612.03016

    See the full article here .

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  • richardmitnick 1:14 pm on December 16, 2016 Permalink | Reply
    Tags: CERN ATLAS, The Trouble with Terabytes, Worldwide LHC Computing Grid   

    From CERN ATLAS: “The Trouble with Terabytes” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    14th December 2016
    Katarina Anthony

    2
    Computing at CERN

    2016 has been a record-breaking year. The LHC surpassed its design luminosity and produced stable beams a staggering 60% of the time – up from 40% in previous years, and even surpassing the hoped for 50% threshold.

    While all of the ATLAS experiment rejoiced – eager to analyse the vast outpouring of data from the experiment – its computing experts had their work cut out for them: “2016 has been quite a challenge,” says Armin Nairz, leader of the ATLAS Tier-0 operations team. Armin’s team is in charge of processing and storing ATLAS data in preparation for distribution to physicists around the world – a task that proved unusually complex this year. “We were well prepared for a big peak in efficiency, but even we did not expect such excellent operation!”

    “Data-taking conditions are constantly changing,” says Armin. “From the detector alignment to the LHC beam parameters, there is never a ‘standard’ set of conditions. One of our key roles is to process this information and provide it along with the main event data.” This job, called the ‘calibration loop’, can take up to 48 hours. Countless teams verify and re-verify the calibrations before they are applied in subsequent bulk reconstruction of the physics data.

    Before 2016, the Tier-0 team would have a 10 to 12 hour break between each LHC beam fill. This gave their servers some breathing room to catch up with demand. “In the weeks leading up to the ICHEP conference, the LHC was working almost too perfectly,” says Armin. “At one point, it operated at 80% efficiency. This meant there were very short breaks between runs; just 2 hours between a beam dump and the next fill.”

    The CERN IT department provided an extra 1000 cores to help the ATLAS team cope with ever-growing demand. However, it soon became clear that that would not be enough: “We had to come up with a new strategy,” explains Armin. “We needed a way to grow Tier-0 without relying on more computers on-site.” Their solution: outsource the data reconstruction to the Worldwide LHC Computing Grid.

    To accomplish this feat, Armin’s Tier-0 team joined forces with the ATLAS Distributed Computing group and the Grid Production team. “Together, we had to train the Grid to process data with a Tier-0 configuration in the much-needed short time scale,” says Armin. “We experimented with lots of different configurations, trying to steer the jobs to the most appropriate sites (i.e. those with the best, quickest machines).”

    This was quite an arduous task for an already-busy team, though it proved very effective. “Despite overwhelming demand during ICHEP, we were able to shepherd copious amounts data into physics results,” says Armin. “In the end, the data presented at the conference was just 2 weeks old!”

    The Tier-0 team will be ready should such a situation arise again. “Although this solution took enormous effort, it was ultimately successful,” concludes Armin. “However, ATLAS computing management are now preparing to add new computing resources in 2017, in the hopes of avoiding a similar situation. We have also used this experience to help improve our reconstruction software and workflow, bettering our performance as the year went on.” After all, an experiment is only as valuable as the data it collects!

    About the Grid

    The Worldwide LHC Computing Grid is a global collaboration of computer centres. It is composed of four levels, or “Tiers”. Each Tier is made up of several computer centres and provides a specific set of services. Between them the tiers process, store and analyse all the data from the Large Hadron Collider.

    ATLAS Tier-0, located at the CERN data centre, has about 800 machines, with approximately 12,000 processing cores. This allows 12,000 jobs to run in parallel, and up to 100,000 jobs are run per day. During data-taking, the ATLAS online data-acquisition system transfers data to Tier-0 at about 2 GB/s, with peaks of 7 GB/s.

    See the full article here .

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  • richardmitnick 4:25 pm on December 6, 2016 Permalink | Reply
    Tags: 2016: an exceptional year for the LHC, , , , CERN ATLAS, , ,   

    From CERN: “2016: an exceptional year for the LHC” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    6 Dec 2016
    Corinne Pralavorio

    1
    This proton-lead ion collision in the ATLAS detector produced a top quark – the heaviest quark – and its antiquark (Image: ATLAS)

    It’s the particles’ last lap of the ring. On 5 December 2016, protons and lead ions circulated in the Large Hadron Collider (LHC) for the last time. At exactly 6.02am, the experiments recorded their last collisions (also known as ‘events’).

    When the machines are turned off, the LHC operators take stock, and the resulting figures are astonishing.

    The number of collisions recorded by ATLAS and CMS during the proton run from April to the end of October was 60% higher than anticipated. Overall, all of the LHC experiments observed more than 6.5 million billion (6.5 x 1015) collisions, at an energy of 13 TeV. That equates to more data than had been collected in the previous three runs combined.

    3
    One of the first proton-lead ion collisions at 8.16 TeV recorded by the ALICE experiment. (Image: ALICE/CERN)

    In technical terms, the integrated luminosity received by ATLAS and CMS reached 40 inverse femtobarns (fb−1), compared with the 25fb−1 originally planned. Luminosity, which measures the number of potential collisions in a given time, is a crucial indicator of an accelerator’s performance.

    “One of the key factors contributing to this success was the remarkable availability of the LHC and its injectors,” explains Mike Lamont, who leads the team that operates the accelerators. The LHC’s overall availability in 2016 was just shy of 50%, which means the accelerator was in ‘collision mode’ 50% of the time: a very impressive achievement for the operators. “It’s the result of an ongoing programme of work over the last few years to consolidate and upgrade the machines and procedures,” Lamont continues.

    4
    An event recorded by the CMS experiment during the LHC’s proton-lead ion run for which no fewer than 449 particles tracks were reconstructed. (Image: CMS/CERN)

    For the last four weeks, the machine has turned to a different type of collision, where lead ions have been colliding with protons. “This is a new and complex operating mode, but the excellent functioning of the accelerators and the competence of the teams involved has allowed us to surpass our performance expectations,” says John Jowett, who is in charge of heavy-ion runs.

    With the machine running at an energy of 8.16 TeV, a record for this assymetric type of collision, the experiments have recorded more than 380 billion collisions. The machine achieved a peak luminosity over seven times higher than initially expected, as well as exceptional beam lifetimes. The performance is even more remarkable considering that colliding protons with lead ions, which have a mass 206 times greater and a charge 82 times higher, requires numerous painstaking adjustments to the machine.

    The physicists are now analysing the enormous amounts of data that have been collected, in preparation for presenting their results at the winter conferences.

    6
    A proton-lead ion collision recorded by the LHCb experiment in the last few days of the LHC’s 2016 run. (Image: LHCb)

    Meanwhile, CERN’s accelerators will take a long break, called the Extended Year End Technical Stop (EYETS) until the end of March 2017. But, while the accelerators might be on holiday, the technical teams certainly aren’t. The winter stop is an opportunity to carry out maintenance on these extremely complex machines, which are made up of thousands of components. The annual stop for the LHC is being extended by two months in 2017 to allow more major renovation work on the accelerator complex and its 35 kilometres of machines to take place. Particles will return to the LHC in spring 2017.

    7
    The integrated luminosity of the LHC with proton-proton collisions in 2016 compared to previous years. Luminosity is a measure of a collider’s efficiency and is proportional to the number of collisions. The integrated luminosity achieved by the LHC in 2016 far surpassed expectations and is double that achieved at a lower energy in 2012. (Image : CERN)

    See the full article here.

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