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  • richardmitnick 1:25 pm on April 24, 2017 Permalink | Reply
    Tags: , , , New ALICE results show novel phenomena in proton collisions, Particle Accelerators, , , Strange quark   

    From ALICE at CERN: “New ALICE results show novel phenomena in proton collisions” 

    CERN
    CERN New Masthead

    CERN ALICE Icon HUGE

    24 Apr 2017.
    Harriet Kim Jarlett

    1
    As the number of particles produced in proton collisions (the blue lines) increase, the more of these so-called strange hadrons are measured (as shown by the orange to red squares in the graph) (Image: ALICE/CERN)

    In a paper published today in Nature Physics , the ALICE collaboration reports that proton collisions sometimes present similar patterns to those observed in the collisions of heavy nuclei. This behaviour was spotted through observation of so-called strange hadrons in certain proton collisions in which a large number of particles are created. Strange hadrons are well-known particles with names such as Kaon, Lambda, Xi and Omega, all containing at least one so-called strange quark. The observed ‘enhanced production of strange particles’ is a familiar feature of quark-gluon plasma, a very hot and dense state of matter that existed just a few millionths of a second after the Big Bang, and is commonly created in collisions of heavy nuclei. But it is the first time ever that such a phenomenon is unambiguously observed in the rare proton collisions in which many particles are created. This result is likely to challenge existing theoretical models that do not predict an increase of strange particles in these events.

    “We are very excited about this discovery,” said Federico Antinori, Spokesperson of the ALICE collaboration. “We are again learning a lot about this primordial state of matter. Being able to isolate the quark-gluon-plasma-like phenomena in a smaller and simpler system, such as the collision between two protons, opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.”

    The study of the quark-gluon plasma provides a way to investigate the properties of strong interaction, one of the four known fundamental forces, while enhanced strangeness production is a manifestation of this state of matter. The quark-gluon plasma is produced at sufficiently high temperature and energy density, when ordinary matter undergoes a transition to a phase in which quarks and gluons become ‘free’ and are thus no longer confined within hadrons. These conditions can be obtained at the Large Hadron Collider by colliding heavy nuclei at high energy. Strange quarks are heavier than the quarks composing normal matter, and typically harder to produce. But this changes in presence of the high energy density of the quark-gluon plasma, which rebalances the creation of strange quarks relative to non-strange ones. This phenomenon may now have been observed within proton collisions as well.

    In particular, the new results show that the production rate of these strange hadrons increases with the ‘multiplicity’ – the number of particles produced in a given collision – faster than that of other particles generated in the same collision. While the structure of the proton does not include strange quarks, data also show that the higher the number of strange quarks contained in the induced hadron, the stronger is the increase of its production rate. No dependence on the collision energy or the mass of the generated particles is observed, demonstrating that the observed phenomenon is related to the strange quark content of the particles produced. Strangeness production is in practice determined by counting the number of strange particles produced in a given collision, and calculating the ratio of strange to non-strange particles.

    Enhanced strangeness production had been suggested as a possible consequence of quark-gluon plasma formation since the early eighties, and discovered in collisions of nuclei in the nineties by experiments at CERN’s Super Proton Synchrotron.

    CERN Super Proton Synchrotron

    Another possible consequence of the quark gluon plasma formation is a spatial correlation of the final state particles, causing a distinct preferential alignment with the shape of a ridge. Following its detection in heavy-nuclei collisions, the ridge has also been seen in high-multiplicity proton collisions at the Large Hadron Collider, giving the first indication that proton collisions could present heavy-nuclei-like properties. Studying these processes more precisely will be key to better understand the microscopic mechanisms of the quark-gluon plasma and the collective behaviour of particles in small systems.

    The ALICE experiment has been designed to study collisions of heavy nuclei. It also studies proton-proton collisions, which primarily provide reference data for the heavy-nuclei collisions. The reported measurements have been performed with 7 TeV proton collision data from LHC run 1.

    See the full article here .

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    Meet CERN in a variety of places:

    CernCourier
    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS
    ATLAS
    CERN/ATLAS detector

    ALICE
    CERN ALICE New

    CMS
    CERN/CMS Detector

    LHCb

    CERN/LHCb

    LHC

    CERN/LHC Map
    CERN LHC Grand Tunnel

    CERN LHC particles


    Quantum Diaries

     
  • richardmitnick 2:16 pm on April 22, 2017 Permalink | Reply
    Tags: , , , Particle Accelerators, , Videos   

    From CMS at CERN: Fantastic Videos 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    These incredible videos are presented in no particular order.,


    An introduction to the CMS Experiment at CERN


    Welcome to LHC season 2: new frontiers in physics at #13TeV


    LHC animation: The path of the protons


    The Large Hadron Collider Returns in the Hunt for New Physics


    Physics Run 2016


    Back to the Big Bang: Inside the Large Hadron Collider – From the World Science Festival


    Higgs boson: what’s next? #13TeV

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    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

    CernCourier
    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New

    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN LHC Map
    CERN LHC Grand Tunnel

    CERN LHC particles

    Quantum Diaries

     
  • richardmitnick 11:56 am on April 20, 2017 Permalink | Reply
    Tags: , , , , New LHC Results Hint At New Physics... But Are We Crying Wolf?, Particle Accelerators,   

    From Ethan Siegel: “New LHC Results Hint At New Physics… But Are We Crying Wolf?” 

    Ethan Siegel
    Apr 20, 2017

    1
    The LHCb collaboration is far less famous than CMS or ATLAS, but the bottom-quark-containing particles they produce holds new physics hints that the other detectors cannot probe. CERN / LHCb Collaboration

    Over at the Large Hadron Collider at CERN, particles are accelerated to the greatest energies they’ve ever reached in history. In the CMS and ATLAS detectors, new fundamental particles are continuously being searched for, although only the Higgs boson has come through. But in a much lesser-known detector — LHCb — particles containing bottom quarks are produced in tremendous numbers. One class of these particles, quark-antiquark pairs where one is a bottom quark, have recently been observed to decay in a way that runs counter to the Standard Model’s predictions. Even though the evidence isn’t very good, it’s the biggest hint for new physics we’ve had from accelerators in years.

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    A decaying B-meson, as shown here, may decay more frequently to one type of lepton pair than the other, contradicting Standard Model expectations. KEK / BELLE collaboration

    KEK Belle detector, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan

    There are two ways, throughout history, that we’ve made extraordinary advances in fundamental physics. One is when an unexplained, robust phenomenon pops up, and we’re compelled to rethink our conception of the Universe. The other is when multiple, competing, but heretofore indistinguishable explanations of the same set of observations are subject to a critical test, where only one explanation emerges as a valid one. Particle physics is at a crossroads right now, because even though there are fundamentally unsolved questions, the energy scales that we can probe with experiments all give results that are perfectly in line with 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.

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    The discovery of the Higgs Boson in the di-photon (γγ) channel at CMS. That ‘bump’ in the data is an unambiguous new particle: the Higgs.

    CERN CMS Higgs Event

    CERN/CMS Detector

    The Higgs boson, discovered earlier this decade, was created over and over at the LHC, with its decays measured in excruciating detail. If there were any hints of departures from the Standard Model — if it decayed into one type of particle more-or-less frequently than predicted — it could be an extraordinary hint of new physics. Similarly, physicists searches exhaustively for new “bumps” where there shouldn’t be any in the data: a signal of a potential new particle. Although they showed up periodically, with some mild significance, they always went away entirely with more and better data.

    4
    The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding, but there are outliers (which is expected) when the error-bars are larger.

    CERN ATLAS Higgs Event

    CERN/ATLAS detector

    Statistically, this is about what you’d expect. If you had a fair coin and tossed it 10 times, you might expect that you’d get 5 heads and 5 tails. Although that’s reasonable, sometimes you’ll get 6 and 4, sometimes you’ll get 8 and 2, and sometimes you’ll get 10 and 0, respectively. If you got 10 heads and 0 tails, you might begin to suspect that the coin isn’t fair, but the odds aren’t that bad: about 0.2% of the time, you’ll have all ten flips give the same result. And if you have 1000 people each flipping a coin ten times, it’s very likely (86%) that at least one of them will get the same result all ten times.

    The Standard Model makes predictions for lots of different quantities — particle production rates, scattering amplitudes, decay probabilities, branching ratios, etc. — for every single particle (both fundamental and composite) that can be created. Literally, there are hundreds of such composite particles that have been created in such numbers, and thousands of quantities like that we can measure. Since we look at all of them, we demand an extremely high level of statistical significance before we’re willing to claim a discovery. In particle physics, the odds of a fluke need to be less than one-in-three-million to get there.

    6
    The standard model calculated predictions (the four colored points) and the LHCb results (black, with error bars) for the electron/positron to muon/antimuon ratios at two different energies. LHCb Collaboration / Tommaso Dorigo

    Earlier this week, the LHCb collaboration announced their greatest departure yet observed from the Standard Model: a difference in the rate of decay of bottom-quark-containing mesons into strange-quark-containing mesons with either a muon-antimuon pair or an electron-positron pairs. In the Standard Model, the ratios should be 1.0 (once mass differences of muons and electrons are taken into account), but they observed a ratio of 0.6. That sure sounds like a big deal, and like it might be a hint of physics beyond the Standard Model!

    7
    The known particles and antiparticles of the Standard Model all have been discovered. All told, they make explicit predictions. Any violation of those predictions would be a sign of new physics, which we’re desperately seeking. E. Siegel

    The case gets even stronger when you consider that the BELLE collaboration, last decade, discovered these decays and began to notice a slight discrepancy themselves. But a closer inspection of the latest data shows that the statistical significance is only about 2.4 and 2.5 sigma, respectively, at the two energies measured. This is about a 1.5% chance of a fluke individually, or about 3.7-sigma significance (0.023% chance of a fluke) combined. Now, 3.7-sigma is a lot more exciting than 2.5-sigma, but it’s still not exciting enough. Given that there were thousands of things these experiments looked at, these results barely even register as “suggestive” of new physics, much less as compelling evidence.

    7
    The ATLAS and CMS diphoton bumps from 2015, displayed together, clearly correlating at ~750 GeV. This suggestive result was significant at more than 3-sigma, but went away entirely with more data. CERN, CMS/ATLAS collaborations; Matt Strassler

    Yet already, just on Wednesday, there were six new papers out (with more surely coming) attempting to use beyond-the-Standard-Model physics to explain this not-even-promising result.

    Why?

    Because, quite frankly, we don’t have any good ideas in place. Supersymmetry, grand unification, string theory, technicolor, and extra dimensions, among others, were the leading extensions to the Standard Model, and colliders like the LHC have yielded absolutely no evidence for any of them. Signals from direct experiments for physics beyond the Standard Model have all yielded results completely consistent with the Standard Model alone. What we’re seeing now is rightly called ambulance-chasing, but it’s even worse than that.

    8
    The Standard Model particles and their supersymmetric counterparts. Non-white-male-American scientists have been instrumental in the development of the Standard Model and its extensions. Claire David

    We know that results like this have a history of not holding up at all; we expect there to be fluctuations like this in the data, and this one isn’t even as significant as the others that have gone away with more and better data. You expect a 2-sigma discrepancy in one out of every 20 measurements you make, and these two are little better than that. Even combined, they’re hardly impressive, and the other things you’d seek to measure about this decay line up with the Standard Model perfectly. In short, the Standard Model is much more likely than not to hold up once more and better data arrives.

    9
    The string landscape might be a fascinating idea that’s full of theoretical potential, but it doesn’t predict anything that we can observe in our Universe. University of Cambridge

    What we’re seeing right now is a response from the community is what we’d expect to an alarm that’s crying “Wolf!” There might be something fantastic and impressive out there, and so, of course we have to look. But we know that, more than 99% of the time, an alarm like this is merely the result of which way the wind blew. Physicists are so bored and so out of good, testable ideas to extend the Standard Model — which is to say, the Standard Model is so maddeningly successful — that even a paltry result like this is enough to shift the theoretical direction of the field.

    A few weeks ago, famed physicist (and supersymmetry-advocate) John Ellis asked the question, Where is Particle Physics going? Unless experiments can generate new, unexpected results, the answer is likely to be “nowhere new; nowhere good” for the indefinite future.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 1:56 pm on April 18, 2017 Permalink | Reply
    Tags: , , , , , LHCb Finds New Hints of Possible Deviations from the Standard Model, Particle Accelerators,   

    From Astro Watch: “LHCb Finds New Hints of Possible Deviations from the Standard Model” 

    Astro Watch bloc

    Astro Watch

    April 18, 2017
    CERN

    1
    CERN LHCb

    The LHCb experiment finds intriguing anomalies in the way some particles decay. If confirmed, these would be a sign of new physics phenomena not predicted by the Standard Model of particle physics. The observed signal is still of limited statistical significance, but strengthens similar indications from earlier studies. Forthcoming data and follow-up analyses will establish whether these hints are indeed cracks in the Standard Model or a statistical fluctuation.

    Today, in a seminar at CERN, the LHCb collaboration presented new long-awaited results on a particular decay of B0 mesons produced in collisions at the Large Hadron Collider. The Standard Model of particle physics predicts the probability of the many possible decay modes of B0 mesons, and possible discrepancies with the data would signal new physics.

    In this study, the LHCb collaboration looked at the decays of B0 mesons to an excited kaon and a pair of electrons or muons. The muon is 200 times heavier than the electron, but in the Standard Model its interactions are otherwise identical to those of the electron, a property known as lepton universality. Lepton universality predicts that, up to a small and calculable effect due to the mass difference, electron and muons should be produced with the same probability in this specific B0 decay. LHCb finds instead that the decays involving muons occur less often.

    While potentially exciting, the discrepancy with the Standard Model occurs at the level of 2.2 to 2.5 sigma, which is not yet sufficient to draw a firm conclusion. However, the result is intriguing because a recent measurement by LHCb involving a related decay exhibited similar behavior.

    While of great interest, these hints are not enough to come to a conclusive statement. Although of a different nature, there have been many previous measurements supporting the symmetry between electrons and muons. More data and more observations of similar decays are needed in order to clarify whether these hints are just a statistical fluctuation or the first signs for new particles that would extend and complete the Standard Model of particles physics. The measurements discussed were obtained using the entire data sample of the first period of exploitation of the Large Hadron Collider (Run 1). If the new measurements indeed point to physics beyond the Standard Model, the larger data sample collected in Run 2 will be sufficient to confirm these effects.

    See the full article here .

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  • richardmitnick 1:13 pm on April 16, 2017 Permalink | Reply
    Tags: , , , , , Particle Accelerators,   

    From Nature: “Muons’ big moment could fuel new physics” 

    Nature Mag
    Nature

    11 April 2017
    Elizabeth Gibney

    1
    The Muon g-2 experiment will look for deviations from the standard model by measuring how muons wobble in a magnetic field. Credit: FNAL

    In the search for new physics, experiments based on high-energy collisions inside massive atom smashers are coming up empty-handed. So physicists are putting their faith in more-precise methods: less crash-and-grab and more watching-ways-of-wobbling. Next month, researchers in the United States will turn on one such experiment. It will make a super-accurate measurement of the way that muons, heavy cousins of electrons, behave in a magnetic field. And it could provide evidence of the existence of entirely new particles.

    The particles hunted by the new experiment, at the Fermi National Laboratory in Batavia, Illinois, comprise part of the virtual soup that surrounds and interacts with all forms of matter. Quantum theory says that short-lived virtual particles constantly ‘blip’ in and out of existence. Physicists already account for the effects of known virtual particles, such as photons and quarks. But the virtual soup might have mysterious, and as yet unidentified, ingredients. And muons could be particularly sensitive to them.

    The new Muon g−2 experiment will measure this sensitivity with unparalleled precision. And in doing so, it will reanalyse a muon anomaly that has puzzled physicists for more than a decade. If the experiment confirms that the anomaly is real, then the most likely explanation is that it is caused by virtual particles that do not appear in the existing physics playbook — 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.

    2
    Adapted from go.nature.com/2naoxaw

    “It would be the first direct evidence of not only physics beyond the standard model, but of entirely new particles,” says Dominik Stöckinger, a theorist at the Technical University of Dresden, Germany, and a member of the Muon g−2 collaboration.

    Physicists are crying out for a successor to the standard model — a theory that has been fantastically successful yet is known to be incomplete because it fails to account for many phenomena, such as the existence of dark matter. Experiments at the Large Hadron Collider (LHC) at CERN, Europe’s particle-physics lab near Geneva, Switzerland, have not revealed a specific chink, despite performing above expectation and carrying out hundreds of searches for physics beyond the standard model. The muon anomaly is one of only a handful of leads that physicists have.

    Measurements of the muon’s magnetic moment — a fundamental property that relates to the particle’s inherent magnetism — could hold the key, because it is tweaked by interactions with virtual particles. When last measured 15 years ago at the Brookhaven National Laboratory in New York, the muon’s magnetic moment was larger than theory predicts.

    BNL RHIC Campus

    BNL/RHIC

    FNAL G-2 magnet from Brookhaven Lab finds a new home in the FNAL Muon G-2 experiment

    Physicists think that interaction with unknown particles, perhaps those envisaged by a theory called supersymmetry, might have caused this anomaly.

    Other possible explanations are a statistical fluke, or a flaw in the theorists᾽ standard-model calculation, which combines the complex effects of known particles. But that is becoming less likely, says Stöckinger, who says that new calculation methods and experimental cross-checks make the theoretical side much more robust than it was 15 years ago.

    “With this tantalizing result from Brookhaven, you really have to do a better experiment,” says Lee Roberts, a physicist at Boston University in Massachusetts, who is joint leader of the Muon g−2 experiment. The Fermilab set-up will use 20 times the number of muons used in the Brookhaven experiment to shrink uncertainty by a factor of 4. “If we agree, but with much smaller error, that will show definitively that there’s some particle that hasn’t been observed anywhere else,” he says.

    To probe the muons, Fermilab physicists will inject the particles into a magnetic field contained in a ring some 14 metres across. Each particle has a magnetic property called spin, which is analogous to Earth spinning on its axis. As the muons travel around the ring at close to the speed of light, their axes of rotation wobble in the field, like off-kilter spinning tops. Combining this precession rate with a measurement of the magnetic field gives the particles’ magnetic moment.

    Since the Brookhaven result, some popular explanations for the anomaly — including effects of hypothetical dark photons — seem to have been ruled out by other experiments, says Stöckinger. “But if you look at the whole range of scenarios for physics beyond the standard model, there are many possibilities.”

    3
    Fermilab is the home of the Muon g−2 experiment.

    Although a positive result would give little indication of exactly what the new particles are, it would provide clues to how other experiments might pin them down. If the relatively large Brookhaven discrepancy is maintained, it can only come from relatively light particles, which should be within reach of the LHC, says Stöckinger, even if they interact so rarely that it takes years for them to emerge.

    Indeed, the desire to build on previous findings is so strong that to avoid possible bias, Fermilab experimenters will process their incoming results ‘blind’ and apply a different offset to each of two measurements that combine to give the magnetic moment. Only once the offsets are revealed will anyone know whether they have proof of new particles hiding in the quantum soup. “Until then nobody knows what the answer is,” says Roberts. “It will be an exciting moment.”

    See the full article here .

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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

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

    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
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    LHC at CERN

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  • richardmitnick 10:25 am on April 2, 2017 Permalink | Reply
    Tags: , , , , Particle Accelerators,   

    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
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

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  • richardmitnick 12:56 pm on March 28, 2017 Permalink | Reply
    Tags: , , , Particle Accelerators, ,   

    From Symmetry: “How to make a discovery” 

    Symmetry Mag

    Symmetry

    03/28/17
    Ali Sundermier

    1
    Artwork by Sandbox Studio, Chicago

    Meenakshi Narain, a professor of physics at Brown University, remembers working on the DZero experiment at Fermi National Accelerator Laboratory near Chicago in the winter of 1994.


    FNAL/Tevatron DZero detector

    She would bring blankets up to her fifth-floor office to keep warm as she sat at her computer going through data in search of the then-undiscovered top quark.

    For weeks, her group had been working on deciphering some extra background that originally had not been accounted for. Their conclusions contradicted the collaboration’s original assumptions.

    Narain, who was a postdoctoral researcher at the time, talked to her advisor about sharing the group’s result. Her advisor told her that if she had followed the scientific method and was confident in her result, she should talk about it.

    “I had a whole sequence of logic and explanation prepared,” Narain says. “When I presented it, I remember everybody was very supportive. I had expected some pushback or some criticism and nothing like that happened.”

    This, she says, is the scientific process: A multitude of steps designed to help us explore the world we live in.

    “In the end the process wins. It’s not about you or me, because we’re all going after the same thing. We want to discover that particle or phenomenon or whatever else is out there collaboratively. That’s the goal.”

    Narain’s group’s analysis was essential to the collaboration’s understanding of a signal that turned out to be the elusive top quark.

    2
    Artwork by Sandbox Studio, Chicago

    The modern hypothesis

    “The scientific method was not invented overnight,” says Joseph Incandela, vice chancellor for research at the University of California, Santa Barbara. “People used to think completely differently. They thought if it was beautiful it had to be true. It took many centuries for people to realize that this is how you must approach the acquisition of true knowledge that you can verify.”

    For particle physicists, says Robert Cahn, a senior scientist at Lawrence Berkeley National Laboratory, the scientific method isn’t so much going from hypothesis to conclusion, but rather “an exploration in which we measure with as much precision as possible a variety of quantities that we hope will reveal something new.

    “We build a big accelerator and we might have some ideas of what we might discover, but it’s not as if we say, ‘Here’s the hypothesis and we’re going to prove or disprove it. If there’s a scientific method, it’s something much broader than that.”

    Scientific inquiry is more of a continuing conversation between theorists and experimentalists, says Chris Quigg, a distinguished scientist emeritus at Fermilab.

    “Theorists in particular spend a lot of time telling stories, making up ideas or elaborating ideas about how something might happen,” he says. “There’s an evolution of our ideas as we engage in dialogue with experiments.”

    An important part of the process, he adds, is that the scientists are trained never to believe their own stories until they have experimental support.

    “We are often reluctant to take our ideas too seriously because we’re schooled to think about ideas as tentative,” Quigg says. “It’s a very good thing to be tentative and to have doubt. Otherwise you think you know all the answers, and you should be doing something else.”

    It’s also good to be tentative because “sometimes we see something that looks tantalizingly like a great discovery, and then it turns out not to be,” Cahn says.

    At the end of 2015, hints appeared in the data of the two general-purpose experiments at the Large Hadron Collider that scientists had stumbled upon a particle 750 times as massive as a proton. The hints prompted more than 500 scientific papers, each trying to tell the story behind the bump in the data.

    “It’s true that if you simply want to minimize wasting your time, you will ignore all such hints until they [reach the traditional uncertainty threshold of] 5 sigma,” Quigg said. “But it’s also true that as long as they’re not totally flaky, as long as it looks possibly true, then it can be a mind-expanding exercise.”

    In the case of the 750-GeV bump, Quigg says, you could tell a story in which such a thing might exist and wouldn’t contradict other things that we knew.

    “It helps to take it from just an unconnected observation to something that’s linked to everything else,” Quigg says. “That’s really one of the beauties of scientific theories, and specifically the current state of particle physics. Every new observation is linked to everything else we know, including all the old observations. It’s important that we have enough of a network of observation and interpretation that any new thing has to make sense in the context of other things.”

    After collecting more data, physicists eventually ruled out the hints, and the theorists moved on to other ideas.

    The importance of uncertainty

    But sometimes an idea makes it further than that. Much of the work scientists put into publishing a scientific result involves figuring out how well they know it: What’s the uncertainty and how do we quantify it?

    “If there’s any hallmark to the scientific method in particle physics and in closely related fields like cosmology, it’s that our results always come with an error bar,” Cahn says. “A result that doesn’t have an uncertainty attached to it has no value.”

    In a particle physics experiment, some uncertainty comes from background, like the data Narain’s group found that mimicked the kind of signal they were looking for from the top quark.

    This is called systematic uncertainty, which is typically introduced by aspects of the experiment that cannot be completely known.

    “When you build a detector, you must make sure that for whatever signal you’re going to see, there is not much possibility to confuse it with the background,” says Helio Takai, a physicist at Brookhaven National Laboratory. “All the elements and sensors and electronics are designed having that in mind. You have to use your previous knowledge from all the experiments that came before.”

    Careful study of your systematic uncertainties is the best way to eliminate bias and get reliable results.

    “If you underestimate your systematic uncertainty, then you can overestimate the significance of the signal,” Narain says. “But if you overestimate the systematic uncertainty, then you can kill your signal. So, you really are walking this fine line in understanding where the issues may be. There are various ways the data can fool you. Trying to be aware of those ways is an art in itself and it really defines the thinking process.”

    Physicists also must think about statistical uncertainty which, unlike systematic uncertainty, is simply the consequence having a limited amount of data.

    “For every measurement we do, there’s a possibility that the measurement is a wrong measurement just because of all the events that happen at random while we are doing the experiment,” Takai says. “In particle physics, you’re producing many particles, so a lot of these particles may conspire and make it appear like the event you’re looking for.”

    You can think of it as putting your hand inside a bag of M&Ms, Takai says. If the first few M&Ms you picked were brown and you didn’t know there were other colors, you would think the entire bag was brown. It wouldn’t be until you finally pulled out a blue M&M that you realized that the bag had more than one color.

    Particle physicists generally want their results to have a statistical significance corresponding to at least 5 sigma, a measure that means that there is only a 0.00003 percent chance of a statistical fluctuation giving an excess as big or bigger than the one observed.

    3
    Artwork by Sandbox Studio, Chicago

    The scientific method at work

    One of the most stunning recent examples of the scientific method – careful consideration of statistical and systematic uncertainties coming together – was announced in 2012 at the moment the spokespersons for the ATLAS and CMS experiments at the LHC revealed the discovery of the Higgs boson.


    CERN CMS Higgs Event


    CERN/CMS Detector


    CERN ATLAS Higgs Event


    CERN/ATLAS detector

    More than half a century of theory and experimentation led up to that moment. Experiments from the 1950s on had accumulated a wealth of information on particle interactions, but the interactions were only partially understood and seemed to come from disconnected sources.

    “But brilliant theoretical physicists found a way to make a single model that gave them a good description of all the known phenomena, says Incandela, who was spokesperson for the CMS experiment during the Higgs discovery. “It wasn’t guaranteed that the Higgs field existed. It was only guaranteed that this model works for everything we do and have already seen, and we needed to see if there really was a boson that we could find that could tell us in fact that that field is there.”

    This led to a generation-long effort to build an accelerator that would reach the extremely high energies needed to produce the Higgs boson, a particle born of the Higgs field, and then two gigantic detectors that could detect the Higgs boson if it appeared.

    Building two different detectors would allow scientists to double-check their work. If an identical signal appeared in two separate experiments run by two separate groups of physicists, chances were quite good that it was the real thing.

    “So there you saw a really beautiful application of the scientific method where we confirmed something that was incredibly difficult to confirm, but we did it incredibly well with a lot of fail-safes and a lot of outstanding experimental approaches,” Incandela says. “The scientific method was already deeply engrained in everything we did to the greatest extreme. And so we knew when we saw these things that they were real, and we had to take them seriously.”

    The scientific method is so engrained that scientists don’t often talk about it by name anymore, but implementing it “is what separates the great scientists from the average scientists from the poor scientists,” Incandela says. “It takes a lot of scrutiny and a deep understanding of what you’re doing.”

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:33 am on March 24, 2017 Permalink | Reply
    Tags: A new gem inside the CMS detector, , , , , Particle Accelerators, , ,   

    From Symmetry: “A new gem inside the CMS detector” 

    Symmetry Mag

    Symmetry

    03/24/17
    Sarah Charley

    1
    Photo by Maximilien Brice, CERN

    This month scientists embedded sophisticated new instruments in the heart of a Large Hadron Collider experiment.

    Sometimes big questions require big tools. That’s why a global community of scientists designed and built gigantic detectors to monitor the high-energy particle collisions generated by CERN’s Large Hadron Collider in Geneva, Switzerland. From these collisions, scientists can retrace the footsteps of the Big Bang and search for new properties of nature.

    The CMS experiment is one such detector. In 2012, it co-discovered the elusive Higgs boson with its sister experiment, ATLAS. Now, scientists want CMS to push beyond the known laws of physics and search for new phenomena that could help answer fundamental questions about our universe. But to do this, the CMS detector needed an upgrade.

    “Just like any other electronic device, over time parts of our detector wear down,” says Steve Nahn, a researcher in the US Department of Energy’s Fermi National Accelerator Laboratory and the US project manager for the CMS detector upgrades. “We’ve been planning and designing this upgrade since shortly after our experiment first started collecting data in 2010.”

    The CMS detector is built like a giant onion. It contains layers of instruments that track the trajectory, energy and momentum of particles produced in the LHC’s collisions. The vast majority of the sensors in the massive detector are packed into its center, within what is called the pixel detector. The CMS pixel detector uses sensors like those inside digital cameras but with a lightning fast shutter speed: In three dimensions, they take 40 million pictures every second.

    For the last several years, scientists and engineers at Fermilab and 21 US universities have been assembling and testing a new pixel detector to replace the current one as part of the CMS upgrade, with funding provided by the Department of Energy Office of Science and National Science Foundation.

    2
    Maral Alyari of SUNY Buffalo and Stephanie Timpone of Fermilab measure the thermal properties of a forward pixel detector disk at Fermilab. Almost all of the construction and testing of the forward pixel detectors occurred in the United States before the components were shipped to CERN for installation inside the CMS detector. Photo by Reidar Hahn, Fermilab

    3
    Stephanie Timpone consults a cabling map while fellow engineers Greg Derylo and Otto Alvarez inspect a completed forward pixel disk. The cabling map guides their task of routing the the thin, flexible cables that connect the disk’s 672 silicon sensors to electronics boards. Maximilien Brice, CERN

    4
    The CMS detector, located in a cavern 100 meters underground, is open for the pixel detector installation. Photo by Maximilien Brice, CERN

    5
    Postdoctoral researcher Stefanos Leontsinis and colleague Roland Horisberger work with a mock-up of one side of the barrel pixel detector next to the LHC’s beampipe.
    Photo by Maximilien Brice, CERN

    6
    Leontsinis watches the clearance as engineers slide the first part of the barrel pixel just millimeters from the LHC’s beampipe. Photo by Maximilien Brice, CERN

    7
    Scientists and engineers lift and guide the components by hand as they prepare to insert them into the CMS detector. Photo by Maximilien Brice, CERN

    8
    Scientists and engineers connect the cooling pipes of the forward pixel detector. The pixel detector is flushed with liquid carbon dioxide to keep the silicon sensors protected from the LHC’s high-energy collisions. Photo by Maximilien Brice, CERN

    The pixel detector consists of three sections: the innermost barrel section and two end caps called the forward pixel detectors. The tiered and can-like structure gives scientists a near-complete sphere of coverage around the collision point. Because the three pixel detectors fit on the beam pipe like three bulky bracelets, engineers designed each component as two half-moons, which latch together to form a ring around the beam pipe during the insertion process.

    Over time, scientists have increased the rate of particle collisions at the LHC. In 2016 alone, the LHC produced about as many collisions as it had in the three years of its first run together. To be able to differentiate between dozens of simultaneous collisions, CMS needed a brand new pixel detector.

    The upgrade packs even more sensors into the heart of the CMS detector. It’s as if CMS graduated from a 66-megapixel camera to a 124-megapixel camera.

    Each of the two forward pixel detectors is a mosaic of 672 silicon sensors, robust electronics and bundles of cables and optical fibers that feed electricity and instructions in and carry raw data out, according to Marco Verzocchi, a Fermilab researcher on the CMS experiment.

    The multipart, 6.5-meter-long pixel detector is as delicate as raw spaghetti. Installing the new components into a gap the size of a manhole required more than just finesse. It required months of planning and extreme coordination.

    “We practiced this installation on mock-ups of our detector many times,” says Greg Derylo, an engineer at Fermilab. “By the time we got to the actual installation, we knew exactly how we needed to slide this new component into the heart of CMS.”

    The most difficult part was maneuvering the delicate components around the pre-existing structures inside the CMS experiment.

    “In total, the full three-part pixel detector consists of six separate segments, which fit together like a three-dimensional cylindrical puzzle around the beam pipe,” says Stephanie Timpone, a Fermilab engineer. “Inserting the pieces in the right positions and right order without touching any of the pre-existing supports and protections was a well-choreographed dance.”

    For engineers like Timpone and Derylo, installing the pixel detector was the last step of a six-year process. But for the scientists working on the CMS experiment, it was just the beginning.

    “Now we have to make it work,” says Stefanos Leontsinis, a postdoctoral researcher at the University of Colorado, Boulder. “We’ll spend the next several weeks testing the components and preparing for the LHC restart.”

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:59 am on March 22, 2017 Permalink | Reply
    Tags: , , , Particle Accelerators, , 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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