Tagged: HEP Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 9:10 am on April 27, 2017 Permalink | Reply
    Tags: , , , , , HEP, , , SCOAP3   

    From CERN: “CERN and the American Physical Society sign an open access agreement for SCOAP3” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    Geneva, 27 April 2017. The European Organization for Nuclear Research (CERN) and the American Physical Society (APS) signed an agreement today for SCOAP3 – the Sponsoring Consortium for Open Access Publishing in Particle Physics. Under this agreement, high-energy physics articles published in three leading journals of the APS will be open access as from January 2018.

    1

    All authors worldwide will be able to publish their high-energy physics articles in Physical Review C, Physical Review D and Physical Review Letters at no direct cost. This will allow free and unrestricted exchange of scientific information within the global scientific community and beyond, for the advancement of science.

    “Open access reflects values and goals that have been enshrined in CERN’s Convention for more than sixty years, such as the widest dissemination of scientific results. We are very pleased that the APS is joining SCOAP3 and we look forward to welcoming more partners for the long-term success of this initiative”, said Fabiola Gianotti, CERN’s Director General.

    APS CEO Kate Kirby commented that, “APS has long supported the principles of open access to the benefit of the scientific enterprise. As a non-profit society publisher and the largest international publisher of high-energy physics content, APS has chosen to participate in the SCOAP3 initiative in support of this community.”

    With this new agreement between CERN and the APS, SCOAP3 will cover about 90 percent of the journal literature in the field of high-energy physics.

    Convened and managed by CERN, SCOAP3 is the largest scale global open access initiative ever built. It involves a global consortium of 3,000 libraries and research institutes from 44 countries, with the additional support of eight research funding agencies. Since its launch in 2014, it has made 15 000 articles by about 20 000 scientists from 100 countries accessible to anyone.
    The initiative is possible through funds made available from the redirection of former subscription monies. Publishers reduce subscription prices for journals participating in the initiative, and those savings are pooled by SCOAP3 partners to pay for the open access costs, for the wider benefit of the community.

    Received via email.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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 1:25 pm on April 24, 2017 Permalink | Reply
    Tags: , , HEP, New ALICE results show novel phenomena in proton collisions, , , , 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 .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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: , , HEP, , , 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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

    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 .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

    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 .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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 1:25 pm on April 14, 2017 Permalink | Reply
    Tags: , , HEP, , ,   

    From Ethan Siegel: “Can muons — which live for microseconds — save experimental particle physics?” 

    Ethan Siegel

    Apr 14, 2017

    You lose whether you use protons or electrons in your collider, for different reasons. Could the unstable muon solve both problems?

    1
    A four-muon candidate event in the ATLAS detector at the Large Hadron Collider. The muon/anti-muon tracks are highlighted in red, as the long-lived muons travel farther than any other unstable particle. Image credit: ATLAS Collaboration / CERN.

    “It does not matter how slowly you go as long as you do not stop.” -Confucius

    High-energy physics is facing its greatest crisis ever. The Standard Model is complete, as all the particles our most successful physics theories have predicted have been discovered.

    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.

    The Large Hadron Collider at CERN, the most energetic particle collider ever developed (with more than six times the energies of any prior collider), discovered the long-sought-after Higgs boson, but nothing else.

    CERN/LHC Map

    CERN LHC Tube


    LHC at CERN

    Traditionally, the way to discover new particles has been to go to higher energies with one of two strategies:

    Collide electrons and positrons, getting a “clean” signal where 100% of the collider energy goes into producing new particles.
    Collide protons and either anti-protons or other protons, getting a messy signal but reaching higher energies due to the heavier mass of the proton.

    Both methods have their limitations, but one unstable particle might give us a third option to make the elusive breakthrough we desperately need: the muon.

    2
    The known particles in the Standard Model. These are all the fundamental particles that have been directly discovered. Image credit: E. Siegel.

    The Standard Model is made up of all the fundamental particles and antiparticles we’ve ever discovered. They include six quarks and antiquarks, each in three colors, three charged leptons and three types of neutrino, along with their antiparticle counterparts, and the bosons: the photon, the weak bosons (W+, W-, Z0), the eight gluons (with color/anticolor combinations attached), and the Higgs boson. While countless different combinations of these particles exist in nature, only a precious few are stable. The electron, photon, proton (made of two up and one down quark), and, if they’re bound together in nuclei, the neutron (with two down and one up quark) are stable, along with their antimatter counterparts. That’s why all the normal matter we see in the Universe is made up of protons, neutrons, and electrons; nothing else with any significant interactions is stable.

    3
    While many unstable particles, both fundamental and composite, can be produced in particle physics, only protons, neutrons (bound in nuclei) and the electron are stable, along with their antimatter counterparts and the photon. Everything else is short-lived. Image credit: Contemporary Physics Education Project (CPEP), U.S. Department of Energy / NSF / LBNL.

    The way you create these unstable particles is by colliding the stable ones together at high enough energies. Because of a fundamental principle of nature — mass/energy equivalence, given by Einstein’s E = mc2 — you can turn pure energy into mass if you have enough of it. (So long as you obey all the other conservation laws.) This is exactly the way we’ve created almost all the other particles of the Standard Model: by colliding particles into one another at enough energy that the energy you get out (E) is high enough to create the new particles (of mass m) you’re attempting to discover.

    4
    The particle tracks emanating from a high energy collision at the LHC in 2014 show the creation of many new particles. It’s only because of the high-energy nature of this collision that new masses can be created.

    We know there are almost certainly more particles beyond the ones we’ve discovered; we expect there to be particle explanations for mysteries like the baryon asymmetry (why there’s more matter than antimatter), the missing mass problem in the Universe (what we suspect will be solved by dark matter), the neutrino mass problem (why they’re so incredibly light), the quantum nature of gravity (i.e., there should be a force-carrying particle for the gravitational interaction, like the graviton), and the strong-CP problem (why certain decays don’t happen), among others. But our colliders haven’t reached the energies necessary to uncover those new particles, if they even exist. What’s even worse: both of the current methods have severe drawbacks that may prohibit us from building colliders that go to significantly higher energies.

    The Large Hadron Collider is the current record-holder, accelerating protons up to energies of 6.5 TeV apiece before smashing them together. The energy you can reach is directly proportional to two things only: the radius of your accelerator (R) and the strength of the magnetic field used to bend the protons into a circle (B). Collide those two protons together, and they hit with an energy of 13 TeV. But you’ll never make a 13 TeV particle colliding two protons at the LHC; only a fraction of that energy is available to create new particles via E = mc². The reason? A proton is made of multiple, composite particles — quarks, gluons, and even quark/antiquark pairs inside — meaning that only a tiny fraction of that energy goes into making new, massive particles.

    5
    A candidate Higgs event in the ATLAS detector. Note how even with the clear signatures and transverse tracks, there is a shower of other particles; this is due to the fact that protons are composite particles. Image credit: The ATLAS collaboration / CERN.

    CERN ATLAS Higgs Event

    CERN/ATLAS detector

    You might think to use fundamental particles instead, then, like electrons and positrons. If you were to put them in the same ring (with the same R) and subject them to the same magnetic field (the same B), you might think you could reach the same energies, only this time, 100% of the energy could make new particles. And that would be true, if it weren’t for one factor: synchrotron radiation. You see, when you accelerate a charged particle in a magnetic field, it gives off radiation. Because a proton is so massive compared to its electric charge, that radiation is negligible, and you can take protons up to the highest energies we’ve ever reached without worrying about it. But electrons and positrons are only 1/1836th of a proton’s mass, and synchrotron radiation would limit them to only about 0.114 TeV of energy under the same conditions.

    6
    Relativistic electrons and positrons can be accelerated to very high speeds, but will emit synchrotron radiation (blue) at high enough energies, preventing them from moving faster. Image credit: Chung-Li Dong, Jinghua Guo, Yang-Yuan Chen, and Chang Ching-Lin, ‘Soft-x-ray spectroscopy probes nanomaterial-based devices’.

    But there’s a third option that’s never been put into practice: use muons and anti-muons. A muon is just like an electron in the sense that it’s a fundamental particle, it’s charged, it’s a lepton, but it’s 206 times heavier than the electron. This is massive enough that synchrotron radiation doesn’t matter for muons or anti-muons, which is great! The only downside? The muon is unstable, with a mean lifetime of only 2.2 microseconds before decaying away.

    5
    The prototype MICE 201-megahertz RF module, with the copper cavity mounted, is shown during assembly at Fermilab. This apparatus could focus and collimate a muon beam, enabling the muons to be accelerated and survive for much longer than 2.2 microseconds. Image credit: Y. Torun / IIT / Fermilab Today.

    That might be okay, though, because special relativity can rescue us! When you bring an unstable particle close to the speed of light, the amount of time that it lives increases dramatically, thanks to the relativistic phenomenon of time dilation. If you brought a muon all the way up to 6.5 TeV of energy, it would live for 135,000 microseconds: enough time to circle the Large Hadron Collider 1,500 times before decaying away. And this time, your hopes would be absolutely true: 100% of that energy, 6.5 TeV + 6.5 TeV = 13 TeV, would be available for particle creation.

    6
    A design plan for a full-scale muon-antimuon collider at Fermilab, the source of the world’s second-most powerful particle accelerator. Image credit: Fermilab.

    We can always build a bigger ring or invent stronger magnets, and we may well do exactly that. But there’s no cure for synchrotron radiation except to use heavier particles, and there’s no cure for energy spreading out among the components of composite particles other than not to use them at all. Muons are unstable and difficult to keep alive for a long time, but as we get to higher and higher energies, that task gets progressively easier. Muon colliders have long been touted as a mere pipe dream, but recent progress by the MICE collaboration — for Muon Ionization Cooling Experiment — has demonstrated that this may be possible after all. A circular muon/anti-muon collider may be the particle accelerator that takes us beyond the LHC’s reach, and, if we’re lucky, into the realm of the new physics we’re so desperately seeking.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “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 11:32 am on April 6, 2017 Permalink | Reply
    Tags: , , HEP, 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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

    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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

    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.

    STEM Icon

    Stem Education Coalition

    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, , , , HEP, , , ,   

    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.

    STEM Icon

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: