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  • richardmitnick 9:03 pm on December 28, 2015 Permalink | Reply
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    From DESY: “ERC Starting Grant for characterising the Higgs boson” 

    DESY
    DESY

    2015/12/28
    No writer credit found

    Temp 1
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    Kerstin Tackmann, a physicist at DESY, is to receive over 1.3 million euros from the European Research Council (ERC) in order to carry out research aimed at a more detailed characterisation of the Higgs boson.

    CERN ATLAS Higgs Event
    Higgs event at ATLAS

    She will use a starting grant to set up a research group to investigate the properties of the Higgs boson in great detail, as part of the international ATLAS Collaboration.

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    ATLAS

    These measurements are an important step towards identifying whether the particle fits the Standard Model of particle physics. The 5-year project is scheduled to begin in 2016.

    Ever since particle physicists working on the big LHC experiments ATLAS and CMS announced, in 2012, the discovery of a particle whose properties corresponded to those of the elusive Higgs boson, particle physics has faced an extremely exciting mystery: does this Higgs boson fit the Standard Model of particle physics, the currently accepted description of the elementary particles that make up matter and the forces acting between them, or will it open the path to a new, higher-level theory.

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    Standard Model of Particle Physics

    Using the data available so far, scientists have already been able to determine the particle’s mass of around 125 gigaelectronvolts (GeV) and its spin of zero to a fairly high degree of accuracy. To obtain even more precise information about additional properties of the particle, the researchers need to analyse far more data from proton-proton collisions in the LHC. They are particularly interested in finding out exactly how the Higgs field, of which the Higgs boson is an indication, lends elementary particles their mass. To answer this question, they have started to analyse the collision data from “LHC Run 2”, which began this summer and which is expected to produce about 15 times as many Higgs bosons as the LHC’s previous run. The analysis of this large amount of collision data will allow far more reliable conclusions to be drawn.

    Kerstin Tackmann intends to devote herself to these questions together with two post-docs and three PhD students, and will be analysing the collisions from Run 2 of the ATLAS detector in great detail. They will be working as part of the ATLAS Collaboration, involving hundreds of scientists from all over the world. Her group is going to concentrate on measuring the kinematic properties of Higgs boson production. The focus will lie especially on the decay of the Higgs boson into two photons or four leptons, which allows very accurate measurements to be made. This is where deviations from the precise predictions of the Standard Model could occur, should the Higgs boson not fit the Standard Model.

    See the full article here .

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

     
  • richardmitnick 8:29 pm on December 16, 2015 Permalink | Reply
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    From CERN: “ATLAS and CMS present their 2015 LHC results” 

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    CERN

    16 Dec 2015
    Corinne Pralavorio

    1
    A 13 TeV collision recorded by ATLAS. The yellow and green bars indicate the presence of particle jets, which leave behind lots of energy in the calorimeters. (Image: ATLAS)

    2
    A 13 TeV proton collision recorded by CMS. The two green lines show two photons generated by the collision. (Image: CMS)

    Particles circulated in the Large Hadron Collider (LHC) on Sunday for the last time in 2015, and, two days later, the two large general-purpose experiments, ATLAS and CMS, took centre stage to present their results from LHC Run 2. These results were based on the analysis of proton collisions at the previously unattained energy of 13 TeV, compared with the maximum of 8 TeV attained during LHC Run 1 from 2010 to 2012.

    The amount of data on which the two experiments’ analyses are based is still limited – around eight times less than that collected during Run 1 – and physicists need large volumes of data to be able to detect new phenomena. Nonetheless, the experimentalists have already succeeded in producing numerous results. Each of the two experiments has presented around 30 analyses, about half of which relate to Beyond-Standard-Model research. The Standard Model is the theory that describes elementary particles and their interactions, but it leaves many questions unanswered. Physicists are therefore searching for signs of Beyond-Standard-Model physics that might help them to answer some of those questions.

    The new ATLAS and CMS results do not show any significant excesses that could indicate the presence of particles predicted by alternative models such as supersymmetry. The two experiments have therefore established new limits for the masses of these hypothetical new particles. Advances in particle physics often come from pushing back these limits. For example, CMS and ATLAS have established new restrictions for the mass of the gluino, a particle predicted by the theory of supersymmetry. This is just one of the many results that were presented on 15 December.

    The two experiments have also observed a slight excess in the diphoton decay channel. Physicists calculate the mass of hypothetical particles that decay to form a pair of photons, and look at how often different masses are seen. If the distribution does not exactly match that expected from known processes, or in other words a bump appears at a specific mass not corresponding to any known particle, it may indicate a new particle being produced and decaying. However, the excess is too small at this stage to draw such a conclusion. We will have to wait for more data in 2016 to find out whether this slight excess is an inconsequential statistical fluctuation or, alternatively, a sign of the existence of a new phenomenon. Find out next time: season 2 is only just beginning.

    The presentations by ATLAS and CMS are available here.

    See the full article here.

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  • richardmitnick 8:11 pm on December 16, 2015 Permalink | Reply
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    From Pauline Gagnon at Quantum Diaries: “If, and really only if…” 

    12.16.15

    Pauline Gagnon
    Pauline Gagnon

    On December 15, at the End-of-the-Year seminar, the CMS and ATLAS experiments from CERN presented their first results using the brand new data accumulated in 2015 since the restart of the Large Hadron Collider (LHC) at 13 TeV, the highest operating energy so far.

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    CMS

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    LHC

    Although the data sample is still only one tenth of what was available at lower energy (namely 4 fb-1 for ATLAS and 2.8-1 fb for CMS collected at 13 TeV compared to 25 fb-1 at 8 TeV for each experiment), it has put hypothetical massive particles within reach. If the LHC were a ladder and particles, boxes hidden on shelves, operating the LHC at higher energy is like having a longer ladder giving us access to higher shelves, a place never checked before. ATLAS and CMS just had their first glimpse at it.

    Both experiments showed how well their detectors performed after several major improvements, including collecting data at twice the rate used in 2012. The two groups made several checks on how known particles behave at higher energy, finding no anomalies. But it is in searches for new, heavier particles that every one hopes to see something exciting. Both groups explored dozens of different possibilities, sifting through billions of events.

    Each event is a snapshot of what happens when two protons collide in the LHC. The energy released by the collision materializes into some heavy and unstable particle that breaks apart mere instants later, giving rise to a mini firework. By catching, identifying and regrouping all particles that fly apart from the collision point, one can reconstruct the original particles that were produced.

    Both CMS and ATLAS found small excesses when selecting events containing two photons. In several events, the two photons seem to come from the decay of a particle having a mass around 750 GeV, that is, 750 times heavier than a proton or 6 times the mass of a Higgs boson.

    CERN ATLAS Higgs Event
    Higgs event at ATLAS

    Since the two experiments looked at dozens of different combinations, checking dozens of mass values for each combination, such small statistical fluctuations are always expected.

    2
    Top part: the combined mass given in units of GeV for all pairs of photons found in the 13 TeV data by ATLAS. The red curve shows what is expected from random sources (i.e. the background). The black dots correspond to data and the lines, the experimental errors. The small bump at 750 GeV is what is now intriguing. The bottom plot shows the difference between black dots (data) and red curve (background), clearly showing a small excess of 3.6σ or 3.6 times the experimental error. When one takes into account all possible fluctuations at all mass values, the significance is only 2.0σ

    What’s intriguing here is that both groups found the same thing at exactly the same place, without having consulted each other and using selection techniques designed not to bias the data. Nevertheless, both experimental groups are extremely cautious, stating that a statistical fluctuation is always possible until more data is available to check this with increased accuracy.

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    CMS has slightly less data than ATLAS at 13 TeV and hence, sees a much smaller effect. In their 13 TeV data alone, the excess at 760 GeV is about 2.6σ, 3σ when combined with the 8 TeV data. But instead of just evaluating this probability alone, experimentalists prefer take into account the fluctuations in all mass bins considered. Then the significance is only 1.2σ, nothing to write home about. This “look-elsewhere effect” takes into account that one is bound to see a fluctuation somewhere when ones look in so many places.

    Theorists show less restrain. For decades, they have known that the Standard Model, the current theoretical model of particle physics, is flawed and have been looking for a clue from experimental data to go further.

    4
    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.

    Many of them have been hard at work all night and eight new papers appeared this morning, proposing different explanations on which new particle could be there, if something ever proves to be there. Some think it could be a particle related to Dark Matter, others think it could be another type of Higgs boson predicted by Supersymmetry or even signs of extra dimensions. Others offer that it could only come from a second and heavier particle. All suggest something beyond the Standard Model.

    Two things are sure: the number of theoretical papers in the coming weeks will explode. But establishing the discovery of a new particle will require more data. With some luck, we could know more by next Summer after the LHC delivers more data. Until then, it remains pure speculation.

    This being said, let’s not forget that the Higgs boson made its entry in a similar fashion. The first signs of its existence appeared in July 2011. With more data, they became clearer in December 2011 at a similar End-of-the-Year seminar. But it was only once enough data had been collected and analysed in July 2012 that its discovery made no doubt. Opening one’s gifts before Christmas is never a good idea.

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  • richardmitnick 7:33 am on December 12, 2015 Permalink | Reply
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    From ATLAS at CERN: “Photo Essay: Impressions from the Control Room” 

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    CERN

    June 12, 2015 [This just became available]
    Abha Eli Phoboo

    As final preparations were made for the start of the Large Hadron Collider’s (LHC) Run 2, the ATLAS Control Room was the centre of activity. Here are images from the three days that were landmark events — first collisions at 900 GeV on 5 May, first test collisions at 13 TeV on 20 May , and 3 June that marked the beginning of physics data-taking at 13 TeV and ATLAS’ journey into unexplored frontiers of physics.

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    5 May: Physicists in the ATLAS Control Room prepare for the first scheduled proton beam collisions to be delivered by the Large Hadron Collider. The beams collided at injection energy or 900 GeV (one proton has a mass of about 1 GeV). IMAGE: Silvia Biondi/The ATLAS Experiment

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    5 May: ATLAS people on shift that morning wait for the LHC Control Room to signal injection of beam. IMAGE: Silvia Biondi/The ATLAS Experiment

    Temp 1
    5 May: ATLAS Run Coordinator Alessandro Polini (left) shares a smile with Spokesperson Dave Charlton as they wait for 900 GeV collisions.
    IMAGE: Matteo Franchini/The ATLAS Experiment

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    5 May: The LHC beam being monitored on one of the many control room desktop monitors. IMAGE: Silvia Biondi/The ATLAS Experiment

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    5 May: A physicist on shift watches as the first collisions at injection energy or 900 GeV burst on the wall of screens in the ATLAS Control Room. IMAGE: Silvia Biondi/The ATLAS Experiment

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    6 May: Display of a proton collision event recorded by ATLAS at 900 GeV or injection energy. Tracks are reconstructed from hits in the inner tracking detector, including the new innermost pixel detector layer, the Insertable B-Layer. No image credit.

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    On 20 May, at 22:24, ATLAS recorded the first 13 TeV test collisions delivered by the Large Hadron Collider. The proton collisions set a new high energy record. IMAGE: Heinz Pernegger/The ATLAS Experiment

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    21 May: Display of a proton collision event recorded by ATLAS at 13 TeV collision energy. Tracks reconstructed from hits in the inner tracking detector are shown to originate from two interaction points, indicating a pile-up event. No image credit.

    10
    3 June: Morning light shines of the mural of a simulated Higgs event perpendicular to the one of the ATLAS detector. This image was taken on the morning when physics data-taking was scheduled to start. The mural is painted on the building that houses the ATLAS Control Room. 100m directly below the building is the cavern where the ATLAS detector sits on the Swiss side of the LHC. IMAGE: Clara Nellist/The ATLAS Experiment

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    3 June: ATLAS physicists gather inside the Control Room to witness the start of the physics data taking at 13 TeV with the ATLAS detector. IMAGE: Silvia Biondi/The ATLAS Experiment

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    3 June: ATLAS Run Coordinator Alex Cerri and Central Trigger Processing expert Julian Glatzer looking at plots that describe proton bunch groups from the LHC. Each LHC orbit has around 3,564 proton bunches spaced at every 25 nanoseconds to fill the 27 km ring. IMAGE: Silvia Biondi/The ATLAS Experiment

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    3 June: Readying for that moment when ATLAS began recording 13 TeV collision data. IMAGE: Pierre Descombe/CERN

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    3 June: Display of a proton collision event recorded by ATLAS with first LHC stable beams at a collision energy of 13 TeV. Tracks reconstructed by the tracking detector are shown as light blue lines, and hits in the layers of the silicon tracking detector are shown as colored filled circles. The four inner layers are part of the silicon pixel detector and the four outer layers are part of the silicon strip detector. The layer closest to the beam, is the IBL. No image credit.

    15
    3 June: The Control Room bursts into applause as ATLAS begins recording data. IMAGE: Emma Ward/The ATLAS Experiment

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  • richardmitnick 7:32 am on July 24, 2015 Permalink | Reply
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    From ATLAS at CERN: “Early Run 2 results test event generator energy extrapolation” 

    CERN New Masthead

    23 July 2015

    ATLAS presented its first measurements of soft strong interaction processes using charged particles produced in proton–proton collisions at 13 TeV centre-of-mass energy delivered by the Large Hadron Collider at CERN. These measurements were performed with a dataset collected beginning of June under special low-luminosity conditions during which the frequency of multiple proton–proton scattering occurring in the same recorded collision event was strongly reduced.

    Measurements like this are important for understanding the collision energy dependence of such processes, as well as ensuring a successful description of the data by the Monte Carlo (MC) event generators. The accuracy of these simulations is critical for their subsequent use in ATLAS searches and measurements.

    Performing these measurements at an unprecedented collision energy with upgraded detector components in such a short scale was a challenge. During the Long Shutdown, many improvements were made to the detector, most relevant among them for these results was the addition of the innermost pixel layer, the insertable B-layer (IBL). Adding the IBL dramatically improved the accuracy of the track reconstruction and the identification of jets originating from bottom quarks, which is important for many searches. The commissioning of the IBL, alignment of different detector components and assessment of passive detector material is still ongoing. Figure 1 shows the number of hits in pixel layer per track. Generally, good agreement is observed, indicating a very good understanding of the ATLAS four-layer pixel system. The minor disagreements stem from the mismatch in simulation and data of the number of modules not working properly during that period of data-taking.

    The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity (which essentially represents the angular position from beam axis) and the dependence of the mean transverse momentum on the charged-particle multiplicity were presented, based on about 9 million events. The events contained at least one charged particle with transverse momentum greater than 500 MeV in the central part of the detector. The data were corrected with minimal model dependence to obtain inclusive distributions. Overall the Monte Carlo models, which were tuned to such similar measurements performed at lower centre-of-mass energies, seem to describe the data reasonably well. Figure 2 shows the mean number of charged particles in the central region compared to previous measurements at different collision energies, together with the MC predictions. The mean number of charged particles increases by a factor of 2.2 when collision energy increases from 0.9 TeV to 13 TeV.

    In the events where the leading track had a transverse momentum of at least 1 GeV, the accompanying activity was studied at the detector level. The azimuthal region perpendicular to the direction of the leading track is most sensitive to this accompanying activity, termed the underlying event (UE). The average number of tracks in each event and their transverse momentum sum are seen to show a gradual rise towards a “plateau” with rising leading track transverse momentum, a trend seen in previous measurements. Figure 3 shows the latter in the transverse region. Compared to 7 TeV results a 20% increase to the UE activity is observed and is predicted well by most of the models.

    These early measurements show a good understanding of the performance the upgraded ATLAS detector as well as the ability of the Monte Carlo event generators to describe the data at new collision energy.

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    Figure 1: Comparison of number of pixel hits distributions in data and simulation.

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    Figure 2: The average charged-particle multiplicity per unit of rapidity for eta= 0 as a function of the centre-of-mass energy.

    3
    Figure 3: Comparison of detector level data and MC predictions for average scalar pT sum density of tracks as a function of leading track pT.

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  • richardmitnick 8:19 pm on July 20, 2015 Permalink | Reply
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    From ATLAS at CERN: “First Run 2 Results to be Presented at EPS” 

    CERN New Masthead

    July 20, 2015

    For the past six weeks, the ATLAS experiment has been recording physics data from 13 TeV proton collisions. In this short time, they have recorded over 7,280 billion collisions – twice the amount recorded in 2010. Detector teams have been calibrating and aligning ATLAS’ various detectors at a remarkable speed. This effort allows ATLAS physicists to exploit the many upgrades made to the detector during Long Shutdown 1.

    ATLAS physicists are combing through this new wealth of data, performing detailed studies of Standard Model processes at unprecedented energies, the Higgs boson and the first searches for as-yet unobserved phenomena.

    The first results using the record-breaking Run 2 data will be presented at the European Physical Society conference on High Energy Physics (EPS-HEP) in Vienna, 22-29 July. It will be an exciting opportunity to see how these first few weeks of data-taking have progressed. Physics briefings will be released throughout the event, highlighting ATLAS’ results from the conference.

    EPS-HEP is the first of the major summer conferences for particle physics, where all of the LHC experiments will be presenting results. About seven hundred researchers from all over the world are expected to attend.

    Further results using Run 2 data will be presented at the Lepton Photon conference, 17-22 August, and the Large Hadron Collider Physics conference (LHCP2015), 31 August-5 September.

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  • richardmitnick 2:22 pm on July 16, 2015 Permalink | Reply
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    From Symmetry: “Something goes bump in the data” 

    Symmetry

    July 16, 2015
    Katie Elyce Jones and Sarah Charley

    The CMS and ATLAS experiments at the LHC see something mysterious, but it’s too soon to pop the Champagne.

    CERN CMS Detector
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    An unexpected bump in data gathered during the first run of the Large Hadron Collider is stirring the curiosity of scientists on the two general-purpose LHC experiments, ATLAS and CMS.

    CMS first reported the bump in July 2014. But because it was small and insignificant, they dismissed it as a statistical fluke. Recently ATLAS confirmed that they also see a bump in roughly the same place, and this time it’s bigger and stronger.

    “Both ATLAS and CMS are developing new search techniques that are greatly improving our ability to search for new particles,” says Ayana Arce, an assistant professor of physics at Duke University. “We can look for new physics in ways we couldn’t before.”

    Unlike the pronounced peak that recently led to the discovery of pentaquarks, these two studies are in their nascent stages. And scientists aren’t quite sure what they’re seeing yet… or if they’re seeing anything at all.

    If this bump matures into a sharp peak during the second run of the LHC, it could indicate the existence of a new heavy particle with 2000 times the mass of a proton. The discovery of a new and unpredicted particle would revolutionize our understanding of the laws of nature. But first, scientists have to rule out false leads.

    “It’s like trying to pick up a radio station,” says theoretical physicist Bogdan Dobrescu of Fermi National Accelerator Laboratory who co-authored a paper on the bump in CMS and ATLAS data. “As you tune the dial, you think you’re beginning to hear voices through the static, but you can’t understand what they’re saying, so you keep tuning until you hear a clear voice.”

    On the heels of the Higgs boson discovery in the first run of the LHC, scientists must navigate a tricky environment where people are hungry for new results while relying on data that is slow to gather and laborious to interpret.

    The data physicists are analyzing are particle decay patterns around 2 TeV, or 2000 GeV.

    “We can’t see short-lived particles directly, but we can reconstruct their mass based on what they transform into during their decay,” says Jim Olsen, a professor of physics at Princeton University. “For instance, we found the Higgs boson because we saw more pairs of W bosons, Z bosons and photons at 125 GeV than our background models predicted.”

    Considering that the heaviest particle of the Standard Model, the top quark, has a mass of 173 GeV, if this bump is real and not a fluctuation, it indicates a significantly heavier particle than those covered in the Standard Model.

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    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.

    While the theories being batted around at this early stage disagree on the particulars, most agree that, so far, this data bump best fits the properties of an extended Standard Model gauge boson.

    The gauge bosons are the force-carrying particles that enable matter particles to interact with each other. The heaviest bosons are the W and Z bosons, which carry the weak force. An extended Standard Model predicts comparable particles at higher energies, heavier versions known as W prime and Z prime (or W’ and Z’). Several theorists suggest the bump at 2 TeV could be a type of W prime.

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    ATLAS data shows an increased number of W and Z boson pairs at 2 TeV. Courtesy of: ATLAS collaboration

    But LHC physicists aren’t practicing their Swedish for the Nobel ceremony yet. Unexpected bumps are common and almost always fizzle out with more data. For instance, in 2003 an international collaboration working on the Belle experiment at the KEK accelerator laboratory in Japan saw an apparent contradiction to the Standard Model’s predictions in the decay patterns of particles containing bottom quarks.

    “It was really striking,” says Olsen. “The probability that the signal was due to sheer statistical fluctuation was only about one in 10,000.”

    Seven years later, after inundating their analysis with heaps of fresh data, the original contradiction from the Belle experiment withered and died, and from its ashes arose a stronger result that perfectly matched the predictions of the Standard Model.

    But scientists also haven’t written off this new bump as a statistical fluctuation. In fact, the closer they look, the more exciting it becomes.

    With most anomalies in the data, one experiment will see it while the other one won’t—a clear indication of a statistical fluctuation. But in this case, both CMS and ATLAS independently reported the same observation. And not only do both experiments see it, they see it at roughly the same energy across several different types of analyses.

    “This is kind of like what we saw with the Higgs,” says JoAnne Hewett, a theoretical physicist at SLAC National Accelerator Laboratory who co-authored a paper theorizing the bump could be a type of W prime particle. “The Higgs just started showing up as 2- to 3-sigma bumps in a few different channels in the two different experiments. But there were also false leads with the Higgs.”

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    CMS data summarizing the search for new heavy particles decaying into several decay channels. The search reveals hints of a structure near 2000 GeV (2 TeV). Courtesy of: CMS collaboration

    Scientists are seeing more Z boson and W boson pairs popping up at 2 TeV than the Standard Model predicts. But besides this curious excess of events, they haven’t identified any sort of clear pattern.

    “Theorists come up with the models that predict the patterns we should see if there is some type of new physics influencing our experimental data,” Olsen says. “So if this bump is new physics, then our models should predict what else we should see.”

    Even though this bump is far too small to signify a discovery and presents no predictable pattern, its presence across multiple different analyses from both CMS and ATLAS is intriguing and suspicious. Scientists will have to patiently wait for more data before they can flesh out what it actually is.

    “We will soon have a lot more data from the second run of the LHC, and both experiments will be able to look more closely at this anomaly,” Arce says. “But I think it would almost be too lucky if we discovered a new particle this soon into the second run of the LHC.”

    The latest results from these two studies will be presented at the European Physical Society conference in Vienna at the end of the month.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 6:41 am on June 6, 2015 Permalink | Reply
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    From ATLAS at CERN’s LHC: “Setting Off To New Energy Horizons” 

    CERN New Masthead

    June 4, 2015
    Andreas Hoecker & Marumi Kado, CERN

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    Display of a proton-proton collision event recorded by ATLAS on 3 June 2015, with the first LHC stable beams at a collision energy of 13 TeV. Tracks reconstructed by the tracking detector are shown as light blue lines, and hits in the layers of the silicon tracking detector are shown as colored filled circles. The four inner layers are part of the silicon pixel detector and the four outer layers are part of the silicon strip detector. The layer closest to the beam, called IBL, is new for Run 2. In the view in the bottom right it is seen that this event has multiple pp collisions. The total number of reconstructed collision vertices is 17 but they are not all resolvable on the scale of this picture..

    After a shutdown of more than two years, Run 2 of the Large Hadron Collider (LHC) is restarting at a centre-of-mass energy of 13 TeV for proton–proton collisions and increased luminosity. This new phase will allow the LHC experiments to explore nature and probe the physical laws governing it at scales never reached before.

    In this first long shutdown, during which the LHC was consolidated, the ATLAS experiment saw a flurry of activity ranging from upgrades and repairs of the detector, its electronics and the trigger system, to a reappraisal of the computing and software used for the data reconstruction and analysis. ATLAS physicists have also used the time without beam to finalise and improve their analyses of the Run-1 data. In spite of the small relative amount of data collected, only 1% of the total dataset expected for the entire LHC programme, the data recorded by ATLAS with collision energies up to 8 TeV have provided a wealth of physics results and led to more than four hundred scientific publications.

    The expectations were high for this unique experimental endeavour represented by the LHC and its ultra-sophisticated particle detectors of which ATLAS is the largest one. The tera (1012) electron-Volt energy scale to which the LHC collisions of high-energetic protons are sensitive was sought to reveal new particles or phenomenon related to the mechanism that gives mass to elementary particles. The most anticipated and acclaimed scenario, and key prediction of the Standard Model, was the Brout-Englert-Higgs mechanism that predicted a spin-zero Higgs boson with a mass in reach of the LHC. Such a boson was discovered in 2012 by the ATLAS and CMS experiments successfully culminating decades of experimental and theoretical effort. This discovery, and a plethora of other important results pushing the frontier of our knowledge, made the LHC Run-1 an astounding success.

    2
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    What’s next? It doesn’t take long, when ambling the corridors along the ATLAS offices at CERN, to realise the suspense that reigns among the physicists running the experiment and preparing the analysis of the first Run-2 collisions. The new data, initially produced at 60% higher collision energy and promising to be several times more abundant than before, have the potential to dramatically extend the results from Run-1. The Higgs boson properties will be measured to much better precision, and new production and decay channels may be observed, further revealing the nature of this particle. Higgs physics will continue to be at the heart of Run-2, but the new data will also allow ATLAS to measure Standard Model processes at unprecedented energies and level of accuracy at hadron colliders, and detect yet unobserved rare processes. High-precision measurements of the masses and couplings of the heaviest known particles, are particularly important as they are indirectly sensitive to new phenomena entering the observed particle reactions through so-called virtual processes.

    These measurements, however, are challenging and will take time to complete. Yet the excitement felt in the ATLAS offices is due to another virtue of the higher collision energy in Run-2: the possibility of directly creating new, heavy particles in the most energetic proton–proton collisions owing to the proportionality relation between energy and mass. There are several reasons to conjecture the existence of such particles. Among these is dark matter, a phenomenon believed to involve physics beyond the Standard Model. Dark matter, if it couples to the known particles, could be produced at the LHC and detected by ATLAS in events with an apparent energy imbalance due to energy taken away by invisible (dark matter) particles. These particles could have any mass and couplings, and we neither know whether the LHC can produce them, nor whether the experiments can detect them, even if they existed. Another motivation for new phenomena beyond the Standard Model lies in an apparent shortcoming of the Higgs mechanism itself. Unlike matter particles, which by virtue of an underlying symmetry appear naturally light with respect to the extremely high energies that are thought to have existed during the earliest moments of the big bang, spin-zero particles such as the Higgs boson in the Standard Model do not have such a protective symmetry. It thus appears unnatural that the Higgs boson is so much lighter than these early energy scales where new phenomena are expected to govern physical laws. A new symmetry, such as the co-called “supersymmetry”, could solve that problem.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Other mechanisms exist; all have in common to introduce new particles of which some may be observable at the LHC, and these new particles could potentially play the role of dark matter as well. ATLAS physicists will therefore mine the new data to deeply and comprehensively search for new physics. The higher collision energy will help to rapidly surpass the sensitivity of the searches conducted during Run-1.

    Ample opportunities but also significant challenges are facing the experimentalists. Critical attention and patience are required for a precise understanding of the new data before drawing conclusions. ATLAS is ready for it.

    See the full article here.

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    Meet CERN in a variety of places:

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New
    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New

    LHC

    CERN LHC New
    CERN LHC Grand Tunnel

    LHC particles

    Quantum Diaries

     
  • richardmitnick 5:03 am on June 2, 2015 Permalink | Reply
    Tags: , CERN ATLAS, , ,   

    From ATLAS at CERN: Fast Forward tp Physics 

    CERN New Masthead

    As ATLAS gears up to record data from proton collisions delivered by the Large Hadron Collider (LHC) at an unprecedented energy level, here are glimpses from the last two years of preparations. For more information about the ATLAS Experiment please visit the official outreach page at http://www.atlas.ch

    Watch, enjoy, learn.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

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    Meet CERN in a variety of places:

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New
    ALICE
    CERN ALICE New

    CMS
    CERN CMS New

    LHCb
    CERN LHCb New

    LHC

    CERN LHC New
    CERN LHC Grand Tunnel

    LHC particles

    Quantum Diaries

     
  • richardmitnick 3:46 pm on May 15, 2015 Permalink | Reply
    Tags: , CERN ATLAS, ,   

    From Physics: “Viewpoint: A More Precise Higgs Boson Mass” 

    Physics LogoAbout Physics

    Physics Logo 2

    Physics

    May 14, 2015
    Chris Quigg, FNAL and ENS

    A new value for the Higgs boson mass will allow stronger tests of the standard model and of theories about the Universe’s stability.

    1
    Figure 1: Values of the top quark and W boson masses measured in experiments (green) and inferred from calculations (blue). The inner and outer ellipses represent 68% and 95% confidence levels, respectively, for the measured and inferred values. Within current experimental and theoretical uncertainties, the two ways of determining the top quark and W boson masses agree. A more precise value of the Higgs mass would narrow the width of the blue ellipses, whereas improved measurements of the top quark and W boson masses would shrink the green ellipses, making for a more incisive test for new physics. (Note, the calculations assume the Higgs mass has a central value of 125.14GeV, which differs insignificantly from the new measurement by ATLAS and CMS, but does not affect the width of the blue ellipses.)

    A great insight of twentieth-century science is that symmetries expressed in the laws of nature need not be manifest in the outcomes of those laws. Consider the snowflake. Its structure is a consequence of electromagnetic interactions, which are identical from any direction, but a snowflake only looks the same when rotated by multiples of 60∘ about a single axis. The full symmetry is hidden by the particular conditions under which the water molecules crystallize. Similarly, a symmetry relates the electromagnetic and weak interactions in the standard model of particle physics, but we know it must be concealed because the weak interactions appear much weaker than electromagnetism.

    2
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    To learn what distinguishes electromagnetism from the weak interactions was an early goal of experiments at CERN’s Large Hadron Collider (LHC).

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

    A big part of the answer was given in mid-2012, when the ATLAS and CMS Collaborations at the LHC announced the discovery of the Higgs boson in the study of proton–proton collisions [1].

    CERN ATLAS New
    ATLAS

    CERN CMS Detector
    CMS

    Now the discovery teams have pooled their data analyses to produce a measurement of the Higgs boson mass with 0.2% precision [2]. The new value enables physicists to make more stringent tests of the electroweak theory and of the Higgs boson’s properties.

    The electroweak theory [3] is a key element of the standard model of particle physics that weaves together ideas and observations from diverse areas of physics [4]. In the theory, interactions are prescribed by gauge symmetries. If nature displayed these symmetries explicitly, the force particles would all be massless, whereas we know experimentally that the weak interactions must—because they are short-ranged—be mediated by massive particles. The so-called Higgs field was introduced to the electroweak theory to hide the gauge symmetry, leading to weak force particles (W± and Z0) that have mass but a photon that is massless.

    The Higgs boson is a spin-zero excitation of the Higgs field and the “footprint” of the mechanism that hides the electroweak gauge symmetry in the standard model. The Higgs boson’s interactions are fully specified in terms of known couplings and masses of its decay products, but the theory does not predict its mass. Instead, experimentalists must measure the energies and momenta of the Higgs boson’s decay products and determine its mass using kinematical equations. Once that mass is known, the rates at which the Higgs boson decays into different particles can be predicted with high precision, and compared with experiment. For a mass in the neighborhood of 125 giga-electron-volts (GeV), the electroweak theory foresees a happy circumstance in which several decay paths occur at large enough rates to be detected.

    ATLAS and CMS are large, broad-acceptance detectors located in multistory caverns about 100 meters below ground [5]. In the discovery run of the LHC, the ATLAS and CMS Collaborations searched for decays of a Higgs boson into bottom-quark–antiquark pairs, tau-lepton pairs, and pairs of electroweak gauge bosons: two photons, W+W−, and Z0Z0. The actual discovery was based primarily on mass peaks associated with either the two-photon final states or Z0Z0 pairs decaying to four-lepton (electrons or muons) final states. These channels, for which the ATLAS and CMS detectors have the best mass resolution, form the basis of their new report.

    Both of the “high-resolution” final states are relatively rare: the standard model predicts that only about 1/4% of Higgs boson decays produce two-photon states; the four-lepton rate is predicted to be nearly 20 times smaller. The two-photon channel exhibits a narrow resonance peak that contains several hundred events per experiment; the Z0Z0 to four-lepton channel yields only a few tens of signal events per experiment. To see these events in the first run of the LHC, the ATLAS and CMS collaborations chose different detector technologies, and therefore different measurement and calibration methods [2]. These differences make pooling the data complicated, but also allow the experimentalists to cross-check systematic uncertainties in their separate measurements. Their combined analyses yield a Higgs boson mass of 125.09±0.24GeV, the precision of which is limited by statistics and by uncertainties in the energy or momentum scale of the ATLAS and CMS detectors.

    The first consequence of the new, precise mass value is sharper predictions, within the standard model, for the relative probabilities of different Higgs boson decay modes and production rates [6]. So far, the measured decay modes and production rates agree with standard-model predictions. The current uncertainties in the measured rates are large, but they will be narrowed in the coming runs at the LHC and at possible future colliders. Evidence of any deviation would suggest that the Higgs boson does not follow the standard model textbook, or that new particles or new forces are implicated in its decays.

    With a precisely known Higgs boson mass MH, theorists can also make more refined predictions of the quantum corrections to many observables, such as the Z0 decay rates. These predictions test the consistency of the electroweak theory as a quantum field theory. Figure 1 illustrates a telling example [7]. The diagonal blue ellipses show the values of the W boson and top quark masses required to reproduce a selection of electroweak observables once MH is fixed. (The narrow and wide ellipses represent 68% and 95% confidence levels, respectively.) The range of masses depends on MH, and the precision with which it is known controls the width of the blue ellipses. The preferred range overlaps the green ellipses, which show the directly measured values of the W boson and top quark masses. In the future, more precise values for the masses of the Higgs boson, W boson, and top quark could unveil a discrepancy that might lead to the discovery of new physics.

    The specific value of MH constrains speculations about physics beyond the standard model, including supersymmetric or composite models. Perhaps most provocative of all is the possibility that the measured value of the mass is special. Quantum corrections influence not just the predictions for observable quantities, but also the shape of the Higgs potential that lies behind electroweak symmetry breaking in the standard model. According to recent analyses, the newly reported value of the Higgs boson mass corresponds to a near-critical situation in which the Higgs vacuum does not lie at the state of lowest energy, but in a metastable state close to a phase transition [8]. This might imply that our Universe is living on borrowed time, or that the electroweak theory must be augmented in some way.

    With LHC Run 2 about to commence, now at higher energies, particle physicists can look forward to a new round of exploration, searches for new phenomena, and refined measurements. Combined analyses and critical evaluations, such as the measurement of the Higgs boson mass discussed here, will help make the most of the data. We still have much to learn about the Higgs boson, the electroweak theory, and beyond.

    Acknowledgments

    Fermilab is operated by Fermi Research Alliance, LLC, under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. I thank the Fondation Meyer pour le développement culturel et artistique for generous support.

    References

    1. G. Aad et al. (ATLAS Collaboration), “Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC,” Phys. Lett. B 716, 1 (2012); S. Chatrchyan et al. (CMS Collaboration), “Observation of a New Boson at a Mass of 125 GeV with the CMS Experiment at the LHC,” 716, 30 (2012)
    2. G. Aad et al. (ATLAS Collaboration†), “Combined Measurement of the Higgs Boson Mass in pp Collisions at s=7 and 8 TeV with the ATLAS and CMS Experiments,” Phys. Rev. Lett. 114, 191803 (2015)
    3. The electroweak theory was developed from a proposal by S. Weinberg, “A Model of Leptons,” Phys. Rev. Lett. 19, 1264 (1967); A. Salam “Weak Electromagnetic Interactions,” in Elementary Particle Theory: Relativistic Groups and Analyticity (Nobel Symposium No. 8), edited by N. Svartholm (Almqvist and Wiksell, Stockholm, 1968), p. 367; http://j.mp/r9dJOo ; The theory is built on the SU(2)L⊗U(1)Y gauge symmetry investigated by S. L. Glashow, “Partial Symmetries of Weak Interactions,” Nucl. Phys. 22, 579 (1961)
    4. C. Quigg, “Electroweak Symmetry Breaking in Historical Perspective,” Ann. Rev. Nucl. Part. Sci.; arXiv:1503.01756
    5. ATLAS Collaboration, “The ATLAS Experiment at the CERN Large Hadron Collider,” JINST 3, S08003 (2008); CMS Collaboration, “The CMS Experiment at the CERN Large Hadron Collider,” 3, S08004 (2008)
    6.S. Heinemeyer et al. (LHC Higgs Cross Section Working Group), Handbook of LHC Higgs Cross Sections: 3. Higgs Properties, Report No. CERN-2013-004; Tables of Higgs boson branching fractions are given at http://j.mp/1OrjQL0
    7. M. Baak et al. (Gfitter Group), “The global electroweak fit at NNLO and prospects for the LHC and ILC,” Eur. Phys. J. C 74, 3046 (2014); a more detailed version of Figure 1 may be found at http://j.mp/1cvuXGQ
    8. D. Buttazzo, G. Degrassi, P. P. Giardino, G. F. Giudice, F. Sala, A. Salvio, and A. Strumia, ”Investigating the Near-Criticality of the Higgs Boson,” J. High Energy Phys. 1312, 089 (2013)

    See the full article here.

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
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