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  • richardmitnick 9:11 am on June 30, 2016 Permalink | Reply
    Tags: , CERN ATLAS, , Muhammad Alhroob   

    From ATLAS at CERN: “A busy day in the life of high energy physicist” Muhammad Alhroob 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLASATLAS

    27th June 2016
    Muhammad Alhroob

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    Muhammad on shift in the ATLAS Control Room. (Image: ATLAS Experiment/CERN)

    Let me start off with a short introduction: my name is Muhammad Alhroob, and I am an international person working at an international organization. I was born and raised in Palestine, which is where I obtained my Bachelor’s degree in Physics. I then travelled to Trieste, Italy to obtain a diploma in theoretical high-energy physics from the Abdus Salam International Center for Theoretical Physics (ICTP). It was named after Abdus Salam, one of the theorists responsible for developing the Standard Model theory of particle physics.

    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 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.

    I carried out my Doctoral studies at Bonn University, Germany, in experimental particle physics with the ATLAS experiment. Currently, I work as a post-doctoral researcher as a member of the ATLAS experiment and I am employed by the University of Oklahoma, USA. I live in France, but my office and work is in Switzerland!

    My work involves analyzing data to try to understand how nature works at the most fundamental level, by searching for new particles and ways in which they interact. Specifically, I am looking at the top quark, which is the heaviest fundamental particle known to exist, with a mass of about 180 times that of a proton.

    The top quark was discovered in 1995 at Fermilab near Chicago, USA.

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    It is predicted by the Standard Model to be produced in two ways: in pairs (a top quark and its antimatter partner) via the strong interaction or singly via the weak interaction. The top quark decays spontaneously to a bottom quark and a W boson, exclusively via the weak interaction. Its heavy mass and very fast decay make it a fantastic for probing the Standard Model.

    I am looking for the top quark when it is produced together with a Z boson. This allows us to measure the strength of the coupling between the top quark and the Z boson, which is a parameter in the Standard Model that has to be determined. This channel allows us to probe for physics beyond the Standard Model.

    This process is also the main background of another extremely important signal: the production of the top quark in association with the Higgs boson (tH). This background has to be measured very precisely and understood. The tH signal will allow us to measure the strength of the coupling between the top quark and the Higgs boson, which is also a free parameter in the Standard Model that needs to be measured. This channel uniquely allows for the structure and nature of the coupling to be studied.

    My work day starts around 9 a.m. as I arrive at CERN and grab a cup of coffee, sometimes with colleagues. Once I am at the office, I read and reply to dozens of emails, edit and debug computer programs, and submit jobs to the Grid. Sometimes my morning involves a chat via Skype or a meeting via video conference. Lunch starts at 12 and can involve physics and detector operation discussion. Therefore, it can sometimes last for as long as 2 hours. My afternoon is usually busy with all kinds of meetings: physics meetings, detector and performance meetings, and ATLAS general meetings. By then it is 5 or 6 p.m. and people have started to leave, giving me the opportunity to stay in the office to concentrate on the job and incorporate the things I learned during the day.

    Afterwards, I need to go home and sleep one or two hours before my night shift in the ATLAS control room starts. I am currently doing shifts to monitor the inner detector of the experiment. It starts at 11 p.m. and ends the next day at 7 a.m. I have as many as two to three night shifts per month. These shifts are extremely important to keep the detector running in good condition and to guarantee the high quality of the collected data.

    When I don’t have a shift, I get a full night’s sleep, hoping for another good day, full of excitement and interesting activities!

    See the full article here .

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  • richardmitnick 6:49 am on May 10, 2016 Permalink | Reply
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    From Cern Atlas: “ATLAS continues to explore the 13 TeV frontier” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    9th May 2016

    ATLAS is back and better than ever! With 13 TeV beams circulating in the Large Hadron Collider, the ATLAS experiment is now recording data for physics. This milestone marks the start of the second year of “Run 2” as ATLAS continues its exploration of 13 TeV energy frontier.

    Anticipation is high for 2016, with the year set to deliver exciting new results for physicists around the world. From precision studies of the Higgs boson to searches for new particles, this year’s data will deepen our understanding of Nature. “We welcome the first 13 TeV collisions of the year with the careful preparation and great expectations of a good friend’s anticipated encounter,” says Alessandro Cerri, ATLAS Run Co-Coordinator. “Together, we are ready for new, exciting explorations!”

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    One of the early collision events with stable beams recorded by ATLAS in 2016. (Image: ATLAS Experiment/CERN)

    Today’s smooth start was thanks to the hard work and dedication of countless ATLAS teams. ATLAS physicists were able to hit the ground running, harnessing last year’s experience running at 13 TeV. “The ATLAS teams have done an incredible job to further improve the performance of the detector and get the systems up and running again in step with the swift start-up of LHC in 2016,” says Alex Oh, ATLAS Run Co-Coordinator. “It’s going to be an exiting year for ATLAS and the other LHC experiments with hopefully great discoveries to be made.”

    “The mission of the data preparation team is to get the best quality data to the physics analysis teams as quickly as possible. We’ve learned from our experience in 2015 and this year we will be faster, with even better data quality,” adds Paul Laycock, ATLAS Data Preparation Coordinator.

    Over the next 6 months of operation with proton beams, the ATLAS experiment will see up to a billion collisions per second. Selecting the most interesting of these collisions is the ATLAS trigger: “It is with great excitement and satisfaction we see the ATLAS trigger system smoothly selecting events for analysis; the many months of preparation and the long nights our experts spent at the control room certainly paid off!” says Anna Sfyrla, ATLAS Trigger Coordinator. “We now need to be patient for more LHC data to come and look into them for the next surprises Nature holds for us.”

    “2015 was like watching the film trailer, there were tantalising glimpses of something amazing happening,” concludes Paul Laycock. “In 2016 we’re looking forward to watching the whole film!”

    See the full article here .

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  • richardmitnick 9:52 am on March 25, 2016 Permalink | Reply
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    From LHC/ATLAS at CERN: “Spring awakening for the ATLAS experiment” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    24th March 2016
    Katarina Anthony

    This morning the Large Hadron Collider (LHC) circulated the first proton-proton beams of 2016 around its 27 kilometre circumference. The beams were met with great enthusiasm in the ATLAS Control Centre as they passed through the ATLAS experiment.

    These beams mark the start of an exciting new period for ATLAS and other CERN experiments. Having seen tantalising but still inconclusive signals in 2015, ATLAS physicists around the world are eagerly awaiting new data to analyse.

    The start of a new run also means the conclusion of a maintenance period, known as the Year-End-Technical-Stop (YETS). This 3 month-long upkeep is vital for the health and well-being of the detectors, ensuring that ATLAS can function impeccably for the 9 straight months of operation that follow.

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    ATLAS uses “beam splash” events to provide simultaneous signals to large parts of the detector, and verify that the readout of different detectors elements are fully synchronized. (Image: ATLAS Experiment © 2016 CERN)

    “This is a normal period of maintenance that happens yearly,” says Michel Raymond, ATLAS Deputy Technical Coordinator. “At ATLAS we use this time to repair and consolidate the detectors first, but also all the infrastructure around that allows us to run the detector.”

    But before their work can begin, there is a lot preparation needed. Although located in an enormous 52,500 m3 cavern, the ATLAS experiment fills that space nearly to the brink. Whatever room is left over is devoted to the cabling and cooling infrastructure that keeps the experiment running. “You cannot just go in and start working on a detector element,” says Raymond. “We first need to move the shielding and cabling to get the experiment into a configuration where the requested detector is accessible.”

    Moving these elements is called “opening” the detector and can take at least 3 weeks. The ATLAS teams have to go slowly and carefully, as they are moving fragile equipment that can weigh anywhere between 100 to 1000 tonnes.

    Once the detector elements are accessible, the teams have only a few weeks to get to work before they need to start closing the detector back up. “Every hour in the cavern is precious,” says Raymond. “We prioritise in advance what operations are the most important, and which can wait for next maintenance period.

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    This display shows one of the ATLAS Experiment’s first splashes on 2016, with beam 1 at 10:26 a.m. on Friday 25th March 2016. (Image: ATLAS Experiment © 2016 CERN)

    During this YETS period, the main priority was the repair of ATLAS’ end-cap magnet bellows. These bellows protect the integrity of the vacuum surrounding ATLAS cooling elements, and are essential for keeping the magnet system cool. They were damaged during a previous maintenance period though continued to work adequately throughout 2015. The damage was successfully repaired during this recent shutdown.

    “After that, we took action on the detector elements, repairing wear-and-tear damage,” says Raymond. “There was a lot of work needed on the muon chambers and the Tile Calorimeters, replacing faulty electronic elements; and a number of gas connections had to be replaced on both sides of the experiment, to avoid leaks.”

    With the work now complete and beams running through the LHC, most of the ATLAS Collaboration has turned their focus to the data. However Michel and his colleagues continue to look forward to their next trip underground. “We’re always planning ahead, thinking about the next shutdown and the ones after that,” concludes Raymond.

    See the full article here .

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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
    No image credit found

    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.

    CERN ATLAS New
    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” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    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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    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.

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    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 New Particle Event

    CERN New Masthead

    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.

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    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

    See the full article here.

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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.

    See the full article here.

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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.

    See the full article here.

    Please help promote STEM in your local schools.

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    Meet CERN in a variety of places:

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    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
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    LHC particles

    Quantum Diaries

     
  • richardmitnick 2:22 pm on July 16, 2015 Permalink | Reply
    Tags: , CERN ATLAS, , , ,   

    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
    CMS

    CERN ATLAS New
    ATLAS

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

    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.

    2
    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.

    2
    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.”

    3
    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.

    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.


     
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