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Showing posts with label CMS. Show all posts
Showing posts with label CMS. Show all posts

Wednesday, December 25, 2013

LHC and Open Access

The CMS experiment at the LHC has released a portion of its data to the public for use in education and outreach. Explore this page to find out more about the data and how to analyse it yourself.


LHC data are exotic, they are complicated and they are big. At peak performance, about one billion proton collisions take place every second inside the CMS detector at the LHC. CMS has collected around 64 petabytes (or over 64,000 terabytes) of analysable data from these collisions so far.

Along with the many published papers, these data constitute the scientific legacy of the CMS Collaboration, and preserving the data for future generations is of paramount importance. “We want to be able to re-analyse our data, even decades from now,” says Kati Lassila-Perini, head of the CMS Data Preservation and Open Access project at the Helsinki Institute of Physics. “We must make sure that we preserve not only the data but also the information on how to use them. To achieve this, we intend to make available through open access our data that are no longer under active analysis. This helps record the basic ingredients needed to guarantee that these data remain usable even when we are no longer working on them.” See: LHC data to be made public via open access initiative

Tuesday, December 13, 2011

Higgs Update Today

Backgrounder:

Guido Tonelli(CMS spokesperson) Higgs update English 1404258

Fabiola Gianotti (ATLAS spokesperson) Higgs update English 1403055


Heuer with Gianotti and Tonelli


See:
See Also:



    Fermilab scientist Don Lincoln describes the concept of how the search for the Higgs boson is accomplished. The latest data is revealed! Several large experimental groups are ht on the trail of this elusive subatomic particle which is thought to explain the origins of particle mass.

    Friday, December 09, 2011

    Tools For Cern Public Annoucement

    CERN PUBLIC SEMINAR

     Tuesday, December 13, 2011 from to (Europe/Zurich)
    at CERN ( Main Auditorium )

    Tuesday, December 13, 2011
    • 14:00 - 14:30 Update on the Standard Model Higgs searches in ATLAS 30'
      Speaker: Fabiola Gianotti
    • 14:30 - 15:00 Update on the Standard Model Higgs searches in CMS 30'
      Speaker: Guido TONELLI
    • 15:00 - 16:00 Joint question session 1h0' 
    Located at Indico Cern Conference

    ***

    See:


    See Also:

    Sunday, November 06, 2011

    LHC trials proton–lead collisions

    Juggling magnetic fields to collide protons and lead

    Physicists at CERN's Large Hadron Collider (LHC) are analysing the results of their first attempt at colliding protons and lead ions. Further attempts at proton–lead collisions are expected over the next few weeks. If these trials are successful, a full-blown experimental programme could run in 2012.

    Since the Geneva lab began experiments with the LHC in 2009, it has mostly been used to send two beams of protons in opposite directions around the 27 km accelerator, with the hope of spotting, among other things, the Higgs boson in the resulting collisions. Two beams of lead ions have also been smashed into each other in order to recreate the hot dense matter, known as a quark–gluon plasma, that was present in the early universe.

    But to fully understand the results of such collisions, physicists need to know the properties of the lead ions before they collide. That is, their "cold state" before vast amounts of heat are released by the collisions. One way to do this, according to Urs Wiedemann at CERN, is to collide protons with lead ions.See:LHC trials proton–lead collisions
     ***
    A lead-ion collision as recorded by the CMS detector at the LHC. © CERN for the benefit of the CMS collaboration.

     The LHC has been smashing lead ions since Sunday, and physicists from the ALICE, ATLAS and CMS experiments are working around the clock to analyze the aftermath of these heavy-ion collisions at record energies and temperatures.* Last week we walked you through the process of creating, accelerating and colliding lead ions. Now we’ll talk about the big question: Why spend one month each year colliding heavy ions in the LHC?See:LHC basics: What we can learn from lead-ion collisions

    ***

    LHC finishes 2011 proton run

    Saturday, November 05, 2011

    Reflections on LHC experiments present latest results at Mumbai conference

    Just following up on ole news to keep abreast of what is going on with CERN.

    LHC experiments present latest results at Mumbai conference

     Geneva, 22 August 2011. Results from the ATLAS and CMS collaborations, presented at the biennial Lepton-Photon conference in Mumbai, India today, show that the elusive Higgs particle, if it exists, is running out of places to hide. Proving or disproving the existence the Higgs boson, which was postulated in the 1960s as part of a mechanism that would confer mass on fundamental particles, is among the main goals of the LHC scientific programme. ATLAS and CMS have excluded the existence of a Higgs over most of the mass region 145 to 466 GeV with 95 percent certainty.

    As well as the Higgs search results, the LHC experiments will be presenting new results across a wide range of physics. Thanks to the outstanding performance of the LHC, the experiments and the Worldwide LHC Computing Grid, some of the current analyses are based on roughly twice the data sample presented at the last major particle physics conference in July.

    ***



    The Latest Word on the Higgs from the Mumbai Conference

     Restructured the post: My preliminary discussion is first, the updates from the talks are now at the end.  The take-away message from the LHC talks: Conversations About Science with Theoretical Physicist Matt Strassler-Posted on

    Tuesday, November 01, 2011

    The Developmental Jet Process

    As a layman I have been going through the research of those better educated then I in order to construct a accurate syntactically written developed scientific process as I have become aware of it. This is what I have been doing for the last number of years so as to get some idea of the scientific process experimentally driven to this point.

    Theoretical development is important to myself,  as well as,   the underlying quest for a foundational perspective of how we can push back perspective with regard to the timeline of the universe in expression.

    This has to be experimentally written in the processes we now use to help formulate an understanding of how the universe came into being by examining local events with the distribution of the cosmological data we are accumulating. A Spherical Cow anyone?


    Jets: Article Updated An update here as well, "Two-Photons: Data and Theory Disagree"

    I do appreciate all those scientist who have been giving their time to educating the public. This is a big thank you for that devotion to the ideal of bringing society forward as to what we as a public are not privy too. As too, being not part of that 3% of the population who are far removed from the work being done in particle research.

    Almost a year ago, I had an e-mail exchange, and planned a phone call, with Maria Spiropulu of CMS. She looked particularly excited about something and the mortals may be learning what the cause was today.

    CMS turned out to be much more "aggressive" relatively to the "conservative" ATLAS detector and it has already provided us with some hints. But what they published today, in the paper called: See:
    CMS: a very large excess of diphotons
    ***

    Measurement of the Production Cross Section for Pairs of Isolated Photons in pp collisions at sqrt(s) = 7 TeV
    The integrated and differential cross sections for the production of pairs of isolated photons is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 36 inverse picobarns is analysed. A next-to-leading-order perturbative QCD calculation is compared to the measurements. A discrepancy is observed for regions of the phase space where the two photons have an azimuthal angle difference, Delta(phi), less than approximately 2.8. 
     ***

    Tscan

    Tscan ("Trivial Scanner") is an event display, traditionally called a scanner, which I developed. It is a program that shows events graphically on the computer screen.

    It was designed to be simple ("trivial") internally, and to have a simple user interface. A lot of importance was given to giving the user a large choice of options to display events in many different ways.

    Tscan proved to be a very useful tool for the development of fitters. A particularly useful feature is the ability to show custom data for every photpmultiplier tube (PMT). Instead of the usual time and charge, it can show expected charge, scattered light, likelihood, chi-squared difference, patches, and any other data that can be prepared in a text format.
    See:Trivial Scanner

    Credit: Super-Kamiokande/Tomasz Barszczak Three (or more?) Cerenkov rings

    Multiple rings of Cerenkov light brighten up this display of an event found in the Super-Kamiokande - neutrino detector in Japan. The pattern of rings - produced when electrically charged particles travel faster through the water in the detector than light does - is similar to the result if a proton had decayed into a positron and a neutral pion. The pion would decay immediately to two gamma-ray photons that would produce fuzzy rings, while the positron would shoot off in the opposite direction to produce a clearer ring. Such kinds of decay have been predicted by "grand unified theories" that link three of nature's fundamental forces - the strong, weak and electromagnetic forces. However, there is so far no evidence for such decays; this event, for example, did not stand up to closer scrutiny.
    See:



    Update

    See Also:



  • 2010 ion run: completed!
  • What Does the Higgs Jet Energy Sound Like?
  • Monday, October 31, 2011

    Gran Sasso and Fermilab

    Gran Sasso

    ***
    deconstruction: soudan mural
    The Soudan mural is next to the 6000-ton MINOS detector. Mural artists: Joseph Giannetti, Leila Giannetti, Mick Pulsifer. Funded by a grant from the University of Minnesota. (Credit: Fermilab Visual Media Services)
    ***
    Fermilab experiment weighs in on neutrino mystery
    Scientists of the MINOS experiment at the Department of Energy’s Fermi National Accelerator Laboratory announced today (June 24) the results from a search for a rare phenomenon, the transformation of muon neutrinos into electron neutrinos. The result is consistent with and significantly constrains a measurement reported 10 days ago by the Japanese T2K experiment, which announced an indication of this type of transformation.

    The results of these two experiments could have implications for our understanding of the role that neutrinos may have played in the evolution of the universe. If muon neutrinos transform into electron neutrinos, neutrinos could be the reason that the big bang produced more matter than antimatter, leading to the universe as it exists today.

    The Main Injector Neutrino Oscillation Search (MINOS) at Fermilab recorded a total of 62 electron neutrino-like events. If muon neutrinos do not transform into electron neutrinos, then MINOS should have seen only 49 events. The experiment should have seen 71 events if neutrinos transform as often as suggested by recent results from the Tokai-to-Kamioka (T2K) experiment in Japan. The two experiments use different methods and analysis techniques to look for this rare transformation.
    To measure the transformation of muon neutrinos into other neutrinos, the MINOS experiment sends a muon neutrino beam 450 miles (735 kilometers) through the earth from the Main Injector accelerator at Fermilab to a 5,000-ton neutrino detector, located half a mile underground in the Soudan Underground Laboratory in northern Minnesota. The experiment uses two almost identical detectors: the detector at Fermilab is used to check the purity of the muon neutrino beam, and the detector at Soudan looks for electron and muon neutrinos. The neutrinos’ trip from Fermilab to Soudan takes about one four hundredths of a second, giving the neutrinos enough time to change their identities.

    For more than a decade, scientists have seen evidence that the three known types of neutrinos can morph into each other. Experiments have found that muon neutrinos disappear, with some of the best measurements provided by the MINOS experiment. Scientists think that a large fraction of these muon neutrinos transform into tau neutrinos, which so far have been very hard to detect, and they suspect that a tiny fraction transform into electron neutrinos.

    The observation of electron neutrino-like events in the detector in Soudan allows MINOS scientists to extract information about a quantity called sin213 (pronounced sine squared two theta one three). If muon neutrinos don’t transform into electron neutrinos, this quantity is zero. The range allowed by the latest MINOS measurement overlaps with but is narrower than the T2K range. MINOS constrains this quantity to a range between 0 and 0.12, improving on results it obtained with smaller data sets in 2009 and 2010. The T2K range for sin213 is between 0.03 and 0.28.
    “MINOS is expected to be more sensitive to the transformation with the amount of data that both experiments have,” said Fermilab physicist Robert Plunkett, co-spokesperson for the MINOS experiment. “It seems that nature has chosen a value for sin213 that likely is in the lower part of the T2K allowed range. More work and more data are really needed to confirm both these measurements.”
    The MINOS measurement is the latest step in a worldwide effort to learn more about neutrinos. MINOS will continue to collect data until February 2012. The T2K experiment was interrupted in March when the severe earth quake in Japan damaged the muon neutrino source for T2K. Scientists expect to resume operations of the experiment at the end of the year. Three nuclear-reactor based neutrino experiments, which use different techniques to measure sin213, are in the process of starting up.
    “Science usually proceeds in small steps rather than sudden, big discoveries, and this certainly has been true for neutrino research,” said Jenny Thomas from University College London, co-spokesperson for the MINOS experiment. “If the transformation from muon neutrinos to electron neutrinos occurs at a large enough rate, future experiments should find out whether nature has given us two light neutrinos and one heavy neutrino, or vice versa. This is really the next big thing in neutrino physics.”
    The MINOS experiment involves more than 140 scientists, engineers, technical specialists and students from 30 institutions, including universities and national laboratories, in five countries: Brazil, Greece, Poland, the United Kingdom and the United States. Funding comes from: the Department of Energy Office of Science and the National Science Foundation in the U.S., the Science and Technology Facilities Council in the U.K; the University of Minnesota in the U.S.; the University of Athens in Greece; and Brazil's Foundation for Research Support of the State of São Paulo (FAPESP) and National Council of Scientific and Technological Development (CNPq).

    Fermilab is a national laboratory supported by the Office of Science of the U.S. Department of Energy, operated under contract by Fermi Research Alliance, LLC.
    For more information about MINOS and related experiments, visit the Fermilab neutrino website: http://www.fnal.gov/pub/science/experiments/intensity/

    See: 

    Intensity Frontier


    See Also: The Reference Frame: CMS: a very large excess of diphotons

    Saturday, October 22, 2011

    CMS Physics Results

    Link on Title.

    • All CMS public results can be found in CDS , and are categorized by subject (group) in this page.
    • Publications and preprints on collision data, ordered by time, are available at this link.
    • Publications on cosmic-ray data can be found here; the paper on muon charge ratio is available here .
    • The complete list of publications is here.
    • Preliminary results on collision data at 0.9, 2.36 and 7 TeV are described in Physics Analysis Summaries; Monte Carlo studies can be found here.
    • Public performance plots are shown in Detector Performance Summaries.

    See Also:CMS Physics Analysis Summaries

    Monday, July 11, 2011

    ALICE Enters New Territory

    A computer screen in the ALICE control room shows an event display on the night of the first heavy-ion collisions in the LHC in November 2010.
    A basic process in QCD is the energy loss of a fast parton in a medium composed of colour charges. This phenomenon, "jet quenching", is especially useful in the study of the QGP, using the naturally occurring products (jets) of the hard scattering of quarks and gluons from the incoming nuclei. A highly energetic parton (a colour charge) probes the coloured medium rather like an X-ray probes ordinary matter. The production of these partonic probes in hadronic collisions is well understood within perturbative QCD. The theory also shows that a parton traversing the medium will lose a fraction of its energy in emitting many soft (low energy) gluons. The amount of the radiated energy is proportional to the density of the medium and to the square of the path length travelled by the parton in the medium. Theory also predicts that the energy loss depends on the flavour of the parton.

    Jet quenching was first observed at RHIC by measuring the yields of hadrons with high transverse momentum (pT). These particles are produced via fragmentation of energetic partons. The yields of these high-pT particles in central nucleus–nucleus collisions were found to be a factor of five lower than expected from the measurements in proton–proton reactions. ALICE has recently published the measurement of charged particles in central heavy-ion collisions at the LHC. As at RHIC, the production of high-pT hadrons at the LHC is strongly suppressed. However, the observations at the LHC show qualitatively new features (see box). The observation from ALICE is consistent with reports from the ATLAS and CMS collaborations on direct evidence for parton energy loss within heavy-ion collisions using fully reconstructed back-to-back jets of particles associated with hard parton scatterings (CERN Courier January/February 2011 p6 and March 2011 p6). The latter two experiments have shown a strong energy imbalance between the jet and its recoiling partner (G Aad et al. 2010 and CMS collaboration 2011). This imbalance is thought to arise because one of the jets traversed the hot and dense matter, transferring a substantial fraction of its energy to the medium in a way that is not recovered by the reconstruction of the jets.See: ALICE enters new territory in heavy-ion collisions

    Click no Image for larger viewing

    Wednesday, March 09, 2011

    Are There Extra Dimensions of Space?

    Are there Extra Dimensions of Space?

    A QGP is formed at the collision point of two relativistically accelerated gold ions in the center of the STAR detector at the relativistic heavy ion collider at the Brookhaven national laboratory.

    Some of these issues in relation to the LHC are what I tried to explain to Searosa.


    Brookhaven National Laboratory

    HOT A computer rendition of 4-trillion-degree Celsius quark-gluon plasma created in a demonstration of what scientists suspect shaped cosmic history.

    Here's what has to be considered. There is a calculated energy value to the collision process. You add that up as all the constituents of that process, and what's left is,  so much energy left to be discerned as particulate expressions as beyond that collision point. This may not be truly an accurate portrayal yet it is one that allows perspective to consider the spaces at such microscopic levels for consideration.

    The perspective of valuations with regard to the LHC is whether or not there is sufficient energy within the confines of LHC experiments in which to satisfy the questions about extra those dimensions. It seems the parameters of those decisions seem to be sufficient?


    Author(s)
    Alex Buche-University of Western Ontario / Perimeter Institute

    Robert Myers-Perimeter Institute
    Aninda Sinha-Perimeter Institute

     

    It is believed that in the first few microseconds after the Big Bang, our universe was dominated by a strongly interacting phase of nuclear matter at extreme temperatures. An impressive experimental program at the Brookhaven National Laboratory on Long Island has been studying the properties of this nuclear plasma with some rather surprising results. We outline how there may be a deep connection between extra-dimensional gravity of String Theory and the fundamental theories of subatomic particles can solve the mystery of the near-ideal fluid properties of the strongly coupled nuclear plasma.

    The QGP has directed attention to a method of expression with regard to that collision point.


    First direct observation of jet quenching.

     

    At the recent seminar, the LHC’s dedicated heavy-ion experiment, ALICE, confirmed that QGP behaves like an ideal liquid, a phenomenon earlier observed at the US Brookhaven Laboratory’s RHIC facility. This question was indeed one of the main points of this first phase of data analysis, which also included the analysis of secondary particles produced in the lead-lead collisions. ALICE's results already rule out many of the existing theoretical models describing the physics of heavy-ions.

    See: 2010 ion run: completed!


    The equations of string theory specify the arrangement of the manifold configuration, along with their associated branes (green) and lines of force known as flux lines (orange). The physics that is observed in the three large dimensions depends on the size and the structure of the manifold: how many doughnut-like "handles" it has, the length and circumference of each handle, the number and locations of its branes, and the number of flux lines wrapped around each doughnut.

    Early on looking at spaces, it was a struggle for me to understand how extra dimensions would be explained. It was easy using a coordinated frame of reference as x,y,z, yet,  how much did you have to go toward seeing that rotation around each of those arrows of direction would add greater depth of perception about such spaces?

    It's easier if you just draw the picture.

    A section of the quintic Calabi–Yau three-fold (3D projection)

    In superstring theory the extra dimensions of spacetime are sometimes conjectured to take the form of a 6-dimensional Calabi–Yau manifold, which led to the idea of mirror symmetry.

     

    The benefit of phenomenological approaches in experimental processes to attempt to answer these theoretical points of views.

     

    The first results on supersymmetry from the Large Hadron Collider (LHC) have been analysed by physicists and some are suggesting that the theory may be in trouble. Data from proton collisions in both the Compact Muon Solenoid (CMS) and ATLAS experiments have shown no evidence for supersymmetric particles – or sparticles – that are predicted by this extension to the Standard Model of particle physics. Will the LHC find supersymmetry Kate McAlpine ?


    Thank you Tommaso Dorigo

     

    Also see:

     

    Beautiful theory collides with smashing particle data."

    Implications of Initial LHC Searches for Supersymmetry"

    More SUSY limits"

    Tuesday, February 22, 2011

    Keeping it Real


    The first results on supersymmetry from the Large Hadron Collider (LHC) have been analysed by physicists and some are suggesting that the theory may be in trouble. Data from proton collisions in both the Compact Muon Solenoid (CMS) and ATLAS experiments have shown no evidence for supersymmetric particles – or sparticles – that are predicted by this extension to the Standard Model of particle physics. Will the LHC find supersymmetry Kate McAlpine ?

    Thank you Tommaso Dorigo

    If such propositions are ever moved to the project of LHC confirmations then the ideals of those who proposed should never be conceived as rats as a commentator writes. It's just not polite.

    I would comment at your blog article but like Cosmic Variance I have been blocked. Oh well:)

    This information is a form of responsible action toward experimental fundamentalism we take as one moves forward.

    Monday, December 13, 2010

    2010 ion run: completed!

    First direct observation of jet quenching.

    At the recent seminar, the LHC’s dedicated heavy-ion experiment, ALICE, confirmed that QGP behaves like an ideal liquid, a phenomenon earlier observed at the US Brookhaven Laboratory’s RHIC facility. This question was indeed one of the main points of this first phase of data analysis, which also included the analysis of secondary particles produced in the lead-lead collisions. ALICE's results already rule out many of the existing theoretical models describing the physics of heavy-ions.
    See: 2010 ion run: completed!

    ***

    After a very fast switchover from protons to lead ions, the LHC has achieved performances that allowed the machine to exceed both peak and integrated luminosity by a factor of three. Thanks to this, experiments have been able to produce high-profile results on ion physics almost immediately, confirming that the LHC was able to keep its promises for ions as well as for protons.

    A seminar on 2 December was the opportunity for the ALICE, ATLAS and CMS collaborations to present their first results on ion physics in front of a packed auditorium. These results are important and are already having a major impact on the understanding of the physics processes that involve the basic constituents of matter at high energies.

    In the ion-ion collisions, the temperature is so high that partons (quarks and gluons), which are usually constrained inside the nucleons, are deconfined to form a highly dense and hot soup known as quark-gluon plasma (QGP). This type of matter existed about 1 millionth of a second after the Big Bang. By studying it, scientists hope to understand the processes that led to the formation of nucleons, which in turn became the nuclei of atoms. See:LHC completes first heavy-ion run

    See Also: Jets: Article Updated

    Sunday, December 12, 2010

    The Compact Muon Solenoid......

    Coordinates: 46°18′34″N 6°4′37″E / 46.30944°N 6.07694°E / 46.30944; 6.07694
    Large Hadron Collider (LHC)
    LHC.svg
    LHC experiments
    ATLAS A Toroidal LHC Apparatus
    CMS Compact Muon Solenoid
    LHCb LHC-beauty
    ALICE A Large Ion Collider Experiment
    TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation
    LHCf LHC-forward
    MoEDAL Monopole and Exotics Detector At the LHC
    LHC preaccelerators
    p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)
    (not marked) Proton Synchrotron Booster
    PS Proton Synchrotron
    SPS Super Proton Synchrotron

    View of the CMS endcap through the barrel sections. The ladder to the lower right gives an impression of scale.
    ......(CMS) experiment is one of two large general-purpose particle physics detectors built on the proton-proton Large Hadron Collider (LHC) at CERN in Switzerland and France. Approximately 3,600 people from 183 scientific institutes, representing 38 countries form the CMS collaboration who built and now operate the detector.[1] It is located in an underground cavern at Cessy in France, just across the border from Geneva.

    Contents

    Background

    Recent collider experiments such as the now-dismantled Large Electron-Positron Collider at CERN and the (as of 2010) still running Tevatron at Fermilab have provided remarkable insights into, and precision tests of the Standard Model of Particle Physics. However, a number of questions remain unanswered.

    A principal concern is the lack of any direct evidence for the Higgs Boson, the particle resulting from the Higgs mechanism which provides an explanation for the masses of elementary particles. Other questions include uncertainties in the mathematical behaviour of the Standard Model at high energies, the lack of any particle physics explanation for dark matter and the reasons for the imbalance of matter and antimatter observed in the Universe.

    The Large Hadron Collider and the associated experiments are designed to address a number of these questions.

    Physics goals

    The main goals of the experiment are:
    The ATLAS experiment, at the other side of the LHC ring is designed with similar goals in mind, and the two experiments are designed to complement each other both to extend reach and to provide corroboration of findings.

    Detector summary

    CMS is designed as a general-purpose detector, capable of studying many aspects of proton collisions at 14 TeV, the center-of-mass energy of the LHC particle accelerator. It contains subsystems which are designed to measure the energy and momentum of photons, electrons, muons, and other products of the collisions. The innermost layer is a silicon-based tracker. Surrounding it is a scintillating crystal electromagnetic calorimeter, which is itself surrounded with a sampling calorimeter for hadrons. The tracker and the calorimetry are compact enough to fit inside the CMS solenoid which generates a powerful magnetic field of 3.8 T. Outside the magnet are the large muon detectors, which are inside the return yoke of the magnet.




    The set up of the CMS. In the middle, under the so-called barrel there is a man for scale. (HCAL=hadron calorimeter, ECAL=electromagnetic calorimeter)

    CMS by layers


    A slice of the CMS detector.
    For full technical details about the CMS detector, please see the Technical Design Report.

    The interaction point

    This is the point in the centre of the detector at which proton-proton collisions occur between the two counter-rotating beams of the LHC. At each end of the detector magnets focus the beams into the interaction point. At collision each beam has a radius of 17 μm and the crossing angle between the beams is 285 μrad.
    At full design luminosity each of the two LHC beams will contain 2,808 bunches of 1.15×1011 protons. The interval between crossings is 25 ns, although the number of collisions per second is only 31.6 million due to gaps in the beam as injector magnets are activated and deactivated.

    At full luminosity each collision will produce an average of 20 proton-proton interactions. The collisions occur at a centre of mass energy of 14 TeV. It is worth noting that the actual interactions occur between quarks rather than protons, and so the actual energy involved in each collision will be lower, as determined by the parton distribution functions.

    The first which ran in September 2008 was expected to operate at a lower collision energy of 10 TeV but this was prevented by the 19 September 2008 shutdown. When at this target level, the LHC will have a significantly reduced luminosity, due to both fewer proton bunches in each beam and fewer protons per bunch. The reduced bunch frequency does allow the crossing angle to be reduced to zero however, as bunches are far enough spaced to prevent secondary collisions in the experimental beampipe.

    Layer 1 – The tracker


    The silicon strip tracker of CMS.
    Immediately around the interaction point the inner tracker serves to identify the tracks of individual particles and match them to the vertices from which they originated. The curvature of charged particle tracks in the magnetic field allows their charge and momentum to be measured.

    The CMS silicon tracker consists of 13 layers in the central region and 14 layers in the endcaps. The innermost three layers (up to 11 cm radius) consist of 100×150 μm pixels, 66 million in total.
    The next four layers (up to 55 cm radius) consist of 10 cm × 180 μm silicon strips, followed by the remaining six layers of 25 cm × 180 μm strips, out to a radius of 1.1 m. There are 9.6 million strip channels in total.
    During full luminosity collisions the occupancy of the pixel layers per event is expected to be 0.1%, and 1–2% in the strip layers. The expected SLHC upgrade will increase the number of interactions to the point where over-occupancy may significantly reduce trackfinding effectiveness.

    This part of the detector is the world's largest silicon detector. It has 205 m2 of silicon sensors (approximately the area of a tennis court) comprising 76 million channels.[2]

    Layer 2 – The Electromagnetic Calorimeter

    The Electromagnetic Calorimeter (ECAL) is designed to measure with high accuracy the energies of electrons and photons.

    The ECAL is constructed from crystals of lead tungstate, PbWO4. This is an extremely dense but optically clear material, ideal for stopping high energy particles. It has a radiation length of χ0 = 0.89 cm, and has a rapid light yield, with 80% of light yield within one crossing time (25 ns). This is balanced however by a relatively low light yield of 30 photons per MeV of incident energy.

    The crystals used have a front size of 22 mm × 22 mm and a depth of 230 mm. They are set in a matrix of carbon fibre to keep them optically isolated, and backed by silicon avalanche photodiodes for readout. The barrel region consists of 61,200 crystals, with a further 7,324 in each of the endcaps.

    At the endcaps the ECAL inner surface is covered by the preshower subdetector, consisting of two layers of lead interleaved with two layers of silicon strip detectors. Its purpose is to aid in pion-photon discrimination.

    Layer 3 – The Hadronic Calorimeter


    Half of the Hadron Calorimeter
    The purpose of the Hadronic Calorimeter (HCAL) is both to measure the energy of individual hadrons produced in each event, and to be as near to hermetic around the interaction region as possible to allow events with missing energy to be identified.

    The HCAL consists of layers of dense material (brass or steel) interleaved with tiles of plastic scintillators, read out via wavelength-shifting fibres by hybrid photodiodes. This combination was determined to allow the maximum amount of absorbing material inside of the magnet coil.

    The high pseudorapidity region (3.0 < | η | < 5.0) is instrumented by the Hadronic Forward detector. Located 11 m either side of the interaction point, this uses a slightly different technology of steel absorbers and quartz fibres for readout, designed to allow better separation of particles in the congested forward region.
    The brass used in the endcaps of the HCAL used to be Russian artillery shells.[3]

    Layer 4 – The magnet

    Like most particle physics detectors, CMS has a large solenoid magnet. This allows the charge/mass ratio of particles to be determined from the curved track that they follow in the magnetic field. It is 13 m long and 6 m in diameter, and its refrigerated superconducting niobium-titanium coils were originally intended to produce a 4 T magnetic field. It was recently announced that the magnet will run at 3.8 T instead of the full design strength in order to maximize longevity.[4]

    The inductance of the magnet is 14 Η and the nominal current for 4 T is 19,500 A, giving a total stored energy of 2.66 GJ, equivalent to about half-a-tonne of TNT. There are dump circuits to safely dissipate this energy should the magnet quench. The circuit resistance (essentially just the cables from the power converter to the cryostat) has a value of 0.1 mΩ which leads to a circuit time constant of nearly 39 hours. This is the longest time constant of any circuit at CERN. The operating current for 3.8 T is 18,160 A, giving a stored energy of 2.3 GJ.

    Layer 5 – The muon detectors and return yoke

    To identify muons and measure their momenta, CMS uses three types of detector: drift tubes (DT), cathode strip chambers (CSC) and resistive plate chambers (RPC). The DTs are used for precise trajectory measurements in the central barrel region, while the CSCs are used in the end caps. The RPCs provide a fast signal when a muon passes through the muon detector, and are installed in both the barrel and the end caps.

    Collecting and collating the data

    Pattern recognition


    Testing the data read-out electronics for the tracker.
    New particles discovered in CMS will be typically unstable and rapidly transform into a cascade of lighter, more stable and better understood particles. Particles travelling through CMS leave behind characteristic patterns, or ‘signatures’, in the different layers, allowing them to be identified. The presence (or not) of any new particles can then be inferred.

    Trigger system

    To have a good chance of producing a rare particle, such as a Higgs boson, a very large number of collisions are required. Most collision events in the detector are "soft" and do not produce interesting effects. The amount of raw data from each crossing is approximately 1 MB, which at the 40 MHz crossing rate would result in 40 TB of data a second, an amount that the experiment cannot hope to store or even process properly. The trigger system reduces the rate of interesting events down to a manageable 100 per second.
    To accomplish this, a series of "trigger" stages are employed. All the data from each crossing is held in buffers within the detector while a small amount of key information is used to perform a fast, approximate calculation to identify features of interest such as high energy jets, muons or missing energy. This "Level 1" calculation is completed in around 1 µs, and event rate is reduced by a factor of about thousand down to 50 kHz. All these calculations are done on fast, custom hardware using reprogrammable FPGAs.

    If an event is passed by the Level 1 trigger all the data still buffered in the detector is sent over fibre-optic links to the "High Level" trigger, which is software (mainly written in C++) running on ordinary computer servers. The lower event rate in the High Level trigger allows time for much more detailed analysis of the event to be done than in the Level 1 trigger. The High Level trigger reduces the event rate by a further factor of about a thousand down to around 100 events per second. These are then stored on tape for future analysis.

    Data analysis

    Data that has passed the triggering stages and been stored on tape is duplicated using the Grid to additional sites around the world for easier access and redundancy. Physicists are then able to use the Grid to access and run their analyses on the data.
    Some possible analyses might be:
    • Looking at events with large amounts of apparently missing energy, which implies the presence of particles that have passed through the detector without leaving a signature, such as neutrinos.
    • Looking at the kinematics of pairs of particles produced by the decay of a parent, such as the Z boson decaying to a pair of electrons or the Higgs boson decaying to a pair of tau leptons or photons, to determine the properties and mass of the parent.
    • Looking at jets of particles to study the way the quarks in the collided protons have interacted.

    Milestones

    1998 Construction of surface buildings for CMS begins.
    2000 LEP shut down, construction of cavern begins.
    2004 Cavern completed.
    10 September 2008 First beam in CMS.
    23 November 2009 First collisions in CMS.
    30 March 2010 First 7 TeV collisions in CMS.

    See also


    References

    1. ^ [1]
    2. ^ CMS installs the world's largest silicon detector, CERN Courier, Feb 15, 2008
    3. ^ CMS HCAL history - CERN
    4. ^ http://iopscience.iop.org/1748-0221/5/03/T03021/pdf/1748-0221_5_03_T03021.pdf Precise mapping of the magnetic field in the CMS barrel yoke using cosmic rays

    External links