The ATLAS detector at the Large Hadron Collider, CERN has been under
construction for more than a decade. It is now largely complete and functional.
This paper will describe the state of the major subsystems of ATLAS. Results
from the brief single beam running period in 2008 will be shown. In addition,
results from a long period of cosmic ray running will be shown. These results
show that ATLAS is prepared to make major new physics discoveries as soon as we
get colliding beams in late 2009.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July
  2009, eConf C09072

Shank, James T.

English

arXiv

The ATLAS detector at the Large Hadron Collider, CERN has been under construction for more than a decade. It is now largely complete and functional. This paper will describe the state of the major subsystems of ATLAS. Results from the brief single beam running period in 2008 will be shown. In addition, results from a long period of cosmic ray running will be shown. These results show that ATLAS is prepared to make major new physics discoveries as soon as we get colliding beams in late 2009.The ATLAS detector at the Large Hadron Collider, CERN has been under construction for more than a decade. It is now largely complete and functional. This paper will describe the state of the major subsystems of ATLAS. Results from the brief single beam running period in 2008 will be shown. In addition, results from a long period of cosmic ray running will be shown. These results show that ATLAS is prepared to make major new physics discoveries as soon as we get colliding beams in late 2009

CERN Document Server

The ATLAS Detector: Status and Results from Cosmic Rays

The ATLAS detector at the Large Hadron Collider, CERN has been under construction for more than a decade. It is now largely complete and functional. This paper will describe the state of the major subsystems of ATLAS. Results from the brief single beam running period in 2008 will be shown. In addition, results from a long period of cosmic ray running will be shown. These results show that ATLAS is prepared to make major new physics discoveries as soon as we get colliding beams in late 2009. 1

James T. Shank

CiteSeerX

The ATLAS detector: Status and results from cosmic rays

The ATLAS detector at the Large Hadron Collider, CERN has been under construction for more than a decade. It is now largely complete and functional. This paper will describe the state of the major subsystems of ATLAS. Results from the brief single beam running period in 2008 will be shown. In addition, results from a long period of cosmic ray running will be shown. These results show that ATLAS is prepared to make major new physics discoveries as soon as we get colliding beams in late 2009

Shank, J

ATL-GEN-PROC-2009-01319October2009Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 1The ATLAS Detector: Status and Results from Cosmic RaysJames T. ShankPhysics Department , Boston University, Boston MA 02215, USAOn Behalf of the ATLAS CollaborationThe ATLAS detector at the Large Hadron Collider, CERN has been under construction for more than a decade.It is now largely complete and functional. This paper will describe the state of the major subsystems of ATLAS.Results from the brief single beam running period in 2008 will be shown. In addition, results from a long periodof cosmic ray running will be shown. These results show that ATLAS is prepared to make major new physicsdiscoveries as soon as we get colliding beams in late 2009.1. IntroductionThe ATLAS detector at CERN is one of two largegeneral purpose detectors at the Large Hadron Col-lider (LHC) [1], a 7 TeV on 7 TeV proton accelerator.It was designed to have excellent tracking, calorimetryand muon spectroscopy over the entire energy range ofthe LHC allowing discovery of any new physics in thatrange. Particular discovery potentials include detailsof the Standard Model and beyond, such as discoveryof the Higgs boson, Supersymmetry (SUSY) and extradimensions. More complete discussion of the physicsdiscovery potential of ATLAS is given in reference [2].The ATLAS detector, under construction for wellover a decade, was ready to record collision events onSeptember 10, 2008 when the LHC started up. Sin-gle beams were circulated around the accelerator andresulting events were recorded in the detector. As iswell known, about one week later, an incident in asuperconducting splice between two dipoles resultedin a shutdown of the machine before any beam colli-sions occurred. This shutdown is still continuing andATLAS has used this time to fix minor problems withthe detector and commission the subsystems with cos-mic ray events. The LHC accelerator is now scheduledto start again in November, 2009. This paper will de-scribe the current state of each subsystem and presentresults of these cosmic ray events showing that ATLASnow fully ready for collision data that is expected laterthis year.2. Overview of the ATLAS DetectorThe ATLAS detector is large in more than one way:the outer dimensions (a cylinder 44m long and 25min diameter) and the collaboration: over 2500 scien-tists from nearly 200 institutes in 37 countries. Anoverview schematic of the detector is shown in Fig. 1where the major subsystems are labelled.The outermost part of ATLAS is the muon spec-trometer and its associated superconducting toroidalmagnets which give ATLAS its name: A ToroidalLHC Apparatus. The key design in the muon spec-trometer is minimal material provided by having air-core toroidal magnets. Next, moving inward is thecalorimeter system consisting of three different tech-nologies. The hadronic calorimeter is a scintillatingtile/iron structure covering η < 1.7. In the endcapregion 1.5 < η < 3.2 hadronic calorimetry is providedby a Copper/Liquid Argon structure providing fourlongitudinal samples. Forward hadronic calorimetryis provided by a Tungsten/Liquid Argon structurecovering 3.2 < η < 4.9 with three longitudinal sam-ples. Electromagnetic calorimetry is provided by aLead/Liquid Argon detector η < 2.5 with three longi-tudinal samples together with a presampler detectorin the η < 1.8 region. Inside of the calorimeter isthe inner detector consisting of a transition radiationtracker (TRT), semiconductor tracker, and a pixel de-tector shown in Fig. 2. The magnetic field for theinner detector is provided by a solenoid magnet pro-viding a 2 Tesla field. The TRT provides e− pi sepa-ration over the energy range 0.5 < E < 150 GeV. Thepixel readout has 80 million channels, another one ofthe superlatives in ATLAS.Complete details of the ATLAS detector are givenin reference [3]3. Single Beam EventsDuring the brief single beam running in September,2008, ATLAS recorded many events from beam halo(interactions with gas in the beam pipe) and so calledbeam-splash events produced when the beam was runinto a collimator approximately 150m upstream of thedetector. The beam splash events provided ratherspectacular event display pictures, such as shown inFig. 3, but also allowed time synchronization betweenthe subsystems and the LHC machine.4. Cosmic ray running in 2008-9Since the end of the beam in Sept. 2008, ATLAShas been accumulating cosmic ray events in all subsys-tems of the detector. Fig. 4 shows the running periods2 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009Figure 1: A schematic view of the ATLAS detector.Figure 2: A schematic view of the ATLAS inner detector.and the state of the magnets during each run. Over200 million events have been recorded.The following sections will review the status of eachATLAS subsystem and show results from these cosmicray runs.5. The Inner DetectorThe ATLAS Inner Detector has three major detec-tor components: the pixel detector, the silicon de-tector and the transition radiation tracker as shownabove in Fig. 2. The pixel detector currently hasapproximately 98% live channels, a hit efficiency ofFigure 3: ATLAS event display for a ”splash” event.≈ 99.8% and a noise occupancy of ≈ 10−10 The Semi-conductor Tracker has over 99% of its 6 million chan-nels operational. The noise occupancy is 1.5 × 10−5in the barrel region and 3.0 × 10−5 in the endcap.The Transition Radiation Tracker (TRT) has 98% ofits 350k channels operational. The transition radia-tion aspects of the TRT have been proven in testbeamstudies where the full turn on of transition radiation isseen and in the cosmic ray runs where the beginningof the turn on is seen. This is shown in Fig. 5 wherethe probability of a signal above the high thresholdis shown as a function of the Lorentz gamma fac-tor of the cosmic muon. Transition radiation photonsProceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 3produced by particles with high gamma-factor causea larger energy deposition in the straw tubes of theTRT.Figure 4: Cosmic ray running in 2008, since the initialturn-on.Figure 5: Muon identification in the Transition RadiationTracker.6. The Calorimeter SystemThe ATLAS Liquid Argon (LAr) calorimeter sys-tems, described above and shown in more detail inFig. 6, has 98.8% of channels operational. TheHadronic Tile Calorimeter has 0.4% dead channels,which will be fixed at the next shutdown. The elec-tronic calibration systems for all the calorimeter sys-tems are fully operational.An example of studies with cosmic rays is shown inFig. 7, where the response of the LAr barrel electro-magnetic calorimeter is shown as a function of pseu-dorapidity. Here the response is the most probablevalue (MPV) determined by two different algorithms.The LArMuID algorithm is a variable size algorithm- only cells above a given threshold are added to thecluster. The 3x3 cluster algorithm is fixed in size.Figure 6: The ATLAS calorimeter system.Also shown are the true cluster response from MonteCarlo simulation and the cell depth. The data matchthe simulation and follow the cell depth as expected.The uniformity of response agrees with the simulationat the 2% level.Figure 7: Signal response to cosmic rays in the LiquidArgon Electromagnetic Calorimeter. The plot shows themost probable energy normalized to the η point between0.3 and 0.47. The Muon SpectrometerThe Muon Spectrometer in ATLAS has two differ-ent technologies for the precision measurement andtwo for the trigger as shown in Fig. 8. The MonitoredDrift Tube (MDT) system together with the CathodeStrip Chambers (CSC) in the forward region providea spatial resolution of 35 − 40 µm. The MDT tubesare ”monitored” by a 12232 channel optical alignmentsystem which provides position accuracy of 30 µm.The MDT system has 0.3% dead channels. The CSCsystem has 1.5% dead channels.4 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009The muon trigger is provided by Resistive PlateChambers (RPC) in the barrel and Thin Gap Cham-bers (TGC) in the endcap. These provide a spatialresolution of 5 − 10mm and a time resolution suffi-cient to allow bunch crossing identification with the 25ns bunch crossing time of the LHC. The RPC systemhas 95.5% channels operational with an additional 3%recoverable during a shutdown. The TGC system is99.8% operational with less than 0.02% noisy chan-nels.Figure 8: The ATLAS Muon Spectrometer.Overall, the muon system provides a stand alone(that is, not combined with inner detector measure-ments) resolution of ∆PT /PT < 10% up to 1 TeV.A typical cosmic ray muon seen in the spectrometeris shown in Fig. 9. As can been seen by the trackbending, the magnetic field was on for this event.Figure 9: The ATLAS event display showing a cosmic raymuon passing through the entire detector. Only the muonchambers with hits are shown for visibility.Effects of the muon alignment system can be seen inFig. 10 where the sagitta measured in the bend planeof the magnet is shown from tracks that go throughone particular chamber in the muon barrel system.The top plot in that figure shows the sagitta with justthe nominal geometry for the chambers that the trackpasses through. The middle plot shows the sagitta af-ter corrections given by the optical alignment system.The bottom plot shows the final distribution obtainedafter applying corrections from track-based alignment.Figure 10: Track residuals in the Muon Spectrometershowing the effect of alignment. a) nominal geometry withno alignment. b) corrections from the optical alignmentsystem applied. c) constraints from track-based alignmentapplied.8. Electrons in Cosmic Ray DataFor this analysis, 3.5 million cosmic ray eventswhich passed the high-level trigger track reconstruc-tion in the barrel inner detector were used. Afterfiltering for electromagnetic cluster candidates withtransverse energy above ∼3 GeV and with a loosetrack match in φ (pointing downwards), about 11 000candidates remain. These remaining events are re-quired to satisfy medium electron cuts (lateral showerProceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 5shapes in first and second layers of EM calorime-ter and track- cluster matching in φ for tracks withat least 25 TRT hits) and are then split into twocategories: a) a sample consisting of 1229 muonbremsstrahlung candidates, with only one track re-constructed in the barrel inner detector b) a sampleconsisting of 85 ionisation electron candidates, withat least two tracks reconstructed in the barrel innerdetectorFigure 11: Ratio of high to low-threshold TRT hits versusE/p, where E is measured in the EM calorimeter and p inthe inner detector, for bremsstrahlung candidates.In Fig. 11, the red boxes correspond to candidatessatisfying additional cuts defined for standard tighthigh−pT electron identification in ATLAS at η ∼ 0.These cuts are pT and η-dependent and the ones ap-plied to most of the events are illustrated by thedashed red lines in figures 11 and 12: 0.8 < E/p < 2.5and high to low-threshold TRT hit ratio > 0.08 (indi-cating the detection of transition radiation producedonly by relativistic particles). Most of the events inthe muon bremsstrahlung sample have small E/p andfew high-threshold TRT hits (only muons above 100GeV momentum are expected to produce transitionradiation). The events with low E/p and high TRTratio contain a large fraction of muons with combinedmomentum measured in the muon spectrometer andID above 100 GeV. Only 19 of the 1229 events satisfythe signal criteria. In contrast, in the electron ioni-sation sample, a large fraction of events, 36 out of atotal of 85, satisfy the signal criteria. These eventsare interpreted as high- energy δ-rays produced in theinner detector volume by the incoming cosmic muons.In Fig. 13, the red curve is the projection of a twodimensional binned maximum likelihood fit to the twodimensional plot in Fig. 12, excluding the signal can-didates. The shape of the projected distributions isobtained from the muon bremsstrahlung sample, butthe parameters are fitted using the ionisation sam-ple. This fit is used as one of the methods to esti-mate the background contamination to the signal. Aclear excess of events with a large ratio of high to low-Figure 12: Ratio of high to low-threshold TRT hits versusE/p, where E is measured in the EM calorimeter and p inthe inner detector, for ionization electron candidates.Figure 13: Distribution of the ratio of the high to low-threshold TRT hits for the 85 ionisation electron candi-dates (black) and for the 49 candidates failing the finalsignal criteria (blue).threshold TRT hits is observed, indicating the pres-ence of an electron signal.Figure 14, shows the Distribution of energy to mo-mentum ratio for the electrons candidates. A clearaccumulation of signal events around E/p = 1 is ob-served, as expected for electrons.A second independent method to estimate thebackground uses the measured ratio of negativelyto positively charged muons coupled to the factthat the electron ionisation signal should contain nopositrons: - the µ−/µ+ ratio obtained from the muonbremsstrahlung sample is ∼ 0.70 - out of the 36 signalcandidates, four have a positive charge, leading to anexpectation of (6.8 ± 3.4) background events in thesignal sample, in good agreement with the estimatefrom the first method. The final sample consists ofthe 32 candidates with measured negative charge.6 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009Figure 14: Distribution of the energy to momentum ratio,E/p, for the electron candidates of Fig. 12, after applyingthe cut indicated on the ratio of high to low-threshold TRThits. The red curve shows the background, estimated asexplained in Fig. 13 and projected on the E/p distribution.The background under the signal is estimated to be (8.7±3.1) events, using this first method described above forFig. 13.9. ConclusionsDue to space limitations, some systems of ATLAShave been omitted from this paper. Notably the trig-ger system which has also been commissioned duringthis period and is also ready for fist collisions. Inaddition, the ATLAS distributed computing system,with its world-wide computing infrastructure, a cru-cial component of the analysis and data processingsystem, has been undergoing continuous commission-ing.In summary, all the subsystems of ATLAS havedemonstrated capability sufficient to ensure physicsdiscoveries as soon as collision data is ready later thisyear.AcknowledgmentsWe are greatly indebted to all CERN’s departmentsand to the LHC project for their immense efforts notonly in building the LHC, but also for their directcontributions to the construction and installation ofthe ATLAS detector and its infrastructure. We ac-knowledge equally warmly all our technical colleaguesin the collaborating Institutions without whom theATLAS detector could not have been built. Further-more we are grateful to all the funding agencies whichsupported generously the construction and the com-missioning of the ATLAS detector and also providedthe computing infrastructure.The ATLAS detector design and construction hastaken about fifteen years, and our thoughts are withall our colleagues who sadly could not see its finalrealisation.We acknowledge the support of ANPCyT, Ar-gentina; Yerevan Physics Institute, Armenia; ARCand DEST, Australia; Bundesministerium fu¨r Wis-senschaft und Forschung, Austria; National Academyof Sciences of Azerbaijan; State Committee on Sci-ence & Technologies of the Republic of Belarus; CNPqand FINEP, Brazil; NSERC, NRC, and CFI, Canada;CERN; NSFC, China; Ministry of Education, Youthand Sports of the Czech Republic, Ministry of Indus-try and Trade of the Czech Republic, and Committeefor Collaboration of the Czech Republic with CERN;Danish Natural Science Research Council; EuropeanCommission, through the ARTEMIS Research Train-ing Network; IN2P3-CNRS and Dapnia-CEA, France;Georgian Academy of Sciences; BMBF, DESY, DFGand MPG, Germany; Ministry of Education and Reli-gion, through the EPEAEK program PYTHAGORASII and GSRT, Greece; ISF, MINERVA, GIF, DIP, andBenoziyo Center, Israel; INFN, Italy; MEXT, Japan;CNRST, Morocco; FOM and NWO, Netherlands; TheResearch Council of Norway; Ministry of Science andHigher Education, Poland; GRICES and FCT, Por-tugal; Ministry of Education and Research, Roma-nia; Ministry of Education and Science of the RussianFederation, Russian Federal Agency of Science andInnovations, and Russian Federal Agency of AtomicEnergy; JINR; Ministry of Science, Serbia; Depart-ment of International Science and Technology Coop-eration, Ministry of Education of the Slovak Repub-lic; Slovenian Research Agency, Ministry of HigherEducation, Science and Technology, Slovenia; Min-isterio de Educacio´n y Ciencia, Spain; The SwedishResearch Council, The Knut and Alice WallenbergFoundation, Sweden; State Secretariat for Educationand Science, Swiss National Science Foundation, andCantons of Bern and Geneva, Switzerland; NationalScience Council, Taiwan; TAEK, Turkey; The Sci-ence and Technology Facilities Council and The Lever-hulme Trust, United Kingdom; DOE and NSF, UnitedStates of America.References1 Details of the Large Hadron Collider can be foundon the main web page: http://lhc.web.cern.ch/lhc/2 Expected Performance of the ATLAS Experiment- Detector, Trigger and Physics, The ATLAS Col-laboration, (Submitted on 28 Dec 2008 (v1), lastrevised 14 Aug. 2009 (v4)) arXiv:0901.0512v3http://arxiv.org/abs/0901.0512v33 The ATLAS Experiment at the CERN LargeHadron Collider. August, 2008. JINST 3 S08003http://www.iop.org/EJ/article/1748-0221/3/08/S08003/jinst8\_08\_s08003.pdf

The ATLAS Detector: Status and Results from Cosmic Rays

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