The start-up of the Large Hadron Collider at CERN marks a new era in particle physics. This paper presents an overview of the ATLAS detector performance and first results extracted from data taken during the early LHC runs

van Vulpen, I.

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DOI 10.1393/ncc/i2010-10649-2Colloquia: TOP2010IL NUOVO CIMENTO Vol. 33 C, N. 4 Luglio-Agosto 2010B-tagging, leptons and missing energy in ATLAS after first dataI. van Vulpen on behalf of the ATLAS CollaborationNikhef, National Institute for Subatomic Physics - Science Park 1051098 XG Amsterdam, The Netherlands(ricevuto il 27 Luglio 2010; approvato il 27 Luglio 2010; pubblicato online il 24 Settembre 2010)Summary. — The start-up of the Large Hadron Collider at CERN marks a newera in particle physics. This paper presents an overview of the ATLAS detectorperformance and first results extracted from data taken during the early LHC runs.PACS 14.65.Ha – Top quarks.1. – IntroductionThe new frontier of particle physics, the high-energy LHC proton-proton collisions atCERN, will allow us to study nature at the smallest distance scales to date and to pursueour main goal: to search for evidence of physics beyond the Standard Model.With the first collisions, each of the ATLAS sub-detectors follows a detailed calibra-tion and alignment program to study the detector response to electrons, muons, photonsand jets. The events used in these calibrations range from minimum bias, through well-predicted low multiplicity (di-)lepton events to more complex event topologies. Thispaper presents the analysis of the first data and current understanding of the detec-tor response(1). A first step towards a claim of a signal from new physics is to showthat the detector response is also understood in more complex event topologies. With acombination of multiple high-pT (b-)jets, isolated high-energy leptons and large missingtransverse energy, the observation and detailed measurement of top quark pair produc-tion is an important milestone in the ATLAS physics program.2. – The Inner Detector: tracking performance and b-taggingThe heart of the ATLAS detector is formed by the Inner Detector (ID): a tracking de-tector inside a 2T magnetic field covering angles up to |η| < 2.5. Tracks are reconstructedusing the ID’s three sub-detectors: space points from the innermost pixel detector andsilicon microstrip detector (SCT) complemented with information from the transitionradiation tracker (TRT) that also provides information on particle identification.(1) All plots and results shown in this paper can be found on the ATLAS public results web-page: https://twiki.cern.ch/twiki/bin/view/Atlas/AtlasResults.c© Societa` Italiana di Fisica 193194 I. VAN VULPEN on behalf of the ATLAS COLLABORATIONη-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Average Number of SCT Hits6.577.588.599.51010.5DataND MC MinBiasATLASPreliminary = 900 GeVsLocal x residual [mm]−0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5Number of hits on tracks050001000015000200002500030000 PreliminaryATLASSCT Barrel = 900 GeVsMonte Carlomμ=0 μmμFWHM/2.35=38 Data 2009mμ=0 μmμFWHM/2.35=43 Fig. 1. – (Colour on-line) Left plot: number of SCT hits on reconstructed tracks in 900GeVdata and non-diffractive minimum bias MC simulation as a function of η. Right plot: unbiasedresidual distribution in x in the SCT barrel for the MC with perfect alignment (open marker)and the alignment obtained with collision data taken in 2009 (solid marker).2.1. Track reconstruction. – Basic track quantities and detector geometry are verywell described by simulation. Figure 1 shows the number of SCT hits on reconstructedtracks in 900GeV data compared to the expectation from non-diffractive minimum biasMonte Carlo (MC) simulation. Tracks were selected within |η| < 2.5, required to have atransverse momentum in excess of 500MeV and have at least 7 SCT hits. The simulationtakes into account the disabled modules in data.A large set of well-reconstructed tracks (pT > 2GeV, |d0| < 10mm and at least 6silicon hits) allows an initial alignment of the ID by minimizing track residuals, defined asthe distance between the measured hit and the prediction from the track extrapolation.Figure 1 shows the unbiased residual distribution in the x-coordinate for all hits-on-track in the SCT barrel for the MC with perfect alignment (open red) and the alignmentobtained with collision data taken in 2009 (solid blue). The distributions show that theresolutions are already close to those from MC expectations. [MeV]π πm400 450 500 550 600 650 700 750 800Entries / 2 MeV0100020003000400050006000 Invariant Mass0SK=900 GeV)sMinimum Bias Stream, Data 2009 ( > 100 MeV, Si hits > 6TBoth tracks: p) > 0.8, flight distance > 0.2 mmθcos(DataSimulationGauss (+poly) fit 0.1 (stat) MeV± =   497.5 μ 0.1 (stat) MeV± =       8.2 σ 0.024 MeV± = 497.614 0KPDG (2009) mATLAS PreliminaryFig. 2. – Invariant mass distribution of two-track vertices around the KS mass.B-TAGGING, LEPTONS AND MISSING ENERGY IN ATLAS AFTER FIRST DATA 195 (L)σL / -20 -10 0 10 20 30Number of jets020406080100120140160180 SV0 selectionData 2010 = 7 TeVs()-1L = 0.4 nbSimulationb jetsc jetslight jetsATLASPreliminaryNumber of jets0dTrack S-40 -30 -20 -10 0 10 20 30 40Number of tracks-110110210310410510610710810 = 7 TeVsData 2010ND min. bias MCATLAS PreliminaryFig. 3. – B-tagging results in 7TeV data: transverse impact parameter significance (left plot)and the three-dimensional decay length significance (right plot).2.2. Primary and secondary vertices. – The interaction point of proton-proton colli-sions is reconstructed on an event-by-event basis using a dedicated primary vertex recon-struction with an efficiency close to 100%. A clear indication of the good performanceand resolution of the secondary vertex tagging is shown in fig. 2 with the data to MCcomparison of the KS peak in the two-vertex-mass distribution, where the flight path ofthe V0 candidate is required to point back to the interaction point.2.3. b-tagging . – In ATLAS, several tagging algorithms try to identify b(c)-quark de-cays. These algorithms are based on testing a non-compatibility of the tracks associatedto the jet to originate from the primary vertex and the existence (and properties) of asecondary vertex. Track quality criteria [1, 2] include the requirement to have at least 7silicon hits. Figure 3 compares data and MC prediction for the transverse impact param-eter significance d0/σd0 for ptracksT > 150MeV (left plot) [1] and the three-dimensionaldecay length significance for ptracksT > 1GeV (right plot) [2]. The initial results on 7TeVdata as shown in the plot show an excellent understanding of the performance.3. – Electron and muon reconstructionThe ability to reconstruct an isolated lepton with large transverse energy is a crucialstep in selecting of candidates from many signals (W, Z, top quarks, new physics models)from the large number of proton-proton collisions. Both electron and muon momentumestimations use extensive information from ID tracking, but require calorimeter and TRT(electron) and the dedicated muon spectrometer information (muons) to obtain a cleanidentification. In this section we show a few key plots in lepton reconstruction andidentification.3.1. Cosmic muons. – In the two years before LHC start-up, several hundred millioncosmic muons that traversed the ATLAS detector have enabled an extensive alignmentand calibration program. Figure 4 shows the transverse momentum resolution. A fit tothe data (curve in plot) reveals that in the region important for top physics, i.e. up to100GeV, the resolution is close to expectation from simulation.196 I. VAN VULPEN on behalf of the ATLAS COLLABORATIONPt (GeV)  10 210 310(Pt)/Ptσ00.020.040.060.080.10.120.140.160.180.2  / ndf 2χ  6.1314 / 10Prob   0.8041p0        0.0230± 0.2808 p1        0.0013± 0.0397 p2        0.0025± 0.0277 ATLAS preliminary2009 cosmic dataFig. 4. – Muon transverse momentum resolution using cosmic muons.3.1.1. Muon reconstruction: Muons in 900GeV data. For data taken at 900GeV dataonly a few muons are expected and only 50 candidates remain after a selection thatincludes track quality criteria and a minimum absolute (transverse) momentum of 4(2.5)GeV [3]. This sample is dominated by muons from π/K decays and ∼ 25% areexpected from semi-leptonic (b-)c-quark decays. The distributions of the pseudo-rapidityand momentum of the candidates are shown in fig. 5, where a good agreement betweendata and MC prediction is observed.3.1.2. Muon reconstruction: Muons in 7TeV data. A well-functioning lepton triggerchain is vital for selecting interesting events from all collisions. As an indication of themuon trigger performance, the left plot of fig. 6 shows the efficiency of the Level-2 muoncombined algorithm in the barrel region (|η| < 1.05) relative to the Level-2 muon spec-trometer algorithm with a nominal threshold set to 4GeV as a function of the transversemomentum of the offline reconstructed combined muon. η -3 -2 -1 0 1 2 3 muons / 0.4  0246810121416Atlas Preliminary 2009 Toroid on 900 GeV Data  non-diffractive minimum bias MC  p [GeV] 0 5 10 15 20 25 30 35 40 muons / 2 [GeV] 024681012141618Atlas Preliminary 2009 Toroid on 900 GeV Data  non-diffractive minimum bias MC Fig. 5. – Distribution of η (left) and momentum (right) of reconstructed muons in 900GeVdata and MC. Distributions are normalized to the number of selected muon candidates. Themomentum is measured by the ID reconstruction or, for combined tracks, is the result of thecombined fit to the ID muon spectrometer track.B-TAGGING, LEPTONS AND MISSING ENERGY IN ATLAS AFTER FIRST DATA 197 [GeV]-μ+μInvariant Mass 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8Entries/0.080 [GeV]051015202530ATLAS PreliminaryData with Poisson ErrorsML FitFig. 6. – Left plot: efficiency of the L2 muon combined algorithm relative to the L2 muonspectrometer algorithm with a nominal threshold set to 4GeV. Right plot: di-muon invariantmass distribution for 320μb−1 at 7TeV using minimum bias trigger.Muons originating from either J/Ψ or Z-boson decays provide a clean sample of muonsand, through the mass constraint, a powerful handle to assess ATLAS’ capability toreconstruct muons. In the absence of a sample of Z-boson decays (we did observe a verynice Z → μ+μ− candidate though), the first analyses exploit the larger statistics fromJ/Ψ decays. We looked for J/Ψ→ μ+μ− by selecting events with two oppositely chargedmuons, each with a momentum above 3GeV. The resulting mass distribution in the first320μb−1 of data at a center-of-mass energy of 7TeV is shown in the right plot of fig. 6.3.2. Electron reconstruction: Electrons in 900GeV data. – As for muons, in 900GeVdata only a limited number of prompt electrons are expected. We observed 879 electroncandidates that are expected to originate from either photon conversions or hadronsfaking an electron signature [4]. The energy in the EM calorimeter of all candidates isshown in the left plot of fig. 7. Identifying conversions using tracking information [5]allows to separate the electrons from the fakes and to test the electron reconstructioncapability of ATLAS. The conversion radius from identified conversion candidates isshown in the right plot of fig. 7, showing that the material description of the ATLAS IDis accurately modeled. [GeV]TE0 2 4 6 8 10 12Entries / 0.5 GeV050100150200250300 = 900 GeV)sData 2009 (Non-diffractive minimum bias MCHadronsElectrons from conversionsATLAS PreliminaryR [mm]0 50 100 150 200 250 300 350 400Entries / 8 mm0510152025303540Entries / 8 mm = 900 GeV)sData 2009 (MC conversion candidatesMC true conversionsMC true Dalitz decays(Non−diffractive minimum bias MC)ATLAS PreliminaryFig. 7. – Transverse energy of the cluster for electron candidates (left) and distribution of thephoton conversion radius integrated over all η for identified conversions (right).198 I. VAN VULPEN on behalf of the ATLAS COLLABORATIONFraction of high-threshold TRT hits0 0.1 0.2 0.3 0.4 0.5Entries / 0.025020406080100120140160180200 = 900 GeV)sData 2009 (Non-diffractive minimum bias MCHadronsElectrons from conversionsATLAS Preliminary−factor    γ10 210 310 410High−threshold probability00.050.10.150.20.25Pion momentum (GeV)1 10Electron momentum (GeV)1 10TRT end−capElectron candidatesGeneric tracks Fit to data Electron candidatesGeneric tracks Electrons (MC)Generic tracks (MC)ATLAS preliminaryFig. 8. – Distribution of fraction of high-threshold TRT hits for candidates with |η| <2 (left) andthe distribution of the high-threshold probability as a function of the particles Lorentz factor γfor electrons and pions separately (right).A unique element of the ATLAS detector is the TRT. It not only provides space-points used for tracking, but is designed to provide electron-pion separation informationusing the (non-)detection of transition radiation that is induced by a traversing particledepending on its Lorentz factor γ. The light electrons are more likely to induce a numberof high-threshold TRT hits as is shown in the left plot of fig. 8. The electron-pionseparation is shown by the difference in response of the TRT to pions and electrons(from conversions) in the right plot of fig. 8.4. – Calorimeter response and reconstructing missing transverse energyA solid and unbiased estimate of missing transverse energy involves understandingboth detector geometry and response to several physics objects to high precision. Herewe present initial results based on calorimeter information [6].4.1. Calorimeter response. – The response of the electromagnetic and hadroniccalorimeters at the electromagnetic (EM) scale is shown in fig. 9. The distribution of cell [GeV]cellE0 2 4 6 8 10Number of cells [/0.05 GeV]-110110210310410510610710810910 = 900 GeV Collision candidatessRandom triggerNon-diffractive minimum bias MC2009 Collisions EM ENDCAPATLAS PreliminaryEM scaleTile Cell Energy [GeV]-1 0 1 2 3 4 5# of Cells / 0.06 GeV / Event-510-410-310-210-110110210310410EM ScaleATLAS Preliminary = 7 TeVsMinBias MC,  = 7 TeVsData,  = 2.36 TeVsData,  = 0.9 TeVsData,  Random TriggerFig. 9. – Distribution of cell energy for the electromagnetic endcap calorimeter (left plot) andthe TileCal hadronic calorimeter (right plot).B-TAGGING, LEPTONS AND MISSING ENERGY IN ATLAS AFTER FIRST DATA 199  [GeV]T EΣ0 10 20 30 40 50 60 70 80 Resolution [GeV]missy,EmissxE00.511.522.533.544.55ATLAS Preliminary T EΣData 2.36 TeV: fit 0.37   T EΣData 900 GeV: fit 0.37   T EΣMC MinBias 900 GeV: fit 0.37   [GeV]missTE0 10 20 30 40 50 60Events / 1 GeV110210310410510610DataMC MinBiasATLAS PreliminaryData April 2010 = 7 TeVs-1= 0.3 nbintL| < 4.5η|Fig. 10. – Resolution of the two components of the missing transverse energy as a function ofthe total sum of transverse energy in 0.9 and 2.36TeV data (left plot) and the distributionof missing transverse energy in 7TeV data sample compared to a normalized MC distribution(right plot).energy(2) in the electromagnetic calorimeter (endcap) in a run of good quality is shownin the left plot that also shows the cell energy distribution from random data (electronicnoise only) as well as the expectation from non-diffractive minimum bias MC events.The distribution of TileCal cells, a hadronic calorimeter, in collision data at 7TeV,2.36TeV, and 0.9TeV is shown in the right plot of fig. 9 where it is superimposed withPythia minimum bias MC and randomly triggered events.4.2. Missing transverse energy . – Using topological clusters, calibrated at the EMscale, the calorimeter response can provide a first estimate of the total missing transverseenergy in an event. The resolution of Emissx and Emissy as a function of the total transverseenergy (∑ET) for minimum bias events in 0.9 and 2.36TeV data is shown in fig. 10 (leftplot) [6]. The line represents a fit to the resolution obtained in the MC simulation.The distribution of missing transverse energy in a data sample of 14.4 million se-lected minimum bias events at 7TeV center-of-mass energy is shown in the right plotof fig. 10 [7]. The expectation from MC simulation is superimposed (histogram) andnormalized to the number of events in data. We observe a remarkable agreement over 6orders of magnitude and, especially important in searches for new physics, the tails seemwell understood.5. – ConclusionsThe early LHC data-taking runs at 0.9, 2.36 and 7TeV center-of-mass have allowed afirst calibration, evaluation of the ATLAS detector operation and ability to reconstructtrajectories of charged particles, high-energy leptons and missing transverse energy. Theagreement between data and MC on several complex physics objects like b-tagging andmissing energy estimation is remarkable at this early stage. It allows us to look withconfidence at more complex event topologies. We are looking forward to seeing signsof top quark pair production in the 2010 LHC runs and hopefully also hints of newphysics.(2) Known noisy cells have been masked.200 I. VAN VULPEN on behalf of the ATLAS COLLABORATIONREFERENCES[1] ATLAS Collaboration, Tracking Studies for b-tagging with 7TeV Collision Data withthe ATLAS Detector, ATLAS-CONF-2010-040 (2010).[2] ATLAS Collaboration, Performance of the ATLAS Secondary Vertex b-taggingAlgorithm in 7TeV Collision Data, ATLAS-CONF-2010-042 (2010).[3] ATLAS Collaboration, Identification of muon candidates in pp collisions at√s =900GeV with the ATLAS detector, ATLAS-CONF-2010-015 (2010).[4] ATLAS Collaboration, Electron and photon reconstruction and identification in ATLAS:expected performance at high energy and results at√s = 900GeV, ATLAS-CONF-2010-005(2010).[5] ATLAS Collaboration, Photon Conversions at√s = 900GeV measured with the ATLASDetector, ATLAS-CONF-2010-007 (2010).[6] ATLAS Collaboration, Performance of the missing transverse energy reconstruction inminimum bias events at√s of 900GeV and 2.36TeV with the ATLAS detector, ATLAS-CONF-2010-008 (2010).[7] ATLAS Collaboration, Performance of the missing transverse energy reconstruction inminimum bias collisions at center-of-mass energy of√s = 7TeV with the ATLAS detector,ATLAS-CONF-2010-039 (2010).

B-tagging, leptons and missing energy in ATLAS after first data

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B-tagging, leptons and missing energy in ATLAS after first data

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