The road with early ATLAS data towards top physics is discussed. This includes a detailed understanding of the detector performance for key components of top quark decays like jets, tracking and lepton identification as well as

measurements on minimum bias and underlying event physics. The status of these studies based on the first tens of nb

−1 is presented

Mättig, P.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2010-10661-6Colloquia: TOP2010IL NUOVO CIMENTO Vol. 33 C, N. 4 Luglio-Agosto 2010First results and status of the ATLAS detector at the LHCP. Ma¨ttig for the ATLAS CollaborationBergische Universita¨t Wuppertal - Wuppertal, Germany(ricevuto il 19 Agosto 2010; approvato il 19 Agosto 2010; pubblicato online il 5 Ottobre 2010)Summary. — The road with early ATLAS data towards top physics is discussed.This includes a detailed understanding of the detector performance for key com-ponents of top quark decays like jets, tracking and lepton identification as well asmeasurements on minimum bias and underlying event physics. The status of thesestudies based on the first tens of nb−1 is presented.PACS 13.85.-t – Hadron-induced high- and super-high-energy interactions (energy> 10GeV).PACS 13.85.Ni – Inclusive production with identified hadrons.PACS 13.85.Qk – Inclusive production with identified leptons, photons, or othernonhadronic particles.PACS 13.87.-a – Jets in large-Q2 scattering.1. – The LHC environmentThe LHC has collided its first proton bunches at√s of 900GeV in November 2009.Since then it has increased the energy to 7TeV and the instantaneous luminosity byseveral orders of magnitude. At the time of TOP2010 some 15 nb−1 had been collectedby ATLAS, at this time of writing a factor about 70 more data has been accumulated.Given the rapid progress in accumulated luminosity, only a snapshot of the fast-movingdata analysis is possible.Even though recent data open a wide spectrum of physics studies, still a substantialamount of work in ATLAS is devoted to understanding detector performance and tooptimise algorithms to characterise physics objects. This will be addressed in this report.In addition measurements of soft QCD physics, i.e. minimum bias and underlying eventstudies, will be presented.2. – The ATLAS detector and its performance goalsThe ATLAS detector [1], being 44m long and 25m in diameter, is the largest detectorat an accelerator ever built. It contains innermost precision tracking devices immersed ina 2 Tesla solenoidal magnetic field, electromagnetic and hadronic calorimetry, and on itsc© Societa` Italiana di Fisica 3132 P. MA¨TTIG for the ATLAS COLLABORATIONFig. 1. – Major processes that lead to an understanding of the components of top quark decays.Also indicated are the typical cross sections at√s = 7TeV.outside a muon spectrometer in a toroidal magnetic field. The ATLAS detector coversalmost the whole solid angle of up to pseudorapidities |η| = 4.9 for the calorimetric and|η| = 2.5 for tracking measurements.From the beginning of LHC collisions, the detector operates with very high efficiencyand all its components have 98–100% of the electronic channels functioning. More than94% of the luminosity produced by LHC in stable beam conditions is recorded by ATLAS.The prerequisite for getting the most physics out of the millions of top quarks thatwill eventually be produced at the LHC and the anticipated new physics is a thoroughunderstanding of the detector performance. The ATLAS strategy is to derive the under-standing of the physics objects as much as possible from data and therefore substantialeffort is devoted to reach this objective. Even though an excellent foundation has beenlaid by test beams and several hundred millions of cosmic ray events, the ultimate pre-cision can only be obtained from collisions, supported by a detailed simulation of thedetector. The road to the top (and beyond) proceeds in the steps shown in fig. 1. Mea-surements of these physics processes will provide the confidence to identify and studytop quark events which in turn eventually will be an efficient tool to calibrate the physicsobjects.3. – Detector performanceExploring top quark events requires all parts of the detector to be well under-stood. Tracking is needed for bottom tagging and lepton identification, electromagneticcalorimetry for electron, the outer muon system for muon identification, and both elec-tromagnetic and hadronic calorimetry for jets. Finally all these measurements have tocome together to determine the missing transverse energy (MET).3.1. Tracking system. – The ATLAS tracking detectors comprise an innermost pixeldetector, a silicon strip detector and the transition radiation tracker. The quality oftrack reconstruction depends largely on alignment, material distribution and hit effi-ciency. Tracks in minimum bias data have been extensively studied —comparing dataand simulation shows a remarkably good agreement.Many hadron resonances have been observed that can be used to determine the qualityof the tracking system. Especially the precisely known and highly abundant K0s mesondecays are used by ATLAS to determine the momentum scale and resolution and toestimate the material before and inside the tracking detector. The reconstructed massFIRST RESULTS AND STATUS OF THE ATLAS DETECTOR AT THE LHC 33x [mm]-400-300-200-100 0 100 200 300 400y [mm]-400-300-200-1000100200300400012345>6MCATLAS Preliminaryx [mm]-400-300-200-100 0 100 200 300 400y [mm]-400-300-200-1000100200300400012345>6DataATLAS Preliminarya)                                                                          b)Fig. 2. – Distribution in the transverse plane of locations of photon conversions γ + X → e+e−in data (left, a) and simulation (right, b).of 0.497 ± 0.001GeV is in perfect agreement with the PDG value. By determining themass as a function of radius, η and the azimuthal angle φ, the amount of material inthe detector is mapped. These studies suggest that the material is largely simulatedcorrectly in the inner detector, and is underestimated by at most 10% in some regions.The material distribution in terms of the radiation lenghts can also be measured usingphoton conversions (fig. 2). Although in general data and simulation agree very well,differences in detail are observed, which will be corrected in the future simulations.The transverse impact parameter d0 of tracks is particularly important for the iden-tification of bottom, and to some extent for charm quarks and τ leptons. The simulationof d0 agrees in general very well with data as a function of the transverse momentumapproaching the goal of σ(d0) ∼ 10μm for high-pT tracks. The high precision in d0translates into an excellent measure of bottom decay vertices. As an example, the dis-tribution of high decay length significances L/σ(L) indicative of bottom quarks (fig. 3a)from a secondary vertex tagger shows the long tail expected for bottom decays. Zoominginto the events with a large decay length significance, the invariant mass of the tracksassociated to the secondary vertex shows nicely the cut-off from the charm mass and thepower and quality of the data (fig. 3b).3.2. Electrons and muons. – Charged leptons can be tested in the decay of resonanceslike J/ψ and eventually the Z0. In fig. 4, as an example, the J/ψ peak is depicted, itsshape being in very good agreement with the expectation. The response of the calorimeterto electrons is checked using photon conversions and found to be in very good agreement.3.3. Jets. – A large number of di- and multiple jet events have been found in ATLAS,with invariant masses extending deep into the TeV range, already now providing probes ofan as yet unchartered territory. The jet energy calibration in ATLAS proceeds in severalsteps. The first one is to exploit isolated charged particles and compare their energy Emeasured in the calorimeter and the momentum p determined with the tracking system.34 P. MA¨TTIG for the ATLAS COLLABORATION [GeV]vtxm0 1 2 3 4 5 6Number of jets0102030405060 SV0 selectionData 2010) -1 = 7 TeV, L = 0.4 nbs(Simulationb jetsc jetslight jetsATLAS Preliminary (L)σL / -20 -10 0 10 20 30Number of jets020406080100120140160180 SV0 selectionData 2010 = 7 TeVs()-1L = 0.4 nbSimulationb jetsc jetslight jetsATLASPreliminarya)                                                                                b)Fig. 3. – Distribution of the decay length significance L/σ(L) in minimum bias events at 7TeV(left, a). Invariant mass of tracks assigned to a secondary vertex with a decay length significancelarger than 7 (right, b).The E/p ratio rises with the momentum between 1 and 20GeV and is described bythe simulation to better than 5%, providing confidence in the quality of the detectordescription. In a next step the simulation is used to apply corrections for jet energiesusing global corrections for the electromagnetic and hadronic parts of the detector anddead material. The observed pjetT distribution agrees very well with the expected one(fig. 5a). In the future corrections will be used which are based on the local energydeposition.3.4. Missing transverse energy . – As already known from the discovery of the Wboson, the measurement of the missing transverse energy is a powerful tool to identify(and measure) the existence of a neutrino and may eventually reveal the existence of adark matter candidate. Its quality is largely determined by the understanding of “hotcells” of electronic noise in the calorimeter. Those can be identified by dedicated studies,only a small fraction of 10−5 of the jets is affected. The remaining jets show a remarkableagreement with the expectation (fig. 5b). [GeV]μμm2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 Candidates / (0.04 GeV)ψJ/020040060080010001200140016001800200022002400Data: 2010ψMC: Prompt J/Fit projectionFit projection of bkg. 90± = 5350 ψJ/N 0.001 GeV±= 3.095 ψJ/m  1 MeV±   = 71 mσ Candidates / (0.04 GeV)ψJ/ATLAS Preliminary-1 L dt = 78 nb∫  = 7 TeVsFig. 4. – J/ψ signal in μ+μ− final states.FIRST RESULTS AND STATUS OF THE ATLAS DETECTOR AT THE LHC 35 [GeV]missTE0 10 20 30 40 50 60Events / 1 GeV110210310410510610GCWDataMC MinBiasATLAS Preliminary = 7 TeVsData 2010   -1Ldt=0.34 nb∫|<4.5η| [GeV]TEM+JESp30 40 100 200 300 [1/GeV]TdN / dp-310-210-110110210310410510 ATLAS Preliminary R=0.6 cluster jets, |y|<2.8tanti-k = 7 TeVsData 2010  MC QCD di-jetsa)b)Fig. 5. – Distribution of jets as a function of transverse momentum pT after calibration (left, a)and distribution of the transverse missing energy (right, b). Both distributions are comparedto the PYTHIA model with MC09 tune.4. – First electroweak physicsAs these discussions on the detector performance underline, ATLAS is prepared forelectroweak physics—all physics objects for the exploration of W± and Z0 bosons andalso top quarks are very well understood. Indeed with increasing luminosities candidateevents for the vector bosons have been observed. At the time of the conference these wereonly a handful, meanwhile several hundred have been found in ATLAS. Top candidateshave only appeared recently with luminosities exceeding 100 nb−1. Candidate events forW → eν and tt¯ → e + μ + jets + X are shown in fig. 6.Fig. 6. – (Colour on-line) Candidate events for W → eν (left, a) and tt¯ production (right, b).The electron in the W-decay is denoted by the yellow line, the MET by the dashed red line.In the case of the top quark pair, one top decays into an electron (green line), the other into amuon (red line).36 P. MA¨TTIG for the ATLAS COLLABORATION5. – Minimum bias physicsWith the increased bunch intensity also the number of concurrent proton-proton in-teractions rises, disturbing the measurements of the interesting hard interactions. Theknowledge of these minimum bias events is therefore of importance to precisely studyhigh-pT physics. Furthermore, the theoretical predictions for these soft QCD events arerather uncertain and further constraints from different c.m. energies and phase spacemay illucidate this physics.The ATLAS strategy to measure minimum bias events is somewhat different fromthe approach taken by other experiments. It is based on the principle to be as muchas possible independent of the physics model. Therefore the data are unfolded to thehadron level and compared to the different models and parameter tunes without applyingcorrections for diffractive components.ATLAS has measured particle densities and event properties for particles in a phasespace region of pT > 500MeV and |η| < 2.5 [2]. Comparing the data to models, the mainconclusions are– The rapidity distributions show (fig. 7a,b) reasonable agreement for PYTHIA [3]with the ATLAS MC09 and MC09c tunes [4], whereas the DW [5] and Perugia0 [6]tunes significantly underestimate the particle yield. The same is true for PHO-JET [7].– All models and tunes have difficulties in describing the shape of the pT spectrum.– Both at 0.9 and, more significantly, at 7TeV, high multiplicity events occur moreoften than predicted by all models. In particular PHOJET underestimates thehigh multiplicity events considerably. Note that diffractive events are expected tocontribute strongest at low multiplicities.– Models tend to overestimate the average pT for high event multiplicities. This isparticularly the case for 7TeV (fig. 7c,d).6. – Underlying eventsApart from the additional pp-interactions, the study of hard parton interactions iscomplicated by the “underlying events”, i.e. residual contributions due to the colourflow among the proton remnants. A first ATLAS analysis at√s 0.9 and 7TeV [8]separates the plane transverse to the beam direction into four quadrants. The “toward”quadrant is defined by the highest pT track. The opposite quadrant is the “away”region, the transverse quadrants “transverse regions”. The idea of this method is thatthese transverse regions are largely unaffected by the hard process and therefore yield anunbiased picture of the underlying event.The particle density as a function of φ, the angle with respect to the leading particlein the transverse plane is shown in fig. 8. The distributions show an accumulation ofparticles around the trigger particle which becomes more and more pronounced with theenergy of the leading particle suggesting an evolving jet structure. The density in thetransverse region also rises with the leading pT . The measurements are compared tothe PHYTHIA MC09 tune. Whereas the transverse region is fairly well described for aleading pT > 1GeV, it significantly underestimates the density in the transverse regionfor higher pT . The difference is more pronounced for 7TeV than at 0.9TeV.FIRST RESULTS AND STATUS OF THE ATLAS DETECTOR AT THE LHC 37Fig. 7. – Particle density as a function of pseudorapidity η (a,b) and the average transversemomentum pT as a function of charged particle multiplicity (c,d). The data are compared tothe PYTHIA model with various tunes and the PHOJET model.38 P. MA¨TTIG for the ATLAS COLLABORATIONFig. 8. – Particle density as a function of the angle in the transverse plane with respect to theleading particle. Note that the distributions are mirrored for the positive and negative φ ranges.7. – ConclusionsThe LHC is progressing fast and its luminosity is used by ATLAS to scrutinize thedetector performance with collision data and to enter the physics of a new energy regime.All studies show that even after data worth a few months, the ATLAS detector is highlyefficient, remarkably well understood and already now close to its design goals.With the current more than 1 pb−1, ATLAS has entered the multi-TeV regime of jetphysics and accumulated a data set of several hundreds of leptonically decaying W±’sand Z0’s, opening the electroweak sector of LHC physics. Whereas at TOP2010 topquark events were eagerly expected, by now several candidates have been found. Topquark physics is on the verge of becoming reality at the LHC with several thousands oftop-quark pairs expected until the end of this year, holding the promise for an excitingphysics harvest at the top and beyond the Standard Model physics.REFERENCES[1] Aad G. et al. (ATLAS Collaboration), JINST, 3 (2008) S08003.[2] Aad G. et al. (ATLAS Collaboration), Phys. Lett. B, 688 (2010) 21.[3] Sjostrand T., Mrenna S. and Skands P., JHEP, 05 (2006) 026.[4] Aad G. et al. (ATLAS Collaboration), ATL-PHYS-PUB-2010-002.[5] Albrow M. G. et al., ArXiv:0610012[hep-ph].[6] Skands P. Z., ArXiv:0905.3418[hep-ph].[7] Engel R., Z. Phys. C, 66 (1995) 203.[8] The ATLAS Collaboration, ATLAS-CONF-2010-029.

First results and status of the ATLAS detector at the LHC

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First results and status of the ATLAS detector at the LHC

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