Adam-Bourdarios, C.

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Lepton Studies with ATLAS and CMS early dataC. Adam-BourdariosTo cite this version:C. Adam-Bourdarios. Lepton Studies with ATLAS and CMS early data. Auge´ E. ; DumarchezJ. ; Pietrzyk B.; Traˆn Thanh Vaˆn J. XLIVth Rencontres de Moriond - QCD and High EnergyInteractions, Mar 2009, La Thuile, Italy. The GIOI publishers, pp.261-264, 2009. <in2p3-00388840>HAL Id: in2p3-00388840http://hal.in2p3.fr/in2p3-00388840Submitted on 20 Nov 2009HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.LEPTON STUDIES WITH ATLAS AND CMS EARLY DATAC. ADAM-BOURDARIOSLaboratoire de l’Acce´le´rateur Line´aire, Orsay, FranceON BEHALF OF THE ATLAS AND CMS COLLABORATIONSLeptons will play a key role in the first measurements performed by the LHC experiments.The ATLAS and CMS detectors are now built, and early estimates of their performance havebeen revisited. The expected first physics signals are described: they will shed light on thelevel of detector understanding and allow a control of the systematics uncertainties, openingthe road to new and precision physics.The last two years have seen the end of the ATLAS and CMS detector assembly. Thisallowed a survey of the detector elements placement accuracy, and a precise cross-check of theamount and position of the dead material. All of this was gradually integrated in the detectorsimulation, which is now believed to be realistic. The Geant 4 simulation and the calibrationprocedure have been extensively validated by a set of Test-Beams. Cosmics data, taken insummer 2008, are being used to study and improve the detector alignment 1. Last but not least,the reconstruction algorithms have now stabilised, in particular for lepton identification.For all these reasons, early estimates made over the years were revisited. Both collaborationshave published detailed reports about the expected performance, to be used as references andguidelines in the coming years 2,3,4. They are both now focusing on the short term physicsprogram, accessible with integrated luminosities of about 100 pb−1.1 Detector Performance for lepton identification1.1 ElectronsThe first challenge for electron reconstruction is the resolution and linearity of the energy mea-surement. After years of very detailed calibration studies, the analysis of Test-Beam data hasproven that the design goals were met, with a local constant term smaller than 0.5 %. Thesecond challenge is the jet rejection: QCD cross sections are such that one expects an electronto jet ratio of 10−5. Both experiments have developed cut-based algorithms, meant to be robustenough to be used at the startup. The efficiencies and rejections obtained by ATLAS are shownin Table 1.1 3: eventually, multivariate techniques will allow to increase the efficiency by 4 to8 %, and the rejection by 40 to 60 %.1.2 MuonsBoth experiments use a combination of the Tracking and Muon detectors to identify and measuretheir momentum between 4 GeV and 1 TeV. Figure 1 shows the ATLAS muon identificationTable 1: Electron identification efficiency and jet rejection of the ATLAS cut-based algorithm, averaged over allET > 17 GeV. The jet rejection factor needed ( 105 ) can be achieved via two sets of cuts.Cuts Efficiency (%) Jet rejectionIsolated electrons Electrons in jetsLoose 87.96 ± 0.07 50.8 ± 0.5 567 ± 1Medium 77.29 ± 0.06 30.7 ± 0.5 2184 ± 13Tight(TRT.) 61.66 ± 0.07 22.5 ± 0.4 (8.9 ± 0.3) 104Tight (isol.) 64.22 ± 0.07 17.3 ± 0.4 (9.8 ± 0.4) 104Figure 1: ATLAS muon identification performance in physics events, as a function of the pseudo-rapidity η.η-3 -2 -1 0 1 2 3efficiency00.51foundgoodATLAS directttStaco(a) muon reconstruction effi-ciencyη-3 -2 -1 0 1 2 3 (/event)ηdN/d-410-310-210-1101>3Tp>10Tp>20Tp>50TpATLAS directttStaco(b) number of fake muonsper eventefficiency and fake rejection 3: for PT > 20 GeV, one expects less that 2.10−3 fake muons perevent.The J/ψ will be one of the first signals reconstructed at the LHC startup. The CMScollaboration expects, with only 3 pb−1, about 7.104 J/ψ → µ+µ− reconstructed decays, withtransverse momenta between 5 and 40 GeV. Making use of the large B lifetime, the promptlyproduced J/ψ will be separated from those issued from B decays, and the cross section measuredwith a systematic uncertainty of 15 %. But the reconstructed dimuon invariant mass will alsobe a probe of the alignment and muon reconstruction quality: it is expected to be 34.2 MeV atthe startup, compared to 29.5 MeV with an ideal alignment 5.1.3 Tausτ reconstruction is tricky and relies ( not for the first data but soon after ) on multivariate tech-niques. ATLAS has followed two approaches: a calorimeter based and a track based algorithm.Typical efficiencies are 30 % for a jet rejection of about 103. With 100 pb−1 one expects clearsignals for Z and W in τ decays. These will eventually be used to set the transverse missingenergy scale to a few % 3.2 Z and W leptonic decays reconstructionThe W and Z production cross sections are proportional to the beam energy and ATLAS ex-pects, at 14 TeV and for 50 pb−1, 2.48 104 Z → e+e− and 2.56 104 Z → µ+µ− reconstructeddecays. Figure 2 shows the reconstructed Z mass in the three leptonic decay channels. These Zdecays are often called candles, because they will initially be used to shed light on our detectorunderstanding, via the tag and probe method: a tight identification is requested for one of theleptons; the second lepton can then be used to measure the efficiency of any identification cut.Figure 2: Reconstructed Z mass in leptonic decaysInvariant Mass Mee (GeV)0 20 40 60 80 100 120 140 160 180 200Events/GeV0500100015002000250030003500Events/GeV  -150 pbExtrapolated BackgroundSignal 50)×QCD MC stat (ATLAS(a) Electron channel)2 (GeV/cµ µm0 20 40 60 80 100 120 140 160 180 2002Events/1 GeV/c-110110210310µ µ -> tt τ τZ->ν µW -> QCD bkgµ µZ->-1CMS preliminary, L = 10 pb(b) Muon channel (GeV)ττVisible m0 20 40 60 80 100 120 140 160 180Expected events in 100pb-1050100150200250ATLASMean= 53.8 GeV= 10.6 GeVσSignalBackground(c) Tau channelFigure 3: Transverse mass (MT ) distribution(a) W → µν channel)2 (GeV/cTM0 20 40 60 80 100 120 140 160 180 200a.u.00.010.020.030.040.050.06νµ→ from WTMµµ→ template from ZTM-1L = 10 pb CMS Preliminary(b) Comparison between realand fake W decays, producedusing Z → µµ events)2 (GeV/cT M0 20 40 60 80 100 120 140 160 180 200 a.u.00.010.020.030.040.050.06Bg Norm Iso < 0.09 Bg Norm Iso > 0.09 -1CMS Preliminary. L= 10 pb(c) QCD background sam-ple, produced by revertingthe isolation cutWith 100 pb−1, such studies will allow to reach an accuracy of about 2 %.Z decays will also be used to spot large scale inhomogeneities that the local calibrationwould not have seen, differences in the energy scale of different detector pieces, and residualmisalignment. With 100 pb−1, ATLAS expects to have enough reconstructed Z decays to checkthe muon momentum resolution and the electron energy scale at the per cent level.W leptonic decays are selected by the presence of an identified isolated lepton, of hightransverse momentum ( typically above 20 GeV ), and some missing energy due to the escapingneutrino ( typically above 25 GeV ). Assuming NLO cross sections, with 10 pb−1, CMS expectsabout 28 103 W → eν and 64 103 W → µν events, the dominant background being QCD events.Two variables can be used to fit the signal and background level: the lepton transverse mo-mentum, and the lepton-neutrino transverse mass, shown in Figure 3. The major issue for suchanalyses being the description of these distributions by the simulation, the CMS collaborationhas shown how one can measure them using data. For the background, this is done by revertingthe lepton isolation cut, to select a pure background control sample. For the signal, one usesZ → l+l− events: if one of the leptons passes the W → l+ν selection cut, the second leptonis “removed” from the event, thus creating fake missing energy. The transverse mass it thenreconstructed. The mass difference between the Z and the W, as well as the neutrino acceptanceare taken into account 5. Figure 3 shows the distributions obtained with 10 pb−1.3 Early physics studies using Z and WThe reactions pp → W + X and pp → Z + X with subsequent W and Z leptonic decays willbe among the first to be measured at the LHC. They are theoretically well understood: thecalculation of the total cross-section at the NNLO has been done and the uncertainty due to themissing higher orders in perturbation theory is estimated to be less than 1 %. Nevertheless, theuncertainty related to the use of different PDF parameterizations is estimated to be about 6 %4.The uncertainty on the signal acceptance is evaluated to be about 2.3 % 3,4. The contributionto this number from the PDF uncertainty has been estimated by varying the eigenvectors of thesame PDF parameterization (CTEQ6.5) and amounts to about 1 % 3.ATLAS has estimated that the Z and W cross sections could be measured with a precision of3 to 5 % with an integrated luminosity of 50 pb−1. To this number, one must add the uncertaintyon the luminosity, which is expected to be about 10 %. With 1 fb−1, the measurement precisionshould go down to 2.5 %, and the uncertainty on the luminosity be 5 %.With 10 fb−1, the precision on the W mass expected by CMS is δmW = 15(stat) ⊕30(experimental) ⊕ 10(theoretical) = 35 MeV. A recent study has revisited the various pos-sible improvements and set a challenging goal of about 7 MeV per decay channel 6. Butthe ATLAS collaboration has concentrated on the early measurements, with 15 pb−1. Inthe electron channel, the fit of the transverse momentum distribution yields a precision ofδmW = 120(statistical) ⊕ 117(systematical) MeV, where the systematics uncertainty is dom-inated by the electron energy scale. In the muon channel, the fit of the transverse mass givesδmW = 57(statistical)⊕ 231(systematical) MeV, where the dominant contribution comes fromthe calibration of the recoil energy . PDF uncertainties contribution is 25 MeV, other theoreticaluncertainties are expected to be smaller.W+ and W− are not produced with the same rapidity distribution. The η distribution ofthe ratio (W+ −W−)/(W+ +W−) is sensitive to PDF’s. With 100 pb−1, the measurementprecision will be comparable to the present CTEQ6 predictions, and the first constraints willthus be set 5.4 ConclusionThe ATLAS and CMS collaboration are obviously eagerly waiting for data, and have used theextra delay to make good progress on the realism of the detector simulation, the understandingof the expected performance, calibration and alignment studies, and to revisit the physics goalsestablished over the years. Even with a reduced centre of mass energy of 10 TeV and about 200pb−1, the copious “candle” signals ( J/ψ, Z and W ) will allow us to shed light on the detectorunderstanding, and perform some interesting measurements.References1. Masaya Ishino and Marina Giunta, same Conference proceedings.2. M.Aharrouche, et al. The ATLAS Experiment at the CERN Large Hadron Collider.JINST 3:S08003,2008.3. Expected Performance of the ATLAS experiment, CERN-OPEN-2008-020.4. CMS Physics Performance TDR, CERN-LHCC-2006-021.5. CMS public notes BPH-070-002, EWK-07-002, EWK-08-0026. N. Besson et al., Eur.Phys.J.C57:627-651,2008.

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