As early physics programs with the ATLAS detector, we present J/psi and Upsilon cross section and spin alignment measurements, and the strategy for the jet energy calibration

HARA, K

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ATL-PHYS-PROC-2009-14011December2009December 10, 2009ATLAS commissioning and early physics of resonance and jet productionK. Hara, for the ATLAS collaborationUniversity of Tsukuba, School of Pure and Applied SciencesAbstractAs early physics programs with the ATLAS detector, we present J/ψ and ϒ productioncross section and spin alignment measurements, and the strategy for the jet energy calibra-tion. 11This note is prepared for the proceedings of the 5-th International Conference on Quarks and Nuclear Physics,Beijing, September 21-26, 2009.1 IntroductionThe ATLAS detector[1] is being commissioned to explore the physics in the energy regime accessible atthe Large Hadron Collider (LHC)[2], the 14 TeV proton-proton collider at CERN. The ATLAS physicsprogram and expected results in the early data taking period are discussed in Ref. [3]. Here we summarizethe J/ψ and ϒ production and jet energy scale determination.The LHC will produce charm and beauty quarks in copious quantities, even in its early low luminosityruns. The J/ψ and ϒ onia will provide important signals for detector calibration and many physics studiesespecially in their leptonic final states which are well separated from the huge hadronic background. Theseparation of prompt and indirect production, for example, will be achieved by means of the tracks’impact parameters. Since such measurements require a good understanding of the detector alignmentand calibration. From the physics point of view, the production mechanism of quarkonium has manyfeatures which are still unexplained and even early measurements can provide important information.The LHC allows us to explore QCD to much shorter distances. Among other studies, the measure-ment of the jet cross section to high momentum is of prime importance to find any deviation from theStandard Model expectation. The jet energy calibration is crucial in order to perform reliable measure-ments.2 J/ψ and ϒ production2.1 OverviewThe CDF J/ψ production cross section measurement [4] highlighted a mysterious aspect in the oniumproduction mechanism, in that the measured value was nearly 50 times larger than the prediction by theColor Singlet Model (CSM)[5, 6], with different pT dependence. The Color Octet Model (COM) wasthen proposed [7] to rescue the situation, where quarkonia produced with color becomes a singlet byradiating soft gluons in longer distance. The COM was successful in describing the cross section andits pT dependence but turned out not to describe the polarization parameter1 α in quarkonium decay[8].The COM predicts positive α whereas the Tevatron measurements are in general negative for J/ψ .The Tevatron measurements, however, are also not yet conclusive. The ϒ spin alignment measure-ment [9] by D0 Run2 is not consistent with CDF Run1 measurement, and both disagree with theoreticalmodels. A recent calculation to NNLO* based on the CSM[10] alone can be consistent with theCDFϒ cross section, providing a negative α prediction consistent partially with D0 measurement. From theexperimental systematics point of view, the limited cosθ   coverage (typically  cosθ    0  6 by CDF)makes the measurement less reliable.2.2 Expected ATLAS performanceWe present the expected quarkonium production measurements in the muonic decay mode. Two triggerschemes will be used, giving two separate data samples. The first is a dimuon trigger, requiring twomuons with pµ1T  6 GeV/c and pµ2T  4 GeV/c. The second is a single muon trigger, requiring onemuon with pT10 GeV/c. The first favors lowcosθ  and the second favors highcosθ  .In the first sample, at the offline level pairs of reconstructed muons are taken to form the J/ψ candi-dates, while in the second we form J/ψ’s from muon plus track pairs. In each case the common vertex ofa dimuon or muon and track is reconstructed at the offline analysis stage to separate the prompt and in-direct production. By requiring the pseudo-proper time to be less than 0.2 ps, prompt Jψ can be selectedat 93% efficiency with 92% purity[3].1The polarization parameter α is defined by dNd cosθ  C32α 	 6  1  α cos2 θ  , where θ  is the angle between µ 	 in the ψrest frame and ψ direction in the laboratory frame.1By combining the two data samples, a wider and flat cosθ   range will be covered as shown in Fig.1, where reliable spin alignment measurements should be possible both for J/ψ and ϒ.Figure 1: cosθ   acceptance of (left) J/ψ and (right) ϒ, shown for µ6µ4 and µ10 triggers. The events aregenerated flat in cosθ   .We have estimated the measurement sensitivity to α at 10 pb  1 of data at  s  1  4 TeV[3]. The massresolution of J/ψ (ϒ) in the dimuon decay mode is 53 (161) MeV, with negligible background consistingmainly of Drell-Yan events. The mass resolution is not degraded in single-muon triggered events, but thehadronic background will be substantial. The signal-to-noise ratio in the mass window is approximatelyone for J/ψ and 1/4 for ϒ. The background subtraction is manageable for J/ψ with 10 pb  1 but is notjustified for ϒ, for which we require more luminosity.The precision of the α measurement is shown in Fig. 2, estimated at 10 pb  1. The input α is recon-structed with a precision of 0.02 to 0.06 in J/ψ pT range of 10pT20 GeV/c, which is comparableto the Tevatron results obtained with  1 fb  1 of data. The determination is less precise for ϒ due to thelimited achievable signal-to-noise ratio.3 Jet energy scaleThe large center-of-mass energy of the LHC allows us to explore the QCD to much shorter distance.Hundreds of jets with ET1 TeV can be produced for 10 pb  1, whereas the maximum ET attainable is0.7 TeV at the Tevatron.The jet production measurement involves a number of theoretical uncertainties such as the partondistribution functions (PDFs), and in the factorization and renormalization scales in the QCD calculation.Among the experimental uncertainties, the jet energy scale (JES) contributes most significantly. Thesteeply falling jet ET spectrum requires good control of the jet energy. At 1 TeV, 1% (5%) energy scaleuncertainty corresponds to 10% (30%) error on the cross section. The typical cross section uncertainty atET  1 TeV due to the QCD scale uncertainty is 10% for pmaxT  2µ2pmaxT , and to the PDF uncertaintyis 15% estimated using CTEQ6, 6.1 PDF set. Comparing with these theoretical uncertainties, control ofJES to 1-2% is regarded as the target to achieve.The jet energy measurement starts from signals recorded in the calorimeter cells, followed by twomajor steps: detector effects corrections (e  g  non-uniform response and non-compensation[3]) and jet-to-particle corrections. The latter step includes the effects such as clustering, initial and final state radia-tion (ISR and FSR), and the underlying event (UE). The final step is to translate the energy to the partonlevel.2Figure 2: Reconstructed α at 10 pb  1 as a function of quarkonium pT, shown for J/ψ for input α  0  1and for (with circles) ϒ for input α  0.The validation of the jet calibration will be performed in-situ using physics processes. QCD dijetevents are used to check the uniformity of the calibration over the detector prior to the absolute JEScalibration, which is performed using energy-balanced processes. Among many possible processes, wepresent the method using the pT balance between the jet and γ /Z boson. At very high jet energies, thestatistics of these events vanishes. After the validation of JES in the limited pT range, higher JES willbe validated using QCD multijet events by balancing the leading energy jet to the momentum sum of theother jets.In the energy range 10ET100  200 GeV, the Z-jet balance method is effective, as shown inFig. 3. The pT of Z is reconstructed from the dilepton into which it decays. A statistical uncertainty of0.8%-1% can be achieved below 200 GeV with an integrated luminosity of 100 pb  1. The method issensitive to the correct modeling of the radiation, given by the difference between ALPGEN and PYTHIAdata samples. A typical JES systematics is 5-10% at low ET due to ISR/FSR and UE uncertainties and1-2% at ET  200 GeV.The energy range 100  200ET500 GeV is covered by the γ-jet balance method (see Fig. 4).The main background is QCD jets, where one of the jets is misidentified as a photon. The leadingjet produced in the hemisphere opposite to the leading photon is examined. Two photon selections areconsidered with tight selection to reduce further the QCD background. At 100 pb  1 a statistical precisionof 1-2% is achievable with another 1-2% from ISR/FSR and UE uncertainties.In the higher energy range (ET500 GeV), the leading jet in multijet events is balanced to the sumof lower energy jets with the JES, which is already calibrated. This method is examined with ALPGENand PYTHIA generators. A statistical uncertainty of 2% is achieved with 1 fb  1 up to 1 TeV while asystematics of 7% is expected due to uncertainty in lower energy jet JES.To further improve the JES, understanding of ISR/FSR and UE is crucial. ATLAS plans to studythese first with the beam[11]. The azimuthal dijet decorrelation is another piece of information whichcan quantify the ISR contribution[12].3Figure 3: Jets energy scale obtainable by Z-jet pT balance method.Figure 4: Jet energy scale obtainable by γ-jet pT balance method.4Figure 5: Jet energy scale obtainable by multijet pT balance method at 1 fb  1.4 SummaryThe ATLAS detector is being commissioned to explore new energy regimes. In early data with 10pb  1 to 1 fb  1 integrated luminosity, ATLAS should provide important information on the quarkoniumproduction mechanism and the jet cross section.References[1] Aad G et al.(ATLAS Collaboration), J. of Instrumentation 2008 3: S08003[2] Evans L and Bryant P (editors), J. of Instrumentation 2008 3: S08001[3] Aad G et al.(ATLAS Collaboration), CERN-OPEN-2008-020[4] Abe F et al. (CDF Collaboration), Phys. Rev. Lett. 1992 69: 3704[5] Bodwin G T, Braaten E And Lepage G P, Phys. Rev.1995 D51: 1125[6] Braaten E and Fleming S, Phys. Rev Lett. 1995 74: 3327[7] Kramer M, Prog. Part. Nucl. Phys. 2001 47: 141[8] Abulencia A et al. (CDF Collaboration), Phys. Rev. Lett. 2007 99: 132001[9] Abazov V M et al. (D0 Collaboration), D0 Note 5089-conf[10] Artoisenet P, Lansberg J P and Maltoni F, Phys. Rev. Lett. 2008 101: 152001[11] Kwee R, talk given at this conference[12] Moraes A et al., ATL-PHYS-PUB-2006-0135

ATLAS commissioning and early physics of resonance and jet production

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