The measurement ofWand Z plus jets final states at the LHC allows for stringent tests of perturbative QCD calculations in the context of the Standard Model and is sensitive to the possible presence of new physics. A study of jet

production in association with a vector boson in proton-proton collisions at a 7TeV center-of-mass energy is presented, using data collected with the CMS detector. The measured jet multiplicity distributions, corrected for efficiency and unfolded for detector effects, are compared with theoretical predictions. The study includes the

measurement of the normalized inclusive rates of jets and of the ratio W/Z. A test of the jet multiplicity scaling at

√s = 7TeV is also presented

Schizzi, A.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2013-11433-6Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Jets produced in association with W and Z bosons in CMSA. Schizzi on behalf of the CMS CollaborationUniversita` di Trieste and INFN, Sezione di Trieste - Trieste, Italyricevuto il 31 Agosto 2012Summary. — The measurement of W and Z plus jets final states at the LHC allowsfor stringent tests of perturbative QCD calculations in the context of the StandardModel and is sensitive to the possible presence of new physics. A study of jetproduction in association with a vector boson in proton-proton collisions at a 7TeVcenter-of-mass energy is presented, using data collected with the CMS detector.The measured jet multiplicity distributions, corrected for efficiency and unfolded fordetector effects, are compared with theoretical predictions. The study includes themeasurement of the normalized inclusive rates of jets and of the ratio W/Z. A testof the jet multiplicity scaling at√s = 7TeV is also presented.PACS 12.38.Aw – General properties of QCD.PACS 14.70.Hp – Z bosons.1. – IntroductionThe study of W and Z vector boson production in association with hadronic jets(V + jets) in proton-proton collisions represents an important test of perturbative QCDcalculations. The production cross section of a Z(W) boson has been calculated atNext-to-Leading-Order (NLO) precision for events with up to 4 associated jets. The un-certainties related to such calculations are not negligible due to their intrinsic complexityand to the uncertainties about the partonic distribution functions of the proton. More-over, V +jets processes represent an irreducible background to some important StandardModel Higgs boson decay channels, to the study of top quark processes and to interestingsearches for new physiscs. A good understanding of these processes is thus of primaryimportance in all those fields. We measured V +jets cross sections in order to provide anexperimental test of the theoretical predictions at the center of mass energy√s = 7TeVof the Large Hadron Collider (LHC). The dataset used for this analysis [1] correspondsto the data collected by the Compact Muon Solenoid (CMS) [2] experiment during the2010 data-taking period and consists of an integrated luminosity of (35.9± 1.4) pb−1.c© CERN on behalf of the CMS Collaboration under CC BY-NC 3.0 178JETS PRODUCED IN ASSOCIATION WITH W AND Z BOSONS IN CMS 1792. – Event selectionThe interesting events for this analysis are selected by means of a dedicated pathof the CMS trigger system, designed to select events containing at least a well isolatedsingle lepton. A trigger-level threshold on the transverse momentum of 17(15)GeV forelectrons (muons) has been used. The events with a positive trigger flag undergo thestandard reconstruction process: electron and muon tracks are built with a Gaussian-Sum-Filter procedure and a Kalman filter respectively, while the collection of hadronicjets is built up with a clustering algorithm starting from Particle Flow candidate particles.Isolated leptons are removed from the jet collection. We require reconstructed leptonsto have a transverse momentum pT > 20GeV, to be in the electromagnetic calorimeteracceptance (|η| < 1.4442 and 1.566 < |η| < 2.5) and to be matched to a trigger candidate.Eventually, we apply selection cuts to a set of variables inherent to the signal qualityand isolation of leptons in the detector and we reject spurious leptons from photonconversions. The set of selection cuts has been optimized on a simulated data samplefor the signal efficiency to be roughly equal to 80%. Events with two selected leptonsin the invariant mass window 60GeV < mll < 120GeV are used to build the Z sample,while events with a single selected lepton with transverse mass mT > 20GeV are used tobuild the W sample. The hadronic jets from the standard clustering algorithm are tunedto the correct energy scale (JES) and corrected for pile-up contributions. We eventuallycompute the jet multiplicity with a threshold on the transverse jet energy ET > 30GeV.3. – SimulationMonte Carlo V + jets events are produced with the matrix element MadGraph [3]generator at parton level. MadGraph is able to compute the matrix element for eventswith a vector boson and up to 4 outgoing partons. The final states are then processed withthe parton shower technique by means of the PYTHIA [4] generator. A similar procedureis used to generate the top quark background processes, while QCD, γ+jets and minimumbias events processes are completely generated within the PYTHIA framework. Particlecrossing and detection through the CMS detector is simulated within the Geant4 [5]framework. The comparison between experimental and simulated data shows indeed agood agreement of the jet multiplicity distributions up to four associated jets.4. – Detection efficiency and signal extractionIn order to get the correct measurement of the V +jets cross sections, it is necessary toaccurately estimate the detection efficiencies of the lepton decay products of the Z boson.All the efficiencies are measured with a data-driven Tag&Probe method which exploitsthe invariant mass distribution of the two selected leptons around the Z mass value. Weextract the signal and background components via an unbinned extended Maximum-Likelihood fit: both the signal and background distribution fractions are modelled withanalytical functions [6]. The efficiency-corrected jet multiplicity distribution has beentreated with an unfolding technique in order to deconvolve the detector effects and toprovide the corresponding particle-level distribution.The same Tag&Probe fitting technique has been exploited to extract the signal yieldfrom the raw Z sample, where the background is very little. The background componentis more prominent in the W sample, with a large fraction of both QCD and tt¯ events not180 A. SCHIZZI on behalf of the CMS COLLABORATIONFig. 1. – Left: exclusive Z+jets cross sections in the electron decay channel. The bottom part ofthe plot shows the behaviour of the cross section scaling: σ(V + ≥ n jets)/(V + ≥ (n− 1) jets).Right: values of the W/Z ratio: σ(W+ ≥ n jets)/σ(Z+ ≥ n jets) in the electron channel.eliminated by the selection cuts. A special template is used to fit the transverse massdistribution of the decay lepton in different channels of b-tagged jet multiplicity.5. – ResultsWe have measured the exclusive vector boson cross sections with respect to theassociated number of jets, providing the results for the following ratios (fig. 1):σ(V + ≥ n jets)/σ(V ) and σ(V + ≥ n jets)/(V + ≥ (n − 1) jets). In particular, thesecond ratio describes the scaling of the cross section as a function of the jet multiplicityand is predicted to be constant by NLO calculations. In order to probe for eventualdeviations from the theory we have compared the scaling with a linear function andthe resulting angular coefficient is compatible with zero. Thus we have no evidence,with the available amount of data, of significative deviations from the theoretical pre-dictions. Eventually, we computed the ratio between W and Z exclusive cross sections:σ(W+ ≥ n jets)/σ(Z+ ≥ n jets). The result of this ratio is essentially independent fromthe associated number of jets, in good agreement with Standard Model predictions.∗ ∗ ∗This work used the tools developed within the Italian MIUR project PRIN2008MHENNA.REFERENCES[1] The CMS Collaboration, JHEP, 01 (2012) 1.[2] The CMS Collaboration, JINST, 3 (2008) S08004.[3] Maltoni F. and Stelzer T., JHEP, 02 (2003) 027.[4] Sjo¨strand T., Mrenna S. and Skands P. Z., JHEP, 05 (2006) 026.[5] GEANT4 Collaboration, Nucl. Instrum. Methods A, 506 (2003) 250.[6] The CMS Collaboration, JHEP, 01 (2011) 1.

Jets produced in association with W and Z bosons in CMS

The measurement of W and Z plus jets final states allows for stringent tests of perturbative QCD calculations in the contest of the Standard Model and is sensitive to the possible presence of new physics. A study of jet production in association with a vector boson in proton-proton collisions at a 7 TeV center-of-mass energy is presented, using data collected with the CMS detector. The measured jet multiplicity distributions corrected for efficiency and unfolded for detector effects are compared with theoretical predictions. The study includes the measurement of the normalized inclusive rates of jets and of the ratio W/Z. A test of the jet multiplicity scaling at sqrt(s) = 7 TeV is also presented

Schizzi, Andrea

CERN Document Server

The measurement of W and Z plus jets final states at the LHC allows for stringent tests of perturbative QCD calculations in the context of the Standard Model and is sensitive to the possible presence of new physics. A study of jet production in association with a vector boson in proton-proton collisions at a 7 TeV center-of-mass energy is presented, using data collected with the CMS detector. The measured jet multiplicity distributions, corrected for efficiency and unfolded for detector effects, are compared with theoretical predictions. The study includes the measurement of the normalized inclusive rates of jets and of the ratio W/Z. A test of the jet multiplicity scaling at sqrt(s) = 7 TeV is also presented

SCHIZZI, ANDREA

Archivio istituzionale della ricerca - Università di Trieste

http://eprints.bice.rm.cnr.it/18088/1/ncc10427.pdf

Jets produced in association with W and Z bosons in CMS

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Archivio istituzionale della ricerca - Università di Trieste