Lepton charge asymmetry in inclusive pp → WX production at

s = 7TeV has been measured by the CMS detector at the LHC, using 36 pb−1 of data. These measurements, performed in both W → eν and W → μν channels, provide new insights into proton structure functions

Taroni, S.

English

Lepton charge asymmetry in inclusive $pp \rightarrow W X$ production at $\sqrt{s}  = 7~\mbox{TeV}$ has been measured by the CMS detector at the LHC, using 36~$\mbox{pb}^{-1}$ of data. These measurements, performed in both $W \rightarrow e \nu$ and $W \rightarrow \mu \nu$ channels, provide new insights into proton structure functions

Taroni, Silvia

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Measurement of the Lepton Charge Asymmetry in Inclusive W Production in pp Collisions at $\sqrt{s}$ = 7 TeV at the CMS experiment

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DOI 10.1393/ncc/i2011-11093-6Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Measurement of the lepton charge asymmetry in inclusiveW production in pp collisions at√s = 7TeV at the CMSexperimentS. Taroni on behalf of the CMS CollaborationUniversita` di Perugia and INFN, Sezione di Perugia - Perugia, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 21 Dicembre 2011)Summary. — Lepton charge asymmetry in inclusive pp → WX production at√s = 7TeV has been measured by the CMS detector at the LHC, using 36 pb−1of data. These measurements, performed in both W → eν and W → μν channels,provide new insights into proton structure functions.PACS 14.70.Fm – W bosons.1. – IntroductionIn pp collisions, W bosons are produced primarily via the processes ud¯ → W+ anddu¯→ W−. Due to the presence of two valence u quarks in the proton, there is an overallexcess of W+ over W− bosons. Measurement of this production asymmetry betweenW+ and W− bosons as a function of boson rapidity can provide new insights on the u/dratio and the sea antiquark densities in the ranges of the Bjo¨rken parameter x [1] probedin pp collisions at√s = 7TeV. However, due to the presence of neutrinos in leptonicW decays the boson rapidity is not directly accessible. The experimentally accessiblequantity is the lepton charge asymmetry, defined to be:A(η) =dσ/dη(W+ → l+ν)− dσ/dη(W− → l−ν)dσ/dη(W+ → l+ν) + dσ/dη(W− → l−ν) ,(1a)where l is the daughter charged lepton, η is the charged lepton pseudorapidity in theCMS lab frame (η = − ln[tan(θ/2)] where θ is the polar angle with the beam), anddσ/dη is the differential cross section for charged leptons from W boson decays. Thelepton charge asymmetry can be used to test SM predictions with high precision. Dueto the V − A structure of the W boson couplings to fermions, theoretical predictionsof the charge asymmetry depend on the transverse momentum (pT ) threshold appliedon the daughter leptons. For this reason, we measure A(η) for two different chargedlepton pT (plT ) thresholds, 25GeV/c and 30GeV/c. The measurement is performed inc© CERN on behalf of the CMS Collaboration under CC BY-NC 3.0 248MEASUREMENT OF THE LEPTON CHARGE ASYMMETRY AT CMS 249Fig. 1. – Comparison of the measured lepton charge asymmetry to different PDF models for a)lepton plT > 30GeV/c and b) lepton plT > 30GeV/c. The error bars include both statistical andsystematic uncertainties. The PDF uncertainty band is corresponding to the 90% confidenceinterval (CI). The bin width for each data point is shown by the filled bars in figure b). Thedata points are placed at the centers of pseudorapidity bins, except that for display purposesthe first three data points are shifted +0.025 (−0.025) for electron (muon).both W → eν and W → μν final states using a dataset corresponding to an integratedluminosity of 36 pb−1 collected by the CMS detector in March-November 2010 [2].2. – Lepton identification and signal extractionA detailed description of the CMS experiment can be found elsewhere [3]. Thismeasurement is based on the excellent performance of the electromagnetic calorimeter(ECAL), of the muon system, and of the tracking system. Electrons are identified asclusters of energy deposited in the ECAL fiducial volume matched to tracks from theinner silicon tracker (silicon tracks). The W → eν candidates are selected by furtherrequiring electrons to be isolated, to have pT > 25GeV/c, |η| < 2.4, and to be associ-ated with one of the electron trigger candidates used to select the electron dataset. Noselections on missing transverse energy (EmissT ) are applied. According to Monte Carlo(MC) simulations, the data sample of selected electrons consists of about 28% QCDbackground events, about 6.5% electroweak (EWK) background events, and about 0.2%tt¯ background events. The events passing the above selection criteria are divided into sixbins of electron pseudorapidity (|ηe|): [0.0, 0.4], [0.4, 0.8], [0.8, 1.2], [1.2, 1.4], [1.6, 2.0],and [2.0, 2.4], for the measurement of the electron charge asymmetry. The fourth bin isreduced to a width of 0.2 in order to exclude the transition region between the ECALbarrel and endcaps.250 S. TARONI on behalf of the CMS COLLABORATIONA binned extended maximum likelihood fit is performed over the EmissT distributionto estimate the W → eν signal yield for electrons (N−) and positrons (N+) in eachpseudorapidity bin. The charge asymmetry is obtained from (N+ −N−)/(N+ + N−).Muons are obtained using a global track fit including both the silicon track hitsand muon chamber hits in order to improve the quality of the reconstructed candi-dates. The muon charge is identified from the signed curvature of the associated silicontrack. To reject the cosmic muons, the tracks have to come from the interaction point(|dxy| < 0.2 cm). The W → μν candidates are selected by further requiring the muon pTto be greater than 25GeV/c, |η| < 2.1, and that the candidate matches one of the muontrigger candidates. No isolation selection is applied. To reject the background, thereis a veto on the presence of other muons having pT > 15GeV/c and |η| < 2.4. FromMC studies, the expected QCD multijet, EWK, and t¯t backgrounds are about 13.0%,6.9%, and 0.3%, respectively. The W → μν signal estimation is done by fitting the dis-tribution of an isolation variable ξ = Σ(ET ) defined as the scalar sum of the transversemomenta of silicon tracks (excluding the muon candidate) and energy deposits in bothECAL and hadronic calorimeter (HCAL) in a cone ΔR < 0.3 around the muon direc-tion. An unbinned extended maximum likelihood fit to the ξ distribution is performedsimultaneously on the W+ → μ+ν and W− → μ−ν candidates to determine the totalW → μν signal yield and the charge asymmetry in each pseudorapidity bin.3. – Asymmetry resultFigure 1 shows a comparison of these asymmetries to predictions from the CT10WPDF model [4] and the MSTW2008NLO PDF model [5]. The central values of bothpredictions are obtained using the MCFM MC [6] and the PDF error bands are estimatedusing the PDF reweighing technique [7].Our data suggest a flatter pseudorapidity dependence of the asymmetry than the PDFmodels studied. The error bars of the experimental values include both the statisticaland the systematic uncertainties. The systematic uncertainties considered for both theelectron and muon channels are mainly due to the lepton charge misidentification rate,possible efficiency differences between the l+ and l−, lepton momentum (energy) scaleand resolution, and signal estimation.In summary, CMS has measured the lepton charge asymmetry in both the W → eνand W → μν channels using a data sample corresponding to an integrated luminosityof 36 pb−1 collected with the CMS detector at the LHC. In each pseudorapidity bin theprecision of the most inclusive measurements is less than 1.6% for both channels. Thishigh precision measurement of the W lepton charge asymmetry at the LHC provides newinputs to the PDF global fits.REFERENCES[1] Bjo¨rken J. D. and Paschos E. A., Phys. Rev., 185 (1969) 1975.[2] CMS Collaboration, JHEP, 04 (2011) 050 and references therein.[3] CMS Collaboration, JINST, 0803 (2008) S08004.[4] Lai H. L. et al., Phys. Rev. D, 82 (2010) 074024.[5] Martin A. D. et al., Eur. Phys. J. C, 63 (2009) 189.[6] Campbell J. M. and Ellis R. K., Phys. Rev. D, 62 (2000) 114012.[7] Bourilkov D., Group R. C. and Whalley M. R., hep-ph/0605240v2 (2006).

Measurement of the lepton charge asymmetry in inclusive

W production in pp collisions at √s = 7TeV at the CMS

experiment

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Measurement of the lepton charge asymmetry in inclusive W production in pp collisions at √s = 7TeV at the CMS experiment

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