The CDF and D0 experiments at the Tevatron have used p-pbar collisions at
sqrt(s)=1.96 TeV to measure the cross section of W and Z boson production using
several leptonic final states. An indirect measurement of the total W width has
been extracted, and the lepton charge asymmetry in Drell-Yan production has
been studied up to invariant masses of 600 GeV/c^2.Comment: 4 pages, 6 figures, proceedings of Moriond QCD 2003, Les Arcs 22-29
  march 2003, to be published by World Scientific (ed. T.T.Vanh

Dorigo, Tommaso

English

arXiv

The CDF and D0 experiments at the Tevatron have used p{bar p} collisions at {radical}s = 1.96 TeV to measure the cross section of W and Z boson production using several leptonic final states. An indirect measurement of the total W width has been extracted, and the lepton charge asymmetry in Drell-Yan production has been studied up to invariant masses of 600 GeV/c{sup 2}

Dorigo, T.

UNT Digital Library

Fermilab FERMILAB-Conf-03/193-E  July 2003W and Z Cross Sections at the TevatronT. Dorigo(on behalf of the CDF and D0 collaborations)Dipartimento di Fisica \G.Galilei", Via Marzolo 8,35131 Padova, ItalyThe CDF and D0 experiments at the Tevatron have used pp collisions atps = 1:96 TeV tomeasure the cross section of W and Z boson production using several leptonic nal states.An indirect measurement of the total W width has been extracted, and the lepton chargeasymmetry in Drell-Yan production has been studied up to invariant masses of 600 GeV=c2.1 IntroductionThe Tevatron collider has undergone a massive upgrade during the last ve years. The construc-tion of a new main injector with a recycler ring, and the improvements done to the antiprotonsource and booster ring promise an increase of instantaneous luminosity by an order of magni-tude over Run I. The CDF and D0 experiments have also recently completed a major upgrade.CDF was refurbished with an entirely new tracking system, with up to 8 silicon layers providingprecise measurement of track parameters close to the beam line, and with a new calorimeter forintermediate rapidities; the muon system has been extended to record tracks up to jj < 1:5.D0 was endowed with a 2T axial eld, and new silicon and ber trackers; mini-drift tubes forforward muons and a new preshower have also been added.The collider experiments have started recording pp interactions with their full functionalityduring 2002, and have used the physics-quality data collected till January 2003 to measure theproduction rate ofW and Z bosons, which constitute the starting point for many high-PT physicsstudies at the Tevatron, and are fundamental \standard candles" with which to understand andcheck detector performance.2 W and Z Cross SectionsW and Z bosons are produced at the Tevatron through qq(0) annihilation. The cleanest signaturesinvolve high-PT electrons or muons: W ! e, , and Z ! e+e , +  all enjoy very smallbackground contaminations. The W !  decay can also be observed with clarity althoughbackgrounds are much larger; other decays are hard to exploit or unobservable.D0 and CDF presented their rst W and Z cross section measurements atps = 1:96 TeVin the summer 2002 1. Many of those results have now been updated using total integratedluminosities of L = 72:0  4:3 pb 1 (CDF) and L = 31:8  3:2 pb 1 (D0).2.1 W Cross Section MeasurementsCDF collects W ! e decays with a triggerEntries  215992, GeV/cTM0 20 40 60 80 100 1202Events per 3GeV/c020040060080010001200140016001800200022002Events per 3GeV/c DATASum MCνµ→W MCµµ→Z MCντ→WQCDCosmic-1CDF Run II Preliminary, 72pbFigure 1: Transverse mass distribution of W ! candidates collected by the CDF II experimentin 72 pb 1 of Run II data.selecting high-ET central electron candidates; af-ter requiring one tight electron with ET > 25 GeVmatched to a track of PT > 10 GeV=c and missingtransverse energy E/T > 25 GeV, 38628 events areleft in the data. The main background source isfrom QCD dijet events where a jet mimics the elec-tron signal and large E/T is due to a second poorlymeasured jet: using events with non-isolated elec-tron candidates or small E/T , 134482672 eventsare estimated from that process. Additional back-grounds from W !  decays (768  22 events) ormisidentied Z ! ee decays (344  17) are esti-mated from Monte Carlo simulations.The acceptance is Ae = 23:40:05 (stat.)0:70(syst.)%; the systematic error is mainly due to theuncertainty in the parton distribution functions (PDF),that aect the fraction of decays yielding centralelectrons by 0:58%, and to the knowledge of theamount of material in the tracking volume (0:29%).The result is WB(W ! e) = 2:64  0:01 (stat.) 0:09 (syst.) 0:16 (lum.)nb, in goodagreement with NNLO calculations 2 (2:731  0:002 nb).CDF also measures WB(W ! ). From central high-PT muon triggers, events with aclean muon candidate are selected if the muon has PT > 20 GeV=c and if E/T > 20 GeV.The transverse mass spectrum of the 21; 599 W candidates is shown in Fig. 1. Backgroundsin this channel include cosmic rays, QCD processes, and misidentied boson decays (Z ! ,W ! ); their sum is estimated at 10:820:18 (stat.) 0:96 (syst.)%. The total acceptance isA = 14:80:1 (stat.) 0:5 (syst.)%; systematics are dominated by the uncertainty in the PDF(0:41%) and by the measurement of the W recoil (0:23%). The result is WB() = 2:64 0:02(stat.)0:12 (syst.) 0:16 (lum.)nb, again in good agreement with NNLO calculations.W !  candidates can also be selected at CDF by collecting events with a E/T > 25 GeVhardware trigger at level 1, complemented by a subsequent software lter for  identication attrigger level 3. Monojet candidates are kept if they have a single ET > 25 GeV jet containingone PT > 4:5 GeV=c charged track in a 10Æ cone and no other track within 30Æ, and if E/T > 25GeV; tight electron events from W ! e decay are explicitly removed. 2345 events pass theselection, with an estimated background of 61261 events mainly due to QCD processes. Fromthose numbers CDF computes WB() = 2:62  0:07 (stat.) 0:21 (syst.) 0:16 (lum.)nb.Moreover, using the previously excluded W ! e signal in the same dataset, it is possible toextract the ratio of coupling constants G=Ge = 0:99 0:04 (stat.) 0:07 (syst.).)2 (GeV/ceeM40 60 80 100 120 140 160 180 2002Evts/ GeV/c050100150200250Opposite Sign (1830) ee DATA→Z  ee  MC →Z CDF Run II Preliminary-1 L dt = 72.0 pb∫ (GeV)-µ+µM0 50 100 150 200 250 300Events / 3 GeV020406080100120140160 D0 Run II PreliminaryDataMCFigure 2: Left: invariant mass distribution of electron-positron pairs collected by CDF. Right: invariant massdistribution of +  pairs collected by D0.2.2 Z Cross Section MeasurementsTo select Z ! ee candidates, CDF requires two central electrons with opposite charge, ET > 25GeV, and PT > 10 GeV=c; they must have invariant mass in the 66 < Mee < 116 GeV=c2range. 1830 evts are thus collected (see Fig. 2, left), with 10  5 estimated from backgroundsources. The total acceptance is Aee = 11:49  0:07 (stat.) 0:64 (syst.)%, where systematicerrors are mostly due to PDF and modeling of tracker material. The cross section is measuredat ZB(Z ! ee) = 267:0  6:3 (stat.) 15:2 (syst.) 16:0 (lum.)pb, higher but consistent withthe NNLO calculation 2 of 250:5  3:8 pb.10 210 31Center of Mass Energy (TeV)σ W, Z × B (pb)DØ and CDF Run2 Preliminarypp_ → W+X → lν+Xpp_ → Z+X → ll+XDØ(µ) Run2DØ(e) Run2CDF(e) Run2CDF(µ) Run2DØ(e)CDF(e) CDF(µ)UA1 UA2DØ(µ)(W)Γ1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8Year1988199019921994199619982000200220042006)µUA1(e+UA2(e)CDF I (e))µD0 I (e+WorldAverage)µCDF II (e+Standard ModelCDF Run II Preliminary-1 L dt = 72.0 pb∫Figure 3: Left: comparison of cross section measurements for W and Z bosons with NNLO calculations (fulllines). Right: comparison of the new CDF result on the W boson width with previous determinations.Both CDF and D0 have updated their measurement of ZB(Z ! ). From 1632 eventscontaining two muon candidates with PT > 20 GeV=c, CDF measures ZB() = 246  6(stat.) 12 (syst.) 15 (lum.)pb. D0 bases the analysis on data collected between September2002 and January 2003. 1585 events are selected by requiring two muon candidates of PT > 15GeV=c, separated in     by more than R > 2:0 (Fig. 2, right); the estimated backgroundis 1:5 1:0%. The result is ZB() = 263:8  6:6 (stat.) 17:3 (syst.) 26:4 (lum.)pb.3 Forward-Backward Dielectron AsymmetryCDF uses 5438 events with 40 < Mee <102FBA-0.6-0.4-0.200.20.40.60.8140 60 100 200 300 600 MC-e+ e→* γZ/band includesseveral theoreticalcalculationsStatisticalTotalCDF Run II Preliminary-1L=72pb∫ (GeV)eeMFBAFigure 4: The forward-backward asymmetry of electron-positronpairs measured by CDF, compared to theory predictions.600 GeV=c2 from the pp ! Z()X !eeX process to measure the forward-backward charge asymmetry AFB overa wide range of Q2. Backgrounds aredue to QCD processes and are estimatedfrom same-sign or non-isolated electron-positron pairs; they amount to N ccQCD =21:55:9 events for central-central pairs,and to N cpQCD = 128  65 events forevents with one forward (jj < 2) elec-tron. The new AFB measurement isshown in Fig. 4; results agree with the-oretical predictions.4 W=Z Ratio and W WidthUsing the cross section measurements in the electron channel presented in Sec. 2, CDF computesthe ratio Re = WB(W ! e)=ZB(Z ! ee) = 9:88  0:24 (stat.)0:47 (syst.), lower butconsistent with NNLO predictions (Rth = 10:66  0:05) 2. Using the LEP measurement ofB(Z ! ee) 3 plus theoretical predictions for W=Z and B(W ! e) 2, the measurement of Recan be converted into an indirect determination of  W = 2:29 0:06 (stat.) 0:10 (syst.) GeV,in good agreement to the most recent average of 2:118  0:042 GeV 3.In addition, using their muon decay results (Sec. 2), CDF nds R = 10:69  0:27 (stat.)0:33 (syst.) and  W = 2:11 0:05 (stat.) 0:07 (syst.) GeV. The electron and muon channelresults can be combined into  W = 2146  78 MeV. Fig. 3 compares that determination toprevious measurements of the W boson width.5 Concluding RemarksW and Z cross sections in pp collisions have been shown to rise with s in accordance with NNLOcalculations. The Drell-Yan asymmetry has been probed up to the invariant mass Mee = 600GeV=c2. From the cross section measurements an indirect determination of the total widthof the W boson has been extracted, in good agreement with Standard Model predictions. Inconclusion, Run II at the Tevatron is starting to deliver: although the CDF and D0 detectorshave not yet been exploited to their fullest potential, the results discussed here have alreadyreached accuracy levels similar to Run I ones.References1. Pub. Proceedings 31st International Conference on High Energy Physics (ICHEP 2002),Amsterdam, The Netherlands, July 24-31, 2002. FERMILAB-CONF-02/256-E.2. W.J. Stirling, private communication.3. K. Hagiwara et al., Phys. Rev. D66 (2002) 010001.

W and Z Cross Sections at the Tevatron

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