An overview of W and Z production in high energy hadron collisions is given. W and Z cross section and asymmetry measurements from CDF and D0 are described, with particular emphasis on recent results. The current status of precision W mass and width measurements is reported. The fundamental physics parameters that can be extracted from these measurements are described, and the relevance of W and Z production studies for the LHC is pointed out

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Waters, D.S.

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W & Z Production and Asymmetries at the TevatronD. S. Waters(for the CDF & DØ collaborations)Department of Physics & Astronomy, University College London, London WC1E 6BT, U.K.Abstract. An overview of W and Z production in high energy hadron collisions is given. W and Z cross section andasymmetry measurements from CDF and DØ are described, with particular emphasis on recent results. The current statusof precision W mass and width measurements is reported. The fundamental physics parameters that can be extracted fromthese measurements are described, and the relevance ofW and Z production studies for the LHC is pointed out.Keywords: W, Z, production, cross section, asymmetryPACS: 13.38.Be, 13.38.Dg, 14.70.Fm, 14.70.Hp1. INTRODUCTIONW and Z bosons are produced in abundance in high energy hadron collisions. Their large mass scale and well knownfermion couplings make their production cross sections accurately calculable in perturbative QCD, and their leptonicdecay modes provide distinctive experimental signatures that are relatively easy to identify, trigger on and separatefrom large backgrounds. In many respects vector boson production has become one of the most important “standardcandles” in experimental hadron collider physics. At the Tevatron, measurements ofW and Z inclusive cross sectionsare routinely used to validate detector and trigger performance and stability. Z → l+l− events in particular, due totheir negligible backgrounds and trigger redundancy as well as precisely known mass, are the most important samplesfor the experimental determination of energy and momentum scales and lepton identification efficiencies. Inclusivevector boson cross section measurements are necessary starting points for more detailed measurements, for exampledifferential cross section measurements, exclusive measurements (e.g. W + n− jet), measurements of rare processes(e.g. diboson production) and precision measurements of vector boson properties. At the Large Hadron Collider (LHC)the use of massive vector bosons as standard candles may be taken one step further with their yields being used toeither compute, or provide an alternative definition of, collider integrated luminosities.In the following sections the basic physics of vector boson production at hadron colliders is reviewed, and therelevant parameters of the CDF and DØ detectors are stated. Measurements of inclusive and differential productioncross sections are discussed in section 4, as well as some of the particle properties that can be extracted from thesemeasurements. Asymmetry measurements in Z and W production are presented in section 5. Finally, the status ofprecision direct measurements of theW boson mass and width at the Tevatron is given in section 6.2. MASSIVE VECTOR BOSON PRODUCTION AT HADRON COLLIDERSFigure 1 indicates the phenomenological ingredients required for a complete description of vector boson productionin hadron collisions. At leading order, a quark-antiquark pair annihilate to produce a W or Z boson, which subse-quently decays to a fermion-antifermion pair. Cross sections are computed as a convolution of partial cross sections(dσqq¯→W/Z→ll¯) over parton distribution functions (PDF’s; fi(x), where x is the fraction of the proton’s momentumcarried by quark flavor i):σpp¯→W/Z→ll¯ =∫∑i, j=u,d,s(,c,b)[ f qi (xp) fq¯j (xp¯)+ fq¯i (xp) fqj (xp¯)]×dσqq¯→W/Z→ll¯ dxp dxp¯ .The longitudinal momentum (or rapidity) distribution of the produced vector bosons depends directly on the PDF’s.The rate and kinematic distributions of the decay fermions additionally depend on the couplings and branching ratios,FERMILAB-PUB-06-538-EFIGURE 1. A schematic of vector boson production in high energy proton-antiproton collisions. Energetic quark-antiquark pairsannihilate to produce aW or Z boson, which subsequently decays, in this case to a lepton-antilepton pair. QCD effects need to betaken into account to give a full description of vector boson production kinematics, as described in the text.as well as the mass (MW/Z) and width (ΓW/Z), of the produced vector boson:dσqq¯→W/Z→ll¯(sˆ,θl ,φl) ∝ couplings ×[1(sˆ−M2W/Z)2+(ΓW/Z sˆ/MW/Z)2],where the parton-parton center of mass energy√sˆ = ECM√xpxp¯.QCD corrections to this process can be divided into perturbative and non-perturbative parts. Perturbative corrections,either in the form of fixed higher order matrix elements or parton showers, modify the vector boson productionkinematics - most notably the vector boson transverse momentum (pT ) distributions - and give rise to final states withmultiple, sometimes high-pT partons. Non-perturbative effects, sometimes understood as intrinsic-kT distributionsfor the colliding partons, are especially important for obtaining a complete description of the low end of the vectorboson pT spectra. Finally, QCD corrections modify the helicity and therefore decay angular distributions of the vectorbosons, a small but measurable effect that is important to take into account for precision measurements.By far the most important electroweak corrections to vector boson production and decay are those due to final statephoton radiation from charged leptons, where the effect on lepton kinematics and identification is appreciable. Initialstate radiation from the colliding quarks and, in the case ofW production, radiation from theW itself, are important toconsider in analyses of high-pT photons produced in association with a vector boson, but have a negligible impact onmore inclusiveW and Z measurements.3. THE CDF AND DØ EXPERIMENTS AT THE TEVATRONCDF and DØ are general purpose ∼ 4pi detectors, efficiently triggering on and identifying leptonic decays ofW andZ bosons over significant regions of phase space [1, 2]. A particular strength of the CDF detector is the excellent cen-tral tracking resolution δ (pT )/pT ≈ 0.0005× pT (GeV/c) [|η | < 1, beam constrained]. The central electromagneticcalorimeter has an energy resolution δ (ET )/ET ≈ 13.5%/√ET ⊕1.5%. Muon chambers cover the pseudorapidity re-gion |η | < 1 and forward calorimeters cover the region out to |η | < 3.6, important for the accurate reconstruction ofmissing-ET . DØ has significantly larger muon acceptance with chamber coverage and accurate momentum determi-nation out to |η |< 2, and excellent hermeticity with calorimeter coverage out to |η |< 4.2.The performance of the Tevatron collider continues to increase, such that at the time of this meeting is has deliveredalmost 1.5 fb−1 to each experiment. The measurements presented here are based on the first ∼ 400 pb−1 of Run IIdata.4. VECTOR BOSON PRODUCTION CROSS SECTIONSThe measurements of inclusiveW and Z production cross-sections in leptonic decay modes provide a benchmark for allanalyses of events containing high pT leptons at the Tevatron. A compilation of both Run I and Run II measurementsfrom CDF and DØ, based on Run II datasets between 72 pb−1 and 350 pb−1, is shown in figure 2, where the goodagreement with predictions calculated at NNLO in QCD can be observed [3].FIGURE 2. A compilation of Run I and Run II Tevatron results on the measurement of (left) σW ×BR(W → lν) and (right)σZ×BR(Z→ l+l−). The measurements are compared to NNLO predictions; see text for further details.In electron and muon decay channels, the best measurements are systematically limited at the 1− 2% level, notincluding an overall 6% uncertainty on the integrated luminosity (completely correlated for all measurements froma single experiment, and partially correlated between CDF and DØ). The largest contributions to the non-luminositysystematic derive from PDF uncertainties, and residual experimental uncertainties on lepton identification efficienciesand backgrounds. Interestingly, with non-luminosity systematic uncertainties of ∼ 2%, and given the uncertainty onthe theoretical cross section predictions of approximately 2%, the CDF measurements are already providing a crosscheck of the integrated luminosity measured for that experiment 1.The ratio ofW to Z leptonic cross sections can be written as:R=σW ×BR(W → lν)σZ×BR(Z→ l+l−) =σWσZ· ΓZΓZ→l+l−· ΓW→lνΓW.With the ratio of inclusive production cross sections taken from a NNLO calculation, the measurement of the Z leptonicbranching ratio from LEP and a Standard Model calculation of theW leptonic decay partial width, a measurement of Rcan therefore be interpreted as an indirect determination of the fullW decay width. The CDF 72 pb−1 analysis resultsin a measurement ΓW = 2.092± 0.042 GeV that is already comparable in precision with the world average result.Analyses are currently being developed with the goal of minimizing the systematic uncertainty on the cross sectionratio rather than the individualW and Z cross sections, in order to further improve the indirectW width determination.Tau decay modes of W and Z bosons are experimentally much more challenging in many respects, includingtriggering, identification and separation from backgrounds. However the physics reward is significant, allowing testsof 3rd generation lepton universality and establishing a benchmark for many searches, most notably the search forthe MSSM Higgs bosons. Both CDF and DØ have recently performed cross section measurements for Z → τ+τ−production, where one τ decays leptonically and the other hadronically [4]. The reconstruction of the hadronic taudecay modes involves combining information from both the tracking and calorimetry, with constraints from the taumass and decay topologies. Figure 3 shows the mass distributions from the two analyses, showing good agreementwith Monte Carlo after applying background corrections. The corresponding cross sections:σZ×BR(Z→ τ+τ−) = 237±15 (stat.)±18 (syst.)±15 (lum.) pb [DØ; 226 pb−1]σZ×BR(Z→ τ+τ−) = 265±20 (stat.)±21 (syst.)±15 (lum.) pb [CDF; 350 pb−1]agree very well with measurements in the electron and muon channels. In addition CDF have a measurement of theW production cross section in the hadronic tau decay channel using 72 pb−1, and a corresponding determination ofthe ratio BR(W → τν)/BR(W → eν) = 0.99±0.04 (stat)±0.07 (syst). An improved measurement of this ratio withmuch higher integrated luminosity will be particularly interesting to compare to measurements from LEP II.1 Integrated luminosity corrections for the DØ experiment are currently being re-evaluated.FIGURE 3. (left) The invariant mass distribution for the muon and tau-track(s) for a DØ analysis of pp¯→ Z(+X)→ τ(µ)τ(h/e);(right) the visible mass for a CDF analysis of pp¯→ Z(+X)→ τ(e)τ(h). See text for further details.4.1. Differential Cross SectionsCDF and DØ have moved on from inclusive cross sections to the measurement of differential cross sections.Measuring the Drell-Yan cross section dσl+l−/dM over as large a mass range as possible controls an importantbackground to searches for new physics in dilepton channels. Both the mass and the rapidity 2 differential crosssection dσl+l−/dY can, with sufficient integrated luminosity, be used to constrain PDF’s and test higher-order QCD.Figure 4 shows measured Drell-Yan mass (Z boson rapidity) differential cross sections in muon (electron) channelsfrom CDF (DØ), both with 337 pb−1 of data. The agreement with theoretical predictions, after careful unfolding ofthe data for smearing effects, is very good.Differential cross section measurements for W ’s are harder than for Z bosons, due to the incomplete kinematicreconstruction of events with neutrinos in the final state. As a first attempt, the CDF experiment has measured thecross section for W production with forward electrons (1.2 ≤ |ηe| ≤ 2.8) in 223 pb−1 of data, using silicon-onlysmall angle tracking and with a different triggering strategy to central leptons. The ratio of central to forwardW crosssections is particularly interesting:Rcentral/ f orwardCDF = 0.925±0.033 .The prediction for this ratio depends on the choice of PDF set:Rcentral/ f orwardCTEQ 6.1 = 0.924±0.037 , Rcentral/ f orwardMRST 01E = 0.941±0.012 .Although not yet significant, there is clearly hope that with more data, this and other differential cross sectionmeasurements at the Tevatron can yield useful inputs to global PDF fits.5. ASYMMETRIESIn addition to cross sections, measured asymmetries are particularly useful for the extraction of underlying physicsparameters such as electroweak couplings and PDF’s. Many systematics cancel in the measurement of asymmetriesas opposed to absolute cross sections, and in certain cases (most notably the search for Z′ bosons), hints of new orunexpected physics may first turn up as asymmetry rather than rate anomalies.2 The rapidity Y is a function of energy and longitudinal momentum: Y = 12 ln(E+pZE−pZ)FIGURE 4. (left) The differential cross section for dimuon pairs as a function of dimuon mass from CDF, normalized to the Zpole cross section. Note the good agreement between data and theory over four orders of magnitude in cross section. (right) Theunit normalized differential cross section for Z production as a function of vector boson rapidity, measured by DØ in the electronchannel.The Drell-Yan forward-backward asymmetry compares the fractions of events for which the negatively chargedlepton is produced in the forward and backward hemispheres, defined in the parton-parton center of mass system:AFB =σF −σBσF +σB, where σF(B) =∫ 1(0)0(−1)dσdcosθ ∗dcosθ ∗ .The result of a recent CDF update to their published AFB analysis [5] is shown in figure 5. The structure around theZ pole, as well as the measured asymmetry at high mass where indications of new physics may first be expected tobe observed, agree well with theoretical predictions. The previous analysis has also used the measured asymmetry toplace constrains on the neutral current light quark couplings. Although less precise, the Tevatron AFB data, along withdata from the analysis of deep inelastic scattering at HERA, do break a degeneracy in the determination of light quarkcouplings at LEP, providing an important confirmation of the Standard Model.The observedW charge asymmetry, defined with respect to the decay lepton direction, is a combined effect of boththe underlyingW production asymmetry and (V −A) decay asymmetry:A(ηl) =dσ+/dηl−dσ−/dηldσ+/dηl +dσ−/dηl= A(yW )⊗ (V −A) .The expectedW charge asymmetry is sensitive to the assumed PDF, in particular the (d/u) ratio at high-x, and benefitsfrom increased statistics with respect to the Z rapidity measurement. Important experimental issues are forwardlepton identification and, crucially, well controlled charge mis-identification rates in order to correct the observedasymmetry for any dilution due to lepton charge mis-assignment. Figure 5 shows a recent W charge asymmetrymeasurement by DØ in the muon channel, where charge mis-identification rates were kept at the 10−4 level out to muonpseudorapidities of 2. Interestingly, the measurement uncertainties are now appreciably smaller than the variation intheoretical predictions that can be generated by varying the input PDF set. This indicates that this data would alreadycontribute additional constraining power to future global PDF fits. CDF have measured the asymmetry using electronsin a smaller dataset [6]. They have additionally shown that certain lepton kinematic regions are more sensitive to PDFvariations than others. Analyses are currently underway to fully exploit all the kinematic information available inWevents with the goal of unfolding directly back to the underlyingW production asymmetry.6. PRECISION MEASUREMENTS OF THEW MASS ANDWIDTHThe precision determination of the W mass is one of the most important measurements to be performed at theTevatron. MW is a critical input to Standard Model fits which can constrain the mass of the unobserved Higgs bosonor, subsequent to a discovery of a Higgs boson, may give indications as to what lies beyond the Standard Model.FIGURE 5. (left) A measurement of the forward-backward asymmetry as a function of di-electron mass in Drell-Yan data atCDF. (right) TheW charge asymmetry measured as a function of the muon pseudorapidity in 230 pb−1 of DØ data. The blue curveis the expectation using the MRST-02 PDF set, while the yellow band is the range of predictions using the CTEQ6.1M error PDFsets. See text for further details.The width of the W is a less sensitive observable in global electroweak fits, but the direct measurement of the Wwidth nevertheless confirms a basic prediction of the Standard Model and provides a useful cross check of indirectmeasurements.The most precise top mass measurements at the Tevatron are at the level of 1%. By contrast the goal for the MWis a measurement substantially better than 0.1%. This highlights the extremely difficult nature of this measurement,requiring exquisite understanding ofW production and decay, detector response and the effect of backgrounds. CDFhave estimated that in the first 200 pb−1 of Run II data, they will measure the W mass with a precision of at least76 MeV, better than the combined Tevatron Run I uncertainty. DØ have started with a direct measurement of theWwidth in 177 pb−1 of Run II data: ΓW = 2.011± 0.093 (stat.)± 0.107 (syst.) GeV. Both experiments are finalizingnew measurements of theW mass, with an ultimate goal for Run II of 30−40 MeV per experiment.7. CONCLUSIONSThe CDF and DØ experiments have completed benchmark measurements of inclusiveW and Z production, as well asdifferential cross section and asymmetry measurements. These measurements are providing important information onfundamentalW and Z properties, as well as parton distribution functions and other aspects of vector boson productionphysics. With the basic signatures and experimental techniques involved in the measurement ofW and Z productionwell understood, attention is now focused on optimizing analyses for sensitivity to PDF’s and other physics parametersof interest. All of this information is being brought to bear in the precision measurement of theW mass in Run II, thefirst results on which are expected soon.Ultimately, measurements of W and Z production at the Tevatron will be used as inputs to the accurate modelingof vector boson production at the LHC, crucial for using these processes to help commission and calibrate LHCdetectors. In general, vector boson production promises to continue to be one of the most useful standard candles ofhadron collider physics.REFERENCES1. D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 032001 (2005).2. V. M. Abazov et al. (DØ Collaboration), "The Upgraded DØ Detector", hep-physics/0507191 (submitted to Nucl. In-strum. Methods Phys. Res. A).3. D. Acosta et al. (CDF Collaboration), Phys. Rev. Lett. 94, 091803 (2005); hep-ex/0508029 (submitted to Phys. Rev. D).4. V. M. Abazov et al. (DØ Collaboration), Phys. Rev. D 71, 072004 (2005); A. Safonov (for the CDF Collaboration),Nucl. Phys. Proc. Suppl. 144 323 (2005);5. D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 052002 (2005).6. D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 051104(R) (2005).

W & Z production and asymmetries at the Tevatron

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W & Z production and asymmetries at the Tevatron

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