The general strategy, as well as channel-specific details, applied to the
measurement of the top quark mass at the Tevatron in Run I are reviewed, and
the combination of the results obtained by the CDF and DO collaborations
presented. The accelerator and detector upgrades for Run II are described, and
expected improvements in the systematic uncertainties on the measurement
evaluated.Comment: Latex (Frascati.sty), 11 pages, 3 figs. Talk given at the 14th
  Rencontres de Physique de la Vallee d'Aoste, La Thuile, Aosta Valley, Italy,
  February 200

Brooijmans, G.

English

arXiv

The general strategy, as well as channel-specific details, applied to the measurement of the top quark mass at the Tevatron in Run I are reviewed, and the combination of the results obtained by the CDF and D0 collaborations presented. The accelerator and detector upgrades for Run II are described, and expected improvements in the systematic uncertainties on the measurement evaluated

Brooijmans, Gustaaf

UNT Digital Library

Fermilab FERMILAB-Conf-00/087-E  CDF/D0  May 2000TOP QUARK MASS MEASUREMENTS AT THE TEVATRONGustaaf BrooijmansFor the CDF and D CollaborationsFermi National Accelerator Laboratory, P.O. Box 500, Batavia, Illinois 60510AbstractThe general strategy, as well as channel-specic details, applied to the measurementof the top quark mass at the Tevatron in Run I are reviewed, and the combination ofthe results obtained by the CDF and D collaborations presented. The acceleratorand detector upgrades for Run II are described, and expected improvements in thesystematic uncertainties on the measurement evaluated.1 IntroductionThe mass of the top quark is a fundamental parameter of the standard model.Its precise measurement will not only improve understanding of the model, but incombination with other experimental data it will also allow to further check themodel's consistency. In addition, combined with the W mass measurement, it canbe used to constrain the mass of the Higgs boson.This paper is organized as follows: Section 2 addresses top quark produc-tion and decay topologies at the Tevatron; top-quark mass measurements performedin Run I are reviewed in Section 3, and the combined CDF and D result is given inSection 4. This is followed in Section 5 by a description of accelerator and detectorupgrades for Run II, and in Section 6 by the prospects for reduction of systematicuncertainties on the mass measurement. Conclusions are drawn in Section 7.2 Top Quark Production and Decay at the TevatronAt the Tevatron, top quarks are produced mainly in pairs. Atps = 1:8 TeV , about90% of the cross section is due to qq annihilation and 10% to gluon fusion. The ttproduction cross section has been measured by both the CDF 1) and D 2) col-laborations to be (mtop = 175 GeV=c2) = 6:5+1:7 1:4 pb, and (mtop = 172 GeV=c2) =5:9  1:7 pb, respectively. These results are in good agreement with the theoreti-cal predictions 3), which range from 4:7 to 5:5 pb. Single top quarks can also beproduced in electroweak processes, but due the smaller cross sections and largerbackgrounds, these have not yet been observed.Because of its large width, the top quark decays to a b quark and a Wboson before it hadronizes. The various possible W boson decays determine theseparate channels for mass measurement: events in which both W bosons decayhadronically comprise the \all hadronic" channel, and account for  44 % of allproduced tt events. Events in which one or both of the W bosons decay to e or form the \lepton+jets" ( 30 %) and \dilepton" ( 4.9 %) channels, respectively.Events in which one or both W bosons decay to  are not used in the mass andcross section measurements.23 Top-Quark Mass Measurements3.1 General StrategyWhile dierent experiments and dierent channels require specic analyses, thesemeasurements have enough in common so that one can speak of a general strategyto extract the mass of the top quark from data. This involves four steps: Apply a set of criteria to select the candidate sample, while being careful tochoose cuts that introduce little bias in the extracted mass. Fit each event's kinematics to the top quark pair production hypothesis, ex-tracting a tted top-quark mass mfit. Signal distributions are generated for many values of mtop, and a likelihood(L) t is used to compare the measured mfit distribution to summed signaland background distributions for each value of mtop. The t determines therelative contributions of signal and background. Fit the obtained lnL versus mtop, and extract the mass at the minimum of lnL.In all cases, tt signal distributions have been generated using the HER-WIG 4) Monte Carlo generator. The background is dominated by multiple-jet pro-duction and the production of a W boson in association with additional jets. Theformer is derived from data while the latter is modeled using the VECBOS 5) eventgenerator.3.2 All-Hadronic ChannelOnly CDF has published a mass measurement 6) in the all-hadronic channel. Eventsare required to have six or more jets, of which at least one is tagged as a b-quarkjet by the detection of a secondary vertex (\SVX tag"). Imposing that the massesof the top and anti-top quark be equal, and that the invariant mass of pairs of jetsforming W bosons equals the W -boson mass, yields a 3-C t. However, for eachevent, multiple combinations are possible in the association of jets to W bosons or bquarks, so the combination resulting in the lowest 2 is chosen as the right kinematict. The sample consists of 136 events (corresponding to an integrated luminosityof 109 pb 1), with the background contamination estimated at 1089 events. Themeasured top mass is m(top) = 186:0 10 (stat) 5:7 (syst) GeV=c2.33.3 Lepton + Jets ChannelThe D lepton + jets analysis 7) requires an isolated central lepton, four or morejets and at least 20 GeV missing transverse energy. Since the neutrino's longitudinalmomentum is not measured, the kinematic t to the tt hypothesis is now a 2C t.D also computes a discriminantD, based on 4 variables weakly correlated with thetop-quark mass. This discriminant is computed through a comparison of signal andbackground using classical methods, as well as a neural network. The likelihood tto determine the top mass is then performed on a two-dimensional grid of D versusthe top mass. Both discriminants yield results that are in good agreement with eachother, which when combined give m(top) = 173:3 5:6 (stat)  5:5 (syst) GeV=c2.The background contamination is estimated to be approximatively 50 out of the 77events that pass all selection criteria.CDF 8) uses a similar procedure, but nds that optimal results can beobtained by dividing the sample into 4 subsamples: events with two SVX tags(5 events), events with a single SVX tag (15 events), events without SVX tagsbut where one jet is identied as a b-quark jet through a soft lepton tag (\SLTtag") (14 events), and nally events without any tags (42 events). The samples aretted separately and the results, shown in Fig. 1, when combined yield m(top) =175:9 5:1 (stat) 5:3 (syst) GeV=c2.3.4 Dilepton ChannelAs indicated in Section 2, the dilepton channel has fairly low statistics. On the otherhand, the background is small, dominated by Z boson decays to charged leptons andparticle misidentication. The diculty arises from the presence of two neutrinos,for which only the combined transverse momentum can be measured. The system isthus kinematically underconstrained, and both experiments use the so-called \neu-trino weighting" technique to measure the top quark mass: for every chosen topquark mass, there is a sampling of allowed neutrino rapidities. Given the neutrinorapidities, the assumed top mass, and the charged-lepton and b-quark momenta,the system can now be solved for the transverse and longitudinal momentum com-ponents of the neutrinos (with a two-fold ambiguity for each neutrino). A weight isthen assigned to each solution based on the agreement between the calculated andmeasured missing transverse momentum. All weights are summed for all possiblecombinations of neutrino rapidities. This is done as a function of top mass value foreach event.D 9) has observed 6 events in a sample corresponding to an integrated4SVX Single TaggedMtop (GeV/c2)-∆log(L)SVX Double TaggedMtop (GeV/c2)-∆log(L)SLT TaggedMtop (GeV/c2)-∆log(L)No Tags (ETJET 4>15 GeV)Mtop (GeV/c2)-∆log(L)Reconstructed Mass (GeV/c2)Events/(10 GeV/c2 )0123456100 200 3000123100 200 300012345100 200 300024681012100 200 300051015125 150 175 2000246125 150 175 200024125 150 175 200024125 150 175 200Figure 1: CDF lepton + jets analysis: reconstructed mass distributions in each ofthe four subsamples (see text). Each plot shows the data (points), the result of thecombined t to top + background (dark shading), and the background component ofthe t (light shading). The insets show the variation of the log-likelihoods with mtopfor the separate ts.luminosity of about 125 pb 1. For each event, the distribution of weights is splitinto ve mass regions, and used to form a vector for calculating the mean massvalue for all events in the sample. Fig. 2 summarizes the results based on theseevents. D also evaluates the mass in this sample using an extended version of theDalitz-Goldstein and Kondo method. Both results are compatible, and the combinedmeasured top-quark mass is m(top) = 168:4 12:3 (stat) 3:6 (syst) GeV=c2.Eight events are observed by CDF 10) (in a sample corresponding to anintegrated luminosity of 109 pb 1). Each event's top mass estimate is determinedfrom the peak of the weight distribution, and the resulting distribution comparedwith Monte Carlo simulation, leading to a measured top-quark mass of m(top) =167:4 10:3 (stat) 4:8 (syst) GeV=c2.3.5 Systematic UncertaintiesThe main contributions to the systematic uncertainties on the Run I top-quarkmass measurements are given in Table 1. The dominant contribution is due to thedetermination of the jet energy scale. Other important contributions come from5012380 120 160 200 240 280mt (GeV)iWi(mt)Σ12131415160 180-ln Lmt (GeV)Figure 2: D dilepton analysis: summed event weight functionPiWi for the da-ta sample (points), the sum of the tted signal and background (solid), and thebackground alone (dashed). The error bars indicate the rms observed for ve-eventsamples in ensemble tests. The inset shows the corresponding t to  lnL, drawn asa solid curve in the region considered in the t.uncertainties in initial and nal state radiation, and modeling of the background.All of these will be discussed in more detail in Section 6.4 Combined CDF and D MeasurementThe Top Averaging Group 11) has combined the 5 mass measurements describedin Section 3. As expected, after taking into account all correlations, the best con-tribution comes from the lepton + jets channel. The combined result is m(top) =174:3 3:2 (stat) 4:0 (syst) GeV=c2, which is compared with the individual mea-surements in Fig. 3.5 Run II UpgradesRun II is formally scheduled to start at the Tevatron on March 1st, 2001. Thiswill follow a shutdown during which both the accelerator complex and the colliderdetectors will have undergone signicant upgrade from their Run I congurations.6Table 1: Summary of main contributions to systematic uncertainties (in GeV=c2) inthe measurement of the mass of the top quark in Run I: a) jet energy scale, b) modelfor signal (mainly initial and nal-state radiation), c) Monte Carlo generator, d)Uranium noise and multiple interactions, e) model for background, and f) methodfor mass tting. Contributions to the uncertainties dier somewhat between the twoexperiments. For example, for CDF measurements, the uncertainty due to multipleinteractions is included in the uncertainty in the jet energy scale.a b c d e fCDF Dilepton 3.8 2.8 0.6 - 0.3 0.7CDF Lepton + Jets 4.4 2.6 0.1 - 1.3 -CDF All Hadronic 5.0 1.8 0.8 - 1.7 0.6D Dilepton 2.4 1.7 - 1.3 1.0 1.1D Lepton + Jets 4.0 1.9 - 1.3 2.5 1.55.1 Upgrade of the Accelerator ComplexIn Run II, the Tevatron is expected to operate at a center of mass energyps =2 TeV , an increase of 200 GeV over Run I. For top quark studies, this yields asubstantial rise of 40 % in the tt cross-section.In addition to that, the completion of the Main Injector should providesignicant improvement in the luminosity. Both experiments are expected to recorddata corresponding to an integrated luminosity of 2 fb 1 in the rst two years,followed, after a brief shutdown, by 5 fb 1 per year until the start up of the LHC.5.2 D UpgradeThe main features of the D upgrade involve the addition of a 2 Tesla solenoidinside the calorimeter, combined with improved tracking and particle identicationcapabilities.Two central tracking systems have been built: an extensive silicon detectorconsisting of barrels for central tracking and disks covering the forward regions, anda scintillating ber tracker between the silicon and the solenoid. The silicon barrelshave four layers mainly of double-sided silicon sensors, while all disks provide twohit projections. The ber tracker consists of 8 coaxial cylinders, and is used in theLevel-1 trigger.In addition to the tracking systems, preshower detectors will be used toidentify electrons and photons. Also, the muon coverage has been extended and anew three-level trigger system is being built.7Figure 3: Individual and combined top-quark mass measurements at the Tevatron.5.3 CDF UpgradeThe upgraded CDF detector will have improved central tracking detectors, as wellas increased coverage in pseudorapidity for lepton identication.The new silicon detector consists of three parts: \Layer-00", silicon barrel-s, and the \Intermediate Silicon Layers (ISL)". Layer-00 is a single layer of siliconlocated very close to the beam, leading to signicant improvement in impact pa-rameter resolution. The barrels have ve layers of double-sided silicon sensors, withthe ISL located at larger radii, providing additional points between the silicon andthe Central Outer Tracker (COT). The COT is an improved version of CDF's RunI central drift chamber: it has 12 layers of wires, less material, and operates withshorter drift times.New end plug calorimeters have been built, giving better coverage for elec-tron identication, and the muon coverage has also been increased. In addition tothis, a time-of-ight system is located between the COT and the solenoid, and anew three-level trigger system is being implemented.6 Systematic Uncertainties on the Top-Quark Mass Expected in Run IIIn this section, we discuss ways to reduce the dominant systematic uncertainties onthe mesurement of the mass of the top quark.86.1 Determination of Jet Energy ScaleFor Run I data, CDF and D use a somewhat dierent approach to determine thejet energy scale. In D, the essential steps of the procedure are the following: theelectromagnetic scale is normalized by the mass of the Z-boson, and the hadronicresponse is then derived by balancing  + jets events. Corrections for the underlyingevent, multiple interactions and noise, are determined from minimum-bias data,while corrections for leakage outside the jet cone are computed based on test-beamdata and Monte Carlo simulation. Finally, the conversion from jet to parton energyis based on Monte Carlo (HERWIG), with separate treatment of light and b-quarkjets. A small remnant pseudo-rapidity dependence is corrected using data from + 1 jet events, and the results are cross-checked using (Z ! ee) + jets events. InD, the required corrections are determined separately and symmetrically for dataand Monte Carlo, and then the relative dierence between data and Monte Carlois added to the systematic uncertainty. Since the top-quark mass is measured bycomparison of data with simulation, this is a valid approach.While many of the steps are similar in CDF, there is one fundamentaldierence: in CDF, the Monte Carlo simulation, and in particular the fragmentation,is tuned to the data, exploiting the available accurate tracking information. As isshown in Table 1, the two dierent approaches do not seem to lead to signicantlydierent results.In Run II, the additional and improved data will yield at least two newhandles on the jet energy scale in tt events. The rst is given by the larger sampleof lepton + jets events with two b-quark tags. In 2 fb 1, this sample is expectedto contain of the order of 300 to 500 events, allowing accurate calibration of lightquark jets through the reconstruction of the W -boson mass. Probably the mostprecise way to determine the calibration of the jet energy scale is given by the largesample of Z ! bb events expected in Run II. This type of events has already beenobserved by CDF in Run I 12), but the sample is unfortunately to small to be usedfor determining the jet energy scale. It is expected that using these events in RunII will allow reduction in this dominant uncertainty to below 1%.6.2 Initial and Final-State RadiationA second sizeable contribution to systematic uncertainty is due to the treatment ofinitial and nal-state radiation. Clearly, additional jets from initial-state radiationshould not be included in the kinematic ts, but those due to nal-state radiationcertainly should be. Unfortunately, such additional jets are the least energetic only9about 55 % of the time. However, theoretical work is progressing, and present andfuture versions of the Monte Carlo generators should reproduce these processes withbetter accuracy. Furthermore, the sample of lepton + jets events with extra jetsin Run II should be large enough to allow more detailed study and comparison toMonte Carlo. This can be done using variables such as the number of additionaljets, relative angles between jets, etc.6.3 BackgroundsMultiple-jet QCD background is determined directly from data, and does not repre-sent a major problem. In Run I, a signicant systematic uncertainty came from theW+jets background, which was modeled using the VECBOS event generator. Inthe meantime, W and Z-boson transverse momentum distributions at the Tevatronhave been studied in detail 13), contributing to our understanding of these vectorboson production processes. Better Monte Carlo generators already exist, so thisuncertainty should be reduced appreciably in Run II.7 ConclusionsIn Run I at the Tevatron, the top quark mass has been measured with an accuracyof 3 %. The measured value is m(top) = 174:3  3:2 (stat)  4:0 (syst) GeV=c2,which corresponds to a top Yukawa coupling top(Q2 = m2top) = 1:00 0:03.In Run II, thanks to the improved accelerators and detectors, statisticswill not be an issue. Furthermore, the additional data will provide better controlof systematic uncertainties, where the main contribution will probably shift to thetreatment of initial and nal-state radiation. A challenging goal will be to reducethe systematic uncertainty on the top quark mass to less than 2 GeV=c2.References1. F. Ptohos (for the CDF Collaboration), talk presented at the International Eu-rophysics Conference on High Energy Physics 99, Tampere, Finland, July 17,1999.2. B. Abbott et al., D Collaboration, Phys. Rev. Lett. 83, 1908 (1999).3. E. Laenen, J. Smith, and W. van Neerven, Phys Lett. B321, 254 (1994);E.L. Berger, H. Contopanagos, Phys. Rev. D54, 3085 (1996); S. Catani,M.L. Mangano, P. Nason and L. Trentadue, Phys. Lett. B378, 329 (1996).104. G. Marchesini et al., Comput. Phys. Commun. 67, 465 (1992).5. F.A. Berends, H. Kuijf, B. Tausk, and W.T. Giele, Nucl. Phys. 357, 32 (1991).6. F. Abe et al., CDF Collaboration, Phys. Rev. Lett. 79, 1992 (1997), and Phys.Rev. Lett. 82, 271 (1999).7. B. Abbott et al., D Collaboration, Phys. Rev. D 58, 052001 (1998), and Phys.Rev. Lett. 79, 1197 (1997).8. F. Abe et al., CDF Collaboration, Phys. Rev. Lett. 80, 2767 (1998), and Phys.Rev. Lett. 82, 271 (1999).9. B. Abbott et al., D Collaboration, Phys. Rev. D 60, 052001 (1999).10. F. Abe et al., CDF Collaboration, Phys. Rev. Lett. 82, 271 (1999).11. L. Demortier et al., The Top Averaging Group, Fermilab-TM-2084 (1999).12. F. Abe et al., CDF Collaboration, FERMILAB-CONF-98-197-E (1998), andhep-ex/9806022.13. B. Abbott et al., D Collaboration, Phys. Rev. Lett. 84, 2792 (2000), and Phys.Rev. D 61, 032004 (2000); B. Abbott et al., D Collaboration, FERMILAB-CONF-99-204-E (1999); F. Abe et al., CDF Collaboration, Phys. Rev. Lett. 79,4760 (1997).11

Top quark mass measurements at the Tevatron

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Top Quark Mass Measurements at the Tevatron

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