The top quark plays an important role in the grand scheme of particle
physics, and is also interesting on its own merits. We present recent results
from CDF on top-quark physics based on 100-200/pb of p-pbar collision data. We
have measured the t-tbar cross section in different decay modes using several
different techniques, and are beginning our studies of top-quark properties.
New analyses for this conference include a measurement of the t-tbar cross
section in the lepton-plus-jets channel using a neural net to distinguish
signal and background events, and measurements of top-quark branching
fractions.Comment: Contribution to the proceedings of the XXXIX Rencontres de Moriond on
  Electroweak Interactions and Unified Theories; 7 pages, 5 figure

Bloom, Kenneth

English

arXiv

The top quark plays an important role in the grand scheme of particle physics, and is also interesting on its own merits. We present recent results from CDF on top-quark physics based on 100-200 pb{sup -1} of p{bar p} collision data. We have measured the t{bar t} cross section in different decay modes using several different techniques, and are beginning our studies of top-quark properties. New analyses for this conference include a measurement of {sigma}{sub t{bar t}} in the lepton-plus-jets channel using a neural net to distinguish signal and background events, and measurements of top-quark branching fractions

Bloom, K.

UNT Digital Library

Fermilab FERMILAB-Conf-04/071-E  June 2004TOP PHYSICS: CDF RESULTSKENNETH BLOOMfor the CDF CollaborationDepartment of Physics, University of Michigan,Ann Arbor, MI 48109, USAThe top quark plays an important role in the grand scheme of particle physics, and is alsointeresting on its own merits. We present recent results from CDF on top-quark physics basedon 100-200 pb 1 of pp collision data. We have measured the tt cross section in dierentdecay modes using several dierent techniques, and are beginning our studies of top-quarkproperties. New analyses for this conference include a measurement of tt in the lepton-plus-jets channel using a neural net to distinguish signal and background events, and measurementsof top-quark branching fractions.Evidence for the top quark was rst reported ten years ago this spring1, but the properties oftop, a fundamental particle in the standard model, have yet to be fully determined. There is stillmuch to learn about the processes of production and decay, and we have not yet measured thequantum numbers of top, which are exactly predicted by the standard model. These propertiesare important not just for the purpose of characterizing top, but also for understanding itsrelationship to other particles in the model. Predictions for the mass of a standard-model Higgsboson depend very strongly on the value of the top-quark mass 2. Because top is so heavy, itmay provide a path to new physics. The top quark must be understood in detail so that we canunderstand whether it is just part of the standard model, or the rst hint of physics beyond it.The top quark is an active topic of study in the CDF Collaboration, which operates adetector that records data from pp collisions atps = 1:96 TeV. The upgraded detector wascommissioned in 2001, and data-taking for physics began in 2002. Since then, CDF has recordedabout 200 pb 1 of analysis-quality data in Run II, roughly double the sample of the previousdata run. Here we present measurements of top-quark production rates and properties in thisdata sample.Most of our studies are done with tt pairs, which are produced through the strong interaction.We typically assume that B(t ! Wb) = 100%. Every tt event thus has two W 's, so weclassify the nal states by the decays of the W 's. \Dilepton" events have both W 's decayingto `; this mode has good signal to noise (S=N  1.5-3.5), but a low detectable event rate(4-6 events/100 pb 1). \Lepton plus jets" events have only one W decaying to `; the rateis higher than for dileptons (25-45 events/100 pb 1) but this channel has worse S=N ( 0.3-3.0). Regardless of how the W decays, tt events have two b quarks; these can be identied bydisplaced vertices (due to the long b-hadron lifetime and large mass) or soft leptons (due tothe large b-hadron semileptonic branching fraction) embedded in the jets produced by quarkhadronization.1 Cross-Section MeasurementsBefore we can study the properties of the top quark, we have to isolate a sample of candidatetop-quark events, and build tools to understand the rates of background events in the sampleand the eÆciency for top-quark selection. Our benchmark for these tools is a measurement ofthe tt production cross section. This production rate is predicted by QCD; a measurement thatagrees with theory gives us condence that we have some control over our tools, and that we infact have top quarks to study.The cross-setion measurement for dileptons is straightforward because the events are so cleanand the backgrounds are relatively small. CDF has two complementary dilepton cross-sectionmeasurements. One explicitly identies the two leptons (electrons or muons), while the otheridenties only one lepton and searches for an additional isolated high-pT track. The formeranalysis has lower background, but the latter has greater acceptance and greater sensitivity to decays of the W . The lepton-lepton analysis observes 13 events (1 ee, 3 , 9 e) on 2:4 0:7background events, while the lepton-track analysis observes 19 events (11 e-track, 8 -track) on7:1  1:2 background { signicant signals in both cases.Cross-section measurements in the lepton-plus-jets mode fall into two categories. The rstincludes counting experiments, in which we predict from rst principles the rate of standard-model non-tt backgrounds in the sample; any excess over that rate is considered the tt signal.Such counting experiments are only done in samples with at least one tagged b jet, where thenon-tt backgrounds are relatively small. The background estimates are data-driven as muchas possible; estimates for rates of QCD-jet and fake-tag backgrounds are entirely derived fromdata, while backgrounds from W plus heavy-avor processes (Wbb, Wcc, Wc) and lower-rateprocesses (dibosons, single top) get assistance from Monte-Carlo information. After checkingthat these methods work in lepton-plus-jets samples with fewer than three jets, where we expectlittle tt, we apply them to the lepton-plus-jets samples with at least three jets, where most ofthe tt events are expected. The distribution of the number of jets in b-tagged events is shownin Figure 1. We use two dierent b-tag techniques. In events where there is at least one jetwith a displaced-vertex tag, we observe 57 events with a background prediction of 23:4 3:0. Inevents where there is at least one jet with a lepton tag, we observe 18 events with a backgroundprediction of 12:9 2:6.Cross-section measurements that make use of kinematic information in candidate events arein the second category. Rather than dead-reckoning the background rate, we use characteristicsof the signal sample itself to help estimate the rate. We build models of kinematic variables forsignal and background events, and then t the distributions observed in the data to a sum of thesemodels, letting the normalizations oat. The estimated fraction of tt events can be convertedinto a cross section. Such analyses can be done with lepton-plus-jets samples where b tags are notrequired, as we use the kinematic information to help separate signal from background insteadof the tags. This exploits the large event sample, but now the backgrounds to tt are larger. Witha t to the distribution of the total transverse energy of the event (HT , see Figure 2), we ndthat this sample has a (134)% tt content. The signal and background HT shapes are modeledNumber of jets in W+jets1 2 3 4Number of tagged events020406080100120140160Number of tagged eventsmistagsWbbWccnon-WWcττ→WW,WZ,ZSingle topσ 1±Tot bkgd )-1Data (162 pbCDF II preliminary≥ 3 jets≥ > 200 for Trequire HNumber of jets in W+jets1 2 3 4Number of tagged events020406080100Number of tagged eventsWbb,Wcc,Fakenon-WDrell-YanWcττ→WW, WZ, ZZ, ZSingle topσ 1 ±Total Bckg )-1 7 pb±Data (126 ≥CDF II PreliminaryFigure 1: Njet distributions in lepton-plus-jets samples which contain vertex tags (left) and lepton tags (right),along with estimates of contributions of standard model non-tt processes.02060events40800 100 200 300 400 500 600HT  (GeV)topW+jetsother EWfWCDF Preliminary (195 pb-1)Njet ≥ 3σtt (pb)QCD0.70.80.91.04 6 82Neural Net Output0 0.2 0.4 0.6 0.8 1events01020304050607080 QCD fakes W+jets Signal Combined Fit Data% 2.9 3.1±ttbar:     17.6 %  0.0  0.0±QCD:     6.3 % 4.6 4.7±W+jets   76.0 nnout )-1CDF Preliminary (195 pb 3≥NjNN outputFigure 2: Distributions of HT (left) and the neural-network output (right) in the lepton-plus-jets sample (withoutrequiring b tags), with models of the physics processes that contribute to this sample.by Monte Carlo simulation, and systematic eects such as our understanding of the jet-energyscale and inputs to the leading-order matrix-element Monte Carlos (such as the Q2 scale) limitour precision. A complementary approach is to perform this measurement in the sample oflepton-plus-jets events that include displaced-vertex-tagged jets; the energy of the leading jet isused as the kinematic variable. Now the signal-to-noise is improved, and we can use a sample ofreal data to model the background { the lepton-plus-jets events without b tags, which should bedepleted of tt events { and thus largely eliminate the systematic uncertainty due to modeling.The tt content of this sample is (67+13 16)%, where the uncertainty is now dominated by statistics.While the kinematics of tt and the dominant W plus jets background process do dier,they only dier modestly, so there is very little separation in any given variable. However,these modest dierences occur in many dierent variables. Thus, we choose several variables(jet energies, invariant masses, event shapes) that are largely uncorrelated, or have dierentcorrelations in signal and background processes, and then develop a neural network from thesevariables to use the information optimally. The distribution of neural-net outputs for the lepton-plus-jets sample (the same sample used in the HT analysis) is also shown in Figure 2. Now thereTable 1: Summary of tt cross-section measurements from CDF. The rst uncertainty is statistical, the second issystematic.Analysis tt (pb)R Ldt (pb)Dilepton { lepton-lepton 8:7+3:9 2:6  1:5 193Dilepton { lepton-track 6:9+2:7 2:4  1:3 200Lepton + jets { kinematic (HT ) 4:7 1:6 1:8 195Lepton + jets { kinematic (NN) 6:7 1:1 1:5 195Lepton + jets { vertex tag 5:6 1:2 0:7 162Lepton + jets { lepton tag 4:1+4:0 2:8  2:2 126Lepton + jets { vertex tag + kinematic 6:0+1:5 1:8  0:8 162Theory (mt = 175 GeV)3 6:7+0:6 0:9is much better separation between the tt signal and the backgrounds, which leads to improvedstatistical and systematic uncertainties. The tt fraction in this sample is (18  3)%, consistentwith, and more accurate than, the value measured in the HT analysis. We believe that suchmultivariate techniques are very promising for isolating tt and other hard-to-extract signals.A summary of the current set of tt cross-section measurements from CDF is given in Table 1and Figure 3, along with the current prediction from theory 3 with mt = 175 GeV. One shouldnot perform a nave average of the values as there are correlations, and one should rememberthat the theory prediction has an additional 1 pb uncertainty due to the uncertainty in mt,but we do see general consistency among the measurements, and agreement with the theoryprediction. This gives us condence that we are in fact observing tt production.In addition, CDF is searching for the production of single top quarks through the electroweakinteraction; the cross section is expected to be about 3 pb. Eventually this will provide a directmeasurement of jVtbj2. We currently set a 95% CL upper limit of 13.7 pb on the total single-topproduction rate.2 Branching fractionsWith the top-quark sample identied, we can begin to measure the properties of top quarks.For instance, we can use the cross-section measurements to study the branching fractions of top.The standard model predicts that B(t ! Wb) is virtually 100%; in particular, we expect thatthere is always a W in the decay. The tt cross-section measurements in fact assume that thereare always two W 's in the nal state; the event rates are corrected for the W branching fractionsto obtain the total tt cross section.We can test this assumption by examining the ratio of measured cross sections for dierenttt nal states, R = ``=`j . R should be consistent with unity if in fact the dilepton andlepton-plus-jets analyses are both examining standard-model tt production. Measuring ratios isappealing to experimenters; R will have smaller systematic uncertainties than the individualcross-section measurements as some common factors will cancel. And of course, R is indepen-dent of any theory prediction for tt, so we can look for new physics by looking for a deviation inthe ratio, rather than by comparing a measured cross section to an uncertain theory prediction.As an example, R is sensitive to decays such as t! H+b. If this process occurs, then the mixof dilepton and lepton-plus-jets events would be dierent from what is expected in the standardmodel, and that mix is sensitive to tan , which controls whether the H+ is more likely to decayto hadrons or to leptons.We estimate R creating a probability distribution based on the observed event rates; thisis shown in Figure 4. We nd R = 1:45+0:83 0:55, and we can limit 0:46 < R < 4:45 at 95% CL.) (pb)t t→ p(pσ0 2 4 6 8 10 12 14CDF Run 2 Preliminary=175tmTop Production Cross SectionsLepton + LeptonLepton + TrackL + Jets: KinematicL + Jets: Kinematic NNL + Jets: Vertex TagL + Jets: Lepton TagL + Jets: Vertex Tag + KinematicFigure 3: Summary of tt cross-section measurements from CDF; the shaded band is the theory prediction.Since this is consistent with standard-model expectations, we can set limits on non-standarddecays of top. These limits are by their nature model dependent, as we need to understandthe eÆciency to detect the non-standard decay. We nd that the branching fraction to anall-hadronic t ! Xb decay is less than 0.46 at 95% CL under the assumption that we detectstandard and non-standard all-hadronic decays with the same eÆciency.We can also examine the other side of the branching-ratio coin { we typically assume thatthere is always a b in each top decay, and in fact all of the cross-section measurements that useb tagging depend on that assumption. We can test that hypothesis by examining the b-tag ratesin identied tt events. These rates depend both on b = B(t ! Wb)=B(t ! Wq) and on thesingle-b tagging eÆciency . The rates of observing two, one or zero tags in tt events depend onthe product b, as the b quark must be produced and subsequently tagged:N2 / (b)2 N1 / 2b(1   b) N0 / (1  b)2 )b =2N1=N2 + 2=12N0=N1 + 1:b is thus determined by the ratios of tag rates. Since we measure the product b, we caneither take the value of  measured in calibration samples and extract b, or just assume thestandard-model value b = 1 and do an in situ cross check of .The above treatment is simplistic; we need to account for our limited acceptance for b jets,for tagged jets that come from non-tt events, and for tagged jets that come from non-b quarks-1 0 1 2 3 4 5 6 7 8 9050010001500200025003000350040004500Expected distribution of cross-section ratio / ndf 2χ  201.7 / 127Constant  8.719±  4205 Mean      0.002788± 1.452 Sigma     0.006934± 0.8415 CDF PreliminaryFigure 4: Probability distribution for R = ``=slj.in tt events. We create a likelihood function that depends on the observed numbers of tags inthe vertex-tagged lepton-plus-jets sample, and nd the most likely value of b. The resultinglikelihood is shown in Figure 5. We measure the most likely value as b = 0:25+0:22 0:18. Thesingle-b tagging eÆciency is 0:45  0:05, and thus b = 0:54+0:49 0:39 { smaller than expected, butconsistent with b = 1. If we assume b = 1, we nd an estimate of  that is consistent with ourmeasurements in calibration samples. In addition, we can set a lower limit of b > 0:12 at 95%CL. If this analysis remains exactly the same as we record more data { no change in backgroundlevels or eÆciencies { we expect to set a lower limit of about 0.70 with 500 pb 1 of integratedluminosity. Of course, we do hope to reduce our backgrounds and increase our eÆciencies as weaccumulate more data.3 MassAs stated above, measurements of the top-quark mass are of great interest right now, as predic-tions for the mass of the Higgs are so sensitive to it. As of the date of this conference, CDF hasperformed two preliminary measurements of the top mass with Run II data.One makes use of the dilepton sample, which has very little background but low event rates.Because of the two neutrinos in the nal state, estimating the top mass is an underconstrainedproblem, and some more information must be added. In addition, we do not know which jetsto match up with which leptons in the nal state, giving an additional combinatorics problem.The current CDF analysis scans over possible directions of the neutrinos, and uses the predictedpT of the tt system (taken from theory) as an event weight. Multiple solutions for the mass arepossible; we choose the one with the greatest kinematic consistency with tt. Using six events in126 pb 1 of data, we measure mt = 175  17stat  8sys GeV, where the systematic uncertaintyis dominated by our limited knowledge of the jet-energy scale.Another mass measurement is made with the lepton-plus-jets sample. We choose the eventswith four jets, at least one of which is vertex-tagged. The event rates are larger here than in thedilepton mode, and while the backgrounds are also larger, they are tolerable. Now that there isonly one neutrino in the nal state, the problem is overconstrained. In each event, we choosethe combination of jets that gives the best consistency with the tt hypothesis in a kinematict. With 22 events in a sample of 108 pb 1, we measure mt = 178+13 9stat  7sys GeV. Again,∈∆b0 0.2 0.4 0.6 0.8 1Likelihood00.00050.0010.00150.0020.00250.0030.00350.004-1x10 Combined likelihood for all samples-1CDF Run II Preliminary 108 pbFigure 5: Likelihood distribution for b in vertex-tagged lepton-plus-jets sample.the systematic uncertainty is dominated by the jet-energy scale. Improved measurements of thetop-quark mass are expected shortly.4 OutlookAt this stage of Run II at the Tevatron, there is little doubt of the top-quark signal at CDF.We have observed tt production in multiple channels (dilepton and lepton plus jets) and withmultiple techniques (dierent b taggers, counting experiments, and ts to kinematic distribu-tions, including a new neural-net-based analysis). All of the tt cross-section measurements areconsistent with the theory prediction, and with each other.Meanwhile, studies of top-quark properties are beginning. At this conference, we presentedthe rst Run II measurements of top-quark branching fractions. We are hard at work on themass measurements: adding in more data, improving our techniques and reducing systematics.We hope to have new results imminently.This is just the beginning of a broad program to characterize the top quark. At CDF, weare preparing for a variety of top-quark studies { measurements of the W helicity, searches fordecays to charged Higgs and for resonant tt production, and studies of angular correlations intop decay. More data and more news about top and its impact on standard and new physicsare expected soon.AcknowledgementsI thank my CDF colleagues for all of their eorts to produce these results, and the conferenceorganizers for putting together a very pleasant week of physics and other activities.1. \Top Quark, Last Piece in Puzzle of Matter, Appears to Be in Place," The New YorkTimes, Page A1, April 26, 1994.2. See, for instance, P. Gambino, \The Top Priority: Precision Electroweak Physics fromLow to High Energy," 21st International Symposium on Lepton and Photon Interactionsat High Energies (LP 03), Batavia, IL, August 2003, hep-ph/0311257.3. M. Cacciari et al., hep-ph/0303085.

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