XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 1 Top physics at the Tevatron Run II Boris Tuchming CEA Saclay, Dapnia/Spp, 91191 Gif-Sur-Yvette, France The pp¯collider Tevatron, with its high center of mass energy of 2 TeV is presently the unique topquarkfactory intheworld. Thousandsoftop-quarkshavebeenproducedduringRunIIa. This givestheopportunitytostudytheproductionandthepropertiesoftheheaviestknownfundamental particle. This article will summarize a sample of recent top quark physics results obtained at the Tevatron. I. INTRODUCTION II. PRODUCTION OF TOP QUARKS AT THE TEVATRON 7 The top quark is the heaviest known fundamental 0 A. Pair production fermionoftheStandardModel(SM)withamassof 0 175 GeV/c2, close to the electroweak breaking scal≃e. 2 This unexpected large mass delayed for several years At the Tevatron, the top quarks are mainly pro- n its discovery. It occurred only 11 years ago at the ducedbypair. Thequarkanti-quarkannihilationcon- a pp¯ collider Tevatron [1, 2], during the so called Run tributes at the level of 85% while the gluon-gluon J ≃ I (1992-1996). annihilation is only 15%. The main diagrams for 9 ≃ these processes are displayed in Figure 1. The cross- 1 Thehighmassofthe topquarkimplies alargecou- plingtotheHiggsboson(Y 1). Thismakesthink section is predicted to be 6.7 pb [4, 5, 6], the level of 2 itcouldplayaparticularrolteopin≃theelectroweakbreak- uncertainty being 12%. v ing mechanism and may be the gatekeeper to physics Typically for ≃1 fb−1 of data and for a channel 0 beyondtheSM.ItalsocauseslargeeffecttotheW bo- with a branching≃ratio of 30% and an acceptance of 3 son propagation through virtual quantum effects, so 15%, 300 events per experiment are expected to be 0 1 thatprecisionmeasurementsofthe topmasstogether observed. 0 with the W mass put strong constraints to the yet to 7 be discovered Higgs boson. Besides, this quark has 0 a very short lifetime and decays before it hadronizes. / x Therefore the bare quark properties are transfered to e his decay products and are not hidden in the spec- - troscopy that we observe for the other heavy flavor p quarks. e h Top physics can be studied at several levels. First : v the production mechanism can be examined, for ex- i ample by looking at the production kinematics or X searching for a resonant production. Secondly the r particle properties, can be studied, such as its mass, a width, lifetime, spin and charge. Finally the decay propertiesandthecouplingscanbestudiedbylooking at the decay products, their kinematics or searching for rare decays. FIG. 1: Main diagrams for top pair production. Asthetopdiscoveryisrecentandonlyafewevents were recorded during Run I, top physics still need to be studiedindetails. Theonlyexistingtopfactoryat this time, is the Tevatron, running at 1.96 TeV since the beginning of Run II in spring 2001. One of the B. Electroweak single top production main physics goal of the Tevatron experiments, CDF and DØ, is the study of the top physics. Single top quarks can be produced via the elec- As of summer 2006, both experiments have about troweakcouplingtotheW boson. Themaindiagrams 1 fb−1 of data available for physics analyzes, which is for these processes are displayed in Figure 2. Since almost 8 times more than what was collected during they have a different final state, the s channel and − Run I. Analysis presented in the following are based t channel are usually distinguished. The s and the onsmallersubsets,from300to750pb−1. This isstill t−channelshaverespectivelycrosssections−ofthe or- − asmallfractionofwhatisexpectedto berecordedby der of 0.88 pb and 1.98 pb which is quite low. Given the end of Run II, in 2009: between 4 and 8 fb−1 [3]. that the background is quite high (there is only one THUPL05 2 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 topasasignature),thisprocesshasnotbeenobserved scale (JES) has also to be known with a good preci- yet and ongoing analysis are searching for it. sion. Identifying the b-jets is an effective way of improving the purity of the selections. The b-tagging makes use of the presence of secondary vertices and tracks with high impact parameters involved by the decays of b-hadrons. Typically for p = 50 GeV, a 50% ef- T ficiency per b-jet is achieved while the mistag rate is about 0.5 1%. − BothCDFandDØanalyzetheirdatausingvarious methods, in the different channels. These numerous FIG. 2: Diagrams for single top production. The s− and analysisareupdated asthe amount ofdata increases. t−channelshavedifferent final states. Only a few examples and the typical selections are discussed in the following. III. TOP PAIR PRODUCTION B. Lepton+jets channels A. Signatures The signature consists of a central spherical ener- getic events, with a high p lepton and a high E . T T Within the Standard Model, the top decays into 6 Four ormore jets can be reconstructed. Among them a W boson and a b quark almost 100% of the time. two jets are expected to arise from b-quark. The different channels arise from the possible decays The cuts to preselect the events are typically: high of the pair of W bosons: either W eν ( 10.7%), p leptontrigger(lepton+jetstriggeratDØ),isolated → ≃ T or W µν ( 10.7%), or W τν ( 10.7%), or leptonwithp >20GeV/c,E >20GeV,4jetswith W q→q¯′ ( 69≃.0%). → ≃ T 6 T p >15 GeV/c. Usually the scalar transverse energy → ≃ T The main channels are then: H defined as the sum of transverse momenta of the T • “evleepnttosnfo+rwjehtsic”hcohnaennWelsd(e≃cay30s%ha)dcroornreicsaplolyndantdo rceacnonbsetruuscetdedtoobijmecptrsovoef tphueriteyv,enatss,tt¯HeTve=ntsPaire|pTexi-| the other one decays into electrons or muons. pected to have a higher HT than the background. These channels have a moderate yield and a To discriminate between signal and backgroundaf- moderatebackgroundarisingfromW+jets pro- ter these preselections, two main approaches are em- duction, Z+jets production, or QCD processes. ployed: either the tagging of b-jets or topological cutswhichmaximizetheuseoftheeventskinematics. “di-lepton” channels ( 4.5%) correspond to Each of these approaches is also a cross-check of the • ≃ eventsforwhichthe twoW decayintoelectrons other one. or muons. These channels are very pure but Theacceptanceistypicallyatthelevelof10to20% have a small yield. The background is due to (for N 3), it is determined with the simulation jets di-bons events (mainly WW+jets production), ≥ (MC), corrected for DATA/MC weights measured on Z+jets and also QCD processes. independent samples. “alljetschannels”( 44%)correspondtoevents The backgrounds arise mainly from W + n jets, • where both W boso≃ns decay hadronically. The Wjjj,Wbbj,Wccj(herej standsforlightpartons,as yield is high, but the background arising from opposed to heavy flavor, b or c). These backgrounds QCD multi-jet production is very large. are determined with the MC, but as the yield suffers fromhightheoreticaluncertainties,thesebackgrounds “tauchannels”( 22%)arisefromeventswhere are normalized to the data. Non-W background is • ≃ atleastoneoftheW decaysintoτ. Astheτ de- mainly QCD where one jet fakes a lepton, it is de- caysarehardtoidentify,especiallyinahadronic termined on the data. The WW+jets background is environment as at Tevatron, these channels are determined using the MC. weakly exploited. Because of the high mass of the top quark, the decay products have high momenta and large angu- 1. Lepton+jets with topological analysis at CDF lar separations. Reconstructing and identifying the productionoftopquarksdemandsreconstructionand Thisanalysisisbasedonanartificialneuralnetwork identification of high transverse momenta (p ) elec- (ANN) which combines seven kinematic variables to T trons, muons, jets, and the measurement of the miss- discriminate against the main background, W+jets. ing transverse energy (E ). Good momentum reso- A likelihood fit of the ANN output distribution al- T lution for these objects6is required and the jet energy lowstodeterminethett¯contentoftheselectedevents THUPL05 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 3 measurement: -1 CDF Preliminary (760 pb ) 350 multijet σ(tt¯)=8.8+−11..21(stat)+−21..03(syst) pb. W+jets tt 300 combined data 04250 N ‡ 3 CDF RUN II Preliminary(695pb-1) 0. jets s/ 200 multijet: 78.0 – 4.2 events nts11220000 Data event150 Wsigjentasl::1 362948..61 –– 4391..76 eevveennttss of Eve11000000 tNDt oi(b8no.-2sWop nbQ)CD 100 r Single Top e b Wc m 880000 Wcc 50 u N Wbb 660000 Mistag 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ANN output 440000 FIG. 3: distribution of the ANN output of the selected 220000 eventsfor the topological lepton+jets analysis at CDF. 00 Sample Events Fitted tt¯ σ(tt¯) WW++11 jjeett WW++22 jjeett WW++33 jjeett WW++‡‡ 44 jjeett JJeett MMuullttiipplliicciittyy W+≥3-jets 2102 324.6±31.6 6.0±0.6±0.9 pb W+≥4-jets 461 166.0±22.1 5.8±0.8±1.3 pb FIG.4: Distribution of thejet multiplicity in lepton+jets dataat CDF after requiring1 b-tag and HT >200 GeV. TABLE I: Results of the CDF topological letpon+jets analysis. as quoted in Table I. The ANN output distribution C. Di-leptons channels forselectedeventsisshowninFigure3. Themeasure- ment based on 760 pb−1 of data reads: The signature consists of a central spherical ener- σ(tt¯)=6.0 0.6 0.9 pb. getic events, with two high pT lepton and high 6ET. ± ± Twooremorejetscanbereconstructed. Amongthem Themainsystematicuncertaintiesforthismeasure- two jets are expected to arise from b-quark. mentarisefromtheJES(8.3%),theW+jetsmodeling The cuts to preselect the events are typically: high (10.2%) and the luminosity measurement (5.8%). p lepton trigger (or di-lepton), presence of isolated T leptons with p > 20 GeV/c, E > 20 GeV, 2 jets T T 6 with p >15 GeV/c and H >120 200 GeV. This T T − 2. Lepton+jets with b-tagging at CDF selection is so pure that the b-tagging is actually not employed. A secondary vertex tag is employ to identify b-jets The acceptance is typically at the level of 10 to in this analysis based on 695 pb−1. Events can have 15%andisdeterminedwiththesimulation(MC),cor- either 1 b-tag or 2 b-tag. The jet multiplic- rected for DATA/MC weight factors. ≥ ≥ ity distribution for one b-tag events is shown in Fig- The backgrounds arise mainly from Z µµ+jets, ure 4. The cross-sections are measured from the 3rd Z ee+jets, Z ττ+jets, WW+jets T→hese back- and 4th jet bin in 1 b-tag events after the cut gro→unds are deter→mined with the MC. Non-W back- ≥ HT >200 GeV: ground consists mainly of QCD events in which jets are faking leptons, it is determined from the data. σ(tt¯)=8.2 0.6(stat) 1.0(syst) pb. ± ± The results for the CDF analysis in 750 pb−1 of Themainsystematicuncertaintiesforthismeasure- data reads: ment arise from the b-tagging uncertainty (6.5%) the luminosity measurement (6.0%) and the parton den- σ(tt¯)=8.3 1.5(stat) 1.0(syst) 0.5(lumi) pb. ± ± ± sity function uncertainty (5.8%). Thett¯yieldcanbe alsoextractedfromthejetmul- For this analysis the main systematic uncertainties tiplicityof 2b-tagevents,butgiventhelargeuncer- arise from the lepton-id (4%), the JES (3.1%) and taintythis≥resultisonlyacross-checkfortheprevious the tt¯modeling (5%). THUPL05 4 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 D. Fully hadronic channel E . This is particularly true in the di-leptons chan- T 6 nels for which two neutrinos carry out unmeasured energy. The fully hadronic channels suffers from a large QCD background and a large combinatoric when one Finally, one of the largest source of experimental tries to match the jets to the W systems or the difficulty is the parton energy measurement. Resolu- t Wbsystem. Therequirementof2b-tagjetsinthe tion effects are typically 85%√ET and go down with ev→ents helps in improving the signal over background increasing statistics. The uncertainty on the jet en- ratio and reducing the combinatorics. But the back- ergy scale is however an irreducible systematics. Re- ground is still very large as shown in Figure 5 where cently, an improved method have been set up to re- the invariant mass spectrum of the reconstructed top duce this uncertainty: using an in-situ calibration of systemisdisplayed,beforeandafter backgroundsub- the JES, a part of its uncertainty scales down with traction. DØ with 360 pb−1 of data measures increasing statistics as well. This method will be discussed in the following sections. σ(tt¯)=12.1 4.9(stat) 4.6(syst) pb. ± ± The main uncertainties arise from the background A. Methodology subtraction (25%), the JES (15%) and the b-tagging efficiency uncertainty (18%). Alotofdifferentanalysishavebeenperformedboth at DØ and CDF. They are different from each other depending on: E. Pair production summary The channel, lepton+jets, or di-leptons of fully • The results of the different channels in both CDF hadronic; andDØaredisplayedinFigure6. ThecombinedCDF result reads: the event selection, for example requiring 0 b- • tag, 1 b-tag or 2-btag; σ(tt¯)=7.3 0.5(stat) 0.6(syst) 0.4(lumi) pb. ± ± ± the jet multiplicity requirements; • Itisworthwhiletonoticethatthelevelofuncertainty is 12%, as low as the theoritical uncertainty. The ac- the method to derive a mass measurement. • curacy is also no more limited by statistics. Thus it can be said that the Tevatron has entered a precision The various methods can be classified in two fami- era regarding the tt¯cross-sectionmeasurement. lies: The template methods consist in choosing a • givenkinematicobservable,creatingsignaltem- IV. MEASUREMENT OF THE TOP MASS plates at different M thanks to the MC, cre- top atingtemplateforbackgroundevents,andusing Measuring the top mass is a complex experimental alikelihoodfittodeterminethebestsignaltem- task. The events are complicatedsince the tt¯produc- plate; this method makes use of the kinematic tion yields six elementary particles in the final state. information only. Even if the b jets have been identified, there is still − a large combinatorics when trying to assign recon- The matrix method consists in building a likeli- structed jets to initial partons. • hood function based upon the PDF, the matrix Besides, these six particles correspond to different elements of tt¯ process, and the transfer func- typeofobjects(jets,b-taggedjets,leptons,ET),which tions which relate detector measurements (eg: 6 have to be well identified and measured. As a conse- jets p ) to the top decay products (eg quarks T quence, the mass measurementdepends onthe simul- p ). Unmeasured quantities are integrated out T taneous understanding of most of the different sub- and a maximum likelihood fit allows to derive detectors (tracking system, electromagnetic calorime- the top mass. This method needs also to be try, hadronic calorimetry, muon system) of CDF or calibrated using MC events. DØ. Anotherdifficultycomesfromthe presenceofback- The different analysis do not suffer from the same ground. Its contamination is sizable so that it can sourcesofuncertainty. Thisallowstochecktheircon- bias the measurementif it is not well understoodand sistency and precision is gained by combining the re- under control. sults. Only three examples (the two most sensitive Othersourcesofdifficultyareencounteredwhenre- and one original analysis) are briefly described in the constructingthekinematicinformationineventswith following. THUPL05 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 5 eV DØ Run II Preliminary (360 pb-1) eV140 DØ Run II Preliminary (360 pb-1) G120 G nations/5 100 DDDDØØØØ DDDDaaaattttaaaa CUCUanannnttaaddggiiddggaaeettddee ssMMuullttii--JJeett EEvveennttss ations/15 110200 DPyØt hDiaa ttat ombi 80 mbin 80 c o c 60 60 40 40 20 20 0 -20 0 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 bjj Mass (GeV) bjj Mass (GeV) FIG.5: Invariantmassoftopsystem(bjj),beforeandafterbackgroundsubtractionintheDØfullyhadroniccross-section analysis. 8 Cacciari et al. JHEP 0404:068 (2004) Assume mt=175 GeV/c2 Kidonakis,Vogt PRD 68 114014 (2003) CDF Preliminary* DØ Run II Preliminary *Dilepton 8.3– 1.5– 1.0– 0.5 (L= 750 pb-1) dilepton (topological) 8.6+–23..72+–11..11pb 230 pb–1 *L(Le=p 7to6n0+ pJbe-t1s): Kinematic ANN 6.0– 0.6– 0.9– 0.3 l+2je30ts p b (–t1opological) 6.7+–11..34+–11..16pb dilepton/l+jets combined 7.1+–11..22+–11..14pb *Lepton+Jets: Vertex Tag 8.2– 0.6– 0.9– 0.5 230 pb–1 (L= 695 pb-1) dilepton (topological) 8.6+–22..03+–11..02pb 370 pb–1 L(eLp= t1o9n3+ Jpebt-1s): Soft Muon Tag 5.3– 3.3 – 11..03– 0.3 ltr3a7c0 kpb/e–1mu combined NEW 8.6+–11..79+–11..11pb l+jets (b-tagged) 8.1+–11..23 pb *MET+Jets: Vertex Tag 6.1– 1.2 – 1.4– 0.4 370 pb–1 (L= 311 pb-1) 0.9 all-jets (b-tagged) 5.2+–22..56+–11..05pb 350 pb–1 *All-hadronic: Vertex Tag 8.0– 1.7 – 3.3– 0.5 all-jets (b-tagged) NEW (L= 311 pb-1) 2.2 360 pb–1 12.1+–44..99+–44..66pb *C(Lo=m 7b6i0n pebd-1) 7.3– 0.5– 0.6– 0.4 mtop = 175 GeV KCiadcocniaarki ise ta anld., VJHogEtP, P0R40D4 6, 80,6 181 (42001044 )(2003) (stat) – (syst) – (lumi) 0 00 22..55 55 77..55 1100 1122..55 1155 1177..55 0 2 4 6 8 10 12 14 s (pp fi tt) (pb) σ(pp→ tt) [pb] FIG. 6: cross-sections measured at CDF and DØ in thevarious channels B. Lepton+jets template method at CDF tion, the 1 b-tag tight selection and the 2 b-tag selection. The Lepton+jets template analysis at CDF has In this channel, the six decay products (if eg the been performed on 680 pb−1 of data. Four exclu- W+ decays semi-leptonically) are expected to be: a sive selections with different purities are performed, leptonℓ+,aneutrino ν,twob-quarksb¯bandtwolight ′ namely the 0-btag selection, the 1 b-tag loose selec- quarks q and q . A kinematic fit allows to choose THUPL05 6 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 CCDDFF RRuunn IIII PPrreelliimmiinnaarryy ((668800 ppbb--11)) This signifies that the systematic uncertainty due to )c the JES as been scaled by 0.6 thanks to the in-situ s (S 2 Dln L=8.0 calibrationand the W mass constraint. A further de- E DJ Dln L=4.5 crease is expected in the future with more statistics. 1 Dln L=2.0 Thesystematicsonthemassmeasurementaresum- 0 Dln L=0.5 marized in Table II. -1 C. Lepton+jets matrix element method at DØ -2 -3 The DØ collaboration has analyzed 380 pb−1 of 116655 117700 117755 118800 118855 MM ((GGeeVV//cc22)) data using a matrix element method in the lep- ttoopp ton+jets channels. Three exclusive samples are selected,namelya0b-tag,1b-tagand2b-tagsamples. FIG.7: Likelihoodcontourinplane(Mtop,∆JES)forthe CDF template method. Thesignalprobabilityforasetofmeasurements(jets, leptons) ~x is built: Systematic Source ∆Mtop(GeV/c2) P(~x,M ,JES)= (1) t b-jet energy scale 0.6 1 Residual JES 0.7 σ Z dσ(~y,Mt) f1(q˜1)f2(q˜2) W(~x,~y,JES)dq˜1dq˜2, Background JES 0.4 ISR 0.5 where q˜ are the initial parton energies, f (q˜) are the i i i FSR 0.2 proton (and anti-proton) parton density function, W PDF 0.3 is the transfer function from true kinematic quanti- Generators 0.2 ties ~y to measured ~x and dσ is the differential cross- Background Shape 0.5 section. b-tagging 0.1 In Equation 1, JES is an additional scaling factor MC statistics 0.3 totheexternallycalibratedJetEnergyScale. Itisleft as a free parameter, so that it can be simultaneously total 1.3 fitted, together with the top mass M . Thus an in- t TABLEII:Systematicuncertaintiesforthetopmassmea- situ calibration is performed, mostly relying on the ′ surement in thelepton+jets channel at CDF. constraint arising from the W qq¯ system. → . In a similar way, a probability is built for background events P and the final likelihood reads: bkg among the various combinations, making use of the b-tag,whichreconstructedjetscorrespondthetheqq′ L= (ftopP(~x,Mt,JES)+(1−ftop)Pbkg(~x,JES)) Y system, which jets correspond to the hadronic top, evt (2) qq′¯b, and which jet should be associated to the “lep- The Likelihood function is shown in Figure 8. tonic top”, bℓ+ν. Thus for each event a top mass, The results of the fit reads: Mqq′¯b =Mlνb,andaW mass,Mqq′,arereconstructed. Templates for these two quantities are prepared M =170.6+0.4(stat+∆JES) 1.4(syst) GeV/c2. from the MC after varying the top mass hypothesis, top −4.7 ± M , and ∆JES which is defined as the deviation top from the usual JES calibration in unit of uncertainty The JES in-situ calibration gives: on this calibration. By comparing the templates to the data, a simul- JES =1.027+−00..003330(stat+Mtop). taneous fit of (M ,∆JES) is performed. Results of top this fit are displayed in Figure 7. The results of this itindicatesa2.7%correctionrelativetotheexternaly fitaretheworldbestsinglemeasurement,atthe time calibrated JES, well compatible with its uncertainty of PIC06. They read (≃3 to 4%). The error on the fit reads +−00..003330. When comparedtotheexternallycalibratedJESuncertainty M =173.4 2.5(stat+∆JES) 1.3(syst)GeV/c2. this indicates a small improvement thanks to the in- top ± ± situ calibration. But this improvements will be more The measured ∆JES is significant with a higher statistics. Thesystematicsonthemassmeasurementaresum- ∆JES = 0.3 0.6(stat+M ). marized in Table III. top − ± THUPL05 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 7 Mass of the Top Quark (*Preliminary) Measurement M [GeV/c2] top ES D0 Run II Preliminary CDF-I di-l 167.4 ± 11.4 J1.12 380pb-1 1.1 0+1+2 Tags D˘ -I di-l 168.4 ± 12.8 1.08 CDF-II di-l* 164.5 ± 5.5 1.06 D˘ -II di-l* 176.6 ± 11.8 1.04 CDF-I l+j 176.1 ± 7.3 1.02 D˘ -I l+j 180.1 ± 5.3 1 CDF-II l+j* 173.4 ± 2.8 0.98 D˘ -II l+j* 170.6 ± 4.6 0.96 CDF-I all-j 186.0 ± 11.5 0.94 c 2 / dof = 8.1 / 8 150 155 160 165 170 175 180 185 M (GeV) top Tevatron Run-I/II* 172.5 ± 2.3 FIG. 8: Likelihood contour in plane (Mtop,JES) for the DØ matrix element method. The different colored region 150 170 190 correspond to steps ∆L=0.5. M [GeV/c2] top Systematic Source ∆Mtop(GeV/c2) FIG. 9: Measurements of thetop mass at CDF and DØ. b-fragmentation 0.7 b-response 0.8 present data sample. However such kind of analysis MC calibration 0.45 will become competitive at the end of Run II, when W background 0.3 ten times more data will be recorded. QCD background 0.3 total 1.4 E. Combined results TABLEIII:Systematicuncertaintiesofthetopmassmea- surement at DØ in the lepton+jets channel with the ma- Nine measurements performed at CDF and DØ trix element method. have been combined to achieve a 1.3% accuracy: . M =172.5 1.3(stat) 1.9(syst) GeV/c2. top ± ± D. Decay length method The various measurements are summarized in Fig- ure 9. This method employed at CDF is very different The chi square of the combination is χ2/dof = from the other ones, as it does not rely on the en- 8.1/8 which demonstrate a good consistency. ergy measurements of outgoing particles. Thus it is It should be stressed that the Run IIa goal was to an important cross-check with reduced impact from obtain a 2.5 GeV/c2 combined uncertainty using the JES uncertainty. 2 fb−1 of data. This goalhas been exceeded. The 1%≃ ThemethodmakesuseoftheBhadrondecaylength precision is in sight for the near future. and the boost of the b-quark in top decays, directly related to the top mass: M2+M2 M2 M F. Impact on the Higgs Boson Mass γ = t b − W 0.4 t. (3) b 2M M ≃ M t b b The top mass is a free parameter in the standard In a lepton+jets sample selected from 695 pb−1 of model. But its mass can be related to various elec- data, the decay length of the b-hadrons are recon- troweak observables thanks to radiative corrections. structed and compared to template simulated with Thusthemeasurementofthetopmassprovidesacon- different top mass hypothesis. The result reads: sistency check of the SM that can be used to probe M =183.6+15.7(stat) 0.3(JES) 5.6(syst)GeV/c2. for new physics. top −13.9 ± ± In particular the top mass plays an important role As expected, the impact of JES uncertainty is very in the radiative corrections to the W boson propaga- small, but the result is statistically limited using the tor: ∆M αM2. As the Higgs boson mass also in- W t THUPL05 8 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 80.70 experimental errors 68% CL: ts 14 DO, 370pb-1 (b) LEP2/Tevatron (today) n Data 80.60 TILeCv/aGtriogna/ZLHC light SUSY ve 1102 |q| = 2e/3 e MSSM |q| = 4e/3 V] f 8 e 80.50 o G M [W 80.40 heavy SUSY ber 46 80.30 MH = 113 GeV um 2 SMMH = 400 GeV both mMoSdSSeMMls N 00 0.5 1 1.5 2 80.20 Heinemeyer, Weiglein ’06 Top quark charge [e] 160 165 170 175 180 185 190 m [GeV] t FIG. 11: Reconstructed top charge in lepton+jets events at DØ. FIG. 10: Error ellipse projections for the top and W masses from current LEP2 and Tevatron data and for fu- tureTevatron/LHCandILCsensitivities. Theseareover- (Q1,Q4)R of charge (−31,−34), Q4 being the heavy laid onto bands of predicted Higgs masses for the SM, as quark observed at Tevatron. In the semi-leptonic well as for different SUSYscenario [8]. mode, the top quark of charge +2 decays into ℓ+ν¯b 3 . while a quark of charge +4 would decay into ℓ+νb . 3 The DØ collaboration has analyzed 370 pb−1 of fluences the W boson mass through radiative correc- data to select a pure lepton+jets+2 b-tagsample. To tions, ∆MWαlog(MH), the combined measurements associatetorightb-jettothe(lepton,E6 T)systemafit of the W and top masses yield constraints onto the to the top mass is performed. SM Higgs boson mass. The charge of a b-jet is defined by weighting the Using the top mass combined measurement pre- contributionofthechargedtracksmatchedtothejets: sented in the previous section, the SM Higgs boson is predicted to have a mass fulfilling (spring 2006): q .pκ Q = i i Ti , with κ=0.6. (4) jet P pκ M =89+42 GeV/c2, i Ti H −30 P and the upper bound at 95%Confidence Level (C.L.) Two observables allow to measure to top charge is: twice per event: Q˜1 = qb+qℓ and Q˜2 = qB+qℓ , | | |− | where b is the b-jet matched to the lepton and B is M <175 GeV/c2 the other b-jet. A fit of MC templates to the data H distribution of Q˜1 and Q˜2 is performed, which yields It is worthwhile to notice that our present data tend the exclusion of Q= 4 at the 93.7% confidence level. 3 to favor Supersymmetry over the Standard Model [8] The distribution is shown in Figure 11. as shown in Figure 10. V. TOP QUARK PROPERTIES B. Top lifetime Other properties than the top mass can be mea- Themeasuredtoplifetimecouldbesignificantlydif- sured at Tevatron to check the SM consistency and ferent from the SM value, cτ = 3.10−10 µm, if for search for new physics. example a long lived decaying particles is involved in the top production. A top lifetime analysis has been performed at CDF using 318 pb−1 of data in a lep- A. Top charge ton+jets+1b-tag selection. The impact parameterof the lepton is used as an observable related to the top WithintheSM,thetopquarkchargeisexpectedto decaylength. Amaximumlikelihoodfitoftheimpact be +2, but one can imagine model in which a heavy parameter from data to MC templates yields the ex- 3 quarkexiststhatcarriesadifferentcharge. Forexam- clusion of long decay length: cτ < 52.5 µm at 95% ple as in [7] one can imagine an exotic right doublet confidence level. THUPL05 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 9 B. Measurement of W helicity In the Standard Model, the (V A) nature of the − electroweakinteractionisalsopresentintheWtbver- tex. As a consequence, almost F+ 0% of the W ≃ arisingfromthetopdecaysshouldhavearight-handed helicity. One also expects F0 70% of longitudinal − ≃ W and F 30% of left-handed W. ≃ By studying the W decay product kinematics, it is possibletomeasuretheW helicityandprobe(V A) − structureoftheelectroweakinteractionatenergyclose to the electroweak symmetry breaking scale. Several variables can be used to relate the event topology to the W helicity: the p ofthe leptons,the decayangle T of the leptons in the top rest frame, cosθ⋆, or the invariant mass M2 = 1(M2 M2 )(1+cosθ⋆). lb 2 t − W AfittotheM2 spectrumobservedfromlepton+jets FIG. 12: Simultaneous measurement of top cross-section lb anddi-lepton top events with 695pb−1 atCDF yields and branching fraction toWb in theDØ experiment. a result perfectly compatible with the SM: VI. WTB VERTEX F+ =0.02 0.07 ± Studying the Wtb vertex is an important test of F+ <0.09 at 95% C.L. the SM which probes the (V A) nature of the weak interaction and also checks th−e unitarity of the CKM At DØ, using 370 pb−1 of data, the study of the ∗ matrix. cosθ spectrum in lepton+jets and di-lepton samples also shows a good agreement with the SM: F+ =0.08 0.08(stat) 0.06(syst) A. Measurement of Vtb ± ± WithintheSMtheCKMmatrixelementisinvolved in the top branching fractions: C. Search for single top BR(t Wb) Vtb 2 0.9980 R= → = | | = BR(t→Wq) |Vtb|2+|Vts|2+|Vtd|2 to 0.9984 As mentioned in Section IIB, the production of (5) single top involves the Wtb coupling in two differ- Tomeasurethisbranchingfraction,DØhasselected ent channels: the s-channel gives rise to a ℓνb¯b fi- a sample of lepton+jets events in 230 pb−1 of data. nal state with kinematic properties different from the By counting the number of selected events and the t-channel and its ℓνqb final state. The high back- numberofb-tagjets,asimultaneousdeterminationof ground arises from W+jets, especially W+b-jets, and Randthett¯cross-sectionisperformed. Sincethereis also from the tt¯production. To discriminate against noaprioriregardingtheb-jetcontentofthe events,a thebackground,multivariatetechniquesareusedboth model indepent cross-sectionis obtained. The results at DØ and CDF: ANN, Likelihood or decision trees. ofthe 2dfit are displayedin Figure12. They arewell ForexampleatDØ, 4likelihoodvariablesarebuilt to in agreement with the Standard Model expectations: discriminate: t-channel against tt¯, t-channel against σ(tt¯)=7.9+1.7 pb W +jets, s-channel against tt¯and s-channel against −1.5 W +jets. Figure 13 shows two out of the four possible distributions for these likelihood variables. Given BR(t Wb) BR(t →Wq) =1.03+−00..1197 the present luminosity, the single top has not been → observedin the data and both DØ and CDF have set limits on the cross-section, which are in agreement The same kind of studies has also been conducted with the SM expectations: at DØ the results arising at CDF. Using 162 pb−1 the CDF collaboration also fromthelikelihoodanalysiswith370pb−1ofdataare: measures a branching fraction in agreement with the SM: s: σ <5.0 pb at 95% C.L. (7) BR(t Wb) BR(t →Wq) =1.12+−00..2119(stat)+−00..1173(syst) (6) t: σ <4.4 pb at 95% C.L. (8) → THUPL05 10 XXVI Physics in Collision, Bu´zios, Rio de Janeiro, 6-9 July 2006 DØ Run II Preliminary, 370 pb-1 DØ Run II Preliminary, 370 pb-1 dd dd220000 elel t-channel (x10) elel t-channel (x10) nt Yint Yi110000 s-channel (x10) nt Yint Yi118800 s-channel (x10) ee ee116600 EvEv tt EvEv tt 8800 W+jets, WW, WZ 114400 W+jets, WW, WZ Multijet 112200 Multijet 6600 Data 110000 Data 8800 4400 6600 2200 4400 2200 00 00 00 00..11 00..22 00..33 00..44 00..55 00..66 00..77 00..88 00..99 11 00 00..11 00..22 00..33 00..44 00..55 00..66 00..77 00..88 00..99 11 SSiinnggllee ttaagg WW++jjeettss//tt--cchhaannnneell LLiikkeelliihhoooodd ffiilltteerr SSiinnggllee ttaagg tttt//tt--cchhaannnneell LLiikkeelliihhoooodd ffiilltteerr FIG.13: t-channeldiscriminantdistributionsforsingletopsearchesatDØ: ontheleftplot: W+jetsfilter,ontheright plot: tt¯filter. At CDF with 695 pb−1 of data, an ANN analysis B. Resonance in top production yields the results: New physics may shows up as a resonance in the s: σ <3.2 pb at 95% C.L. (9) production of top pair. The events are selected in a lepton+jets sample and the invariant mass of the tt¯ pair is reconstructed. No excess is observed by either t: σ <3.1 pb at 95% C.L. (10) DØin370pb−1orCDFin680pb−1,sothatlimitcan beputontheresonantproduction. InaLepto-phobic ′ s+t: σ <3.4 pb at 95% C.L. (11) top-color model, the heavy Z is excluded following Prospect studies have been performed at CDF, MZ′ >725 GeV/c2 at 95% C.L. (CDF 680 pb−1) whichindicatethatwith1.5fb−1,thesingletopcould beobservedatthe3σlevel. Ononehandthesestudies do notaccountfor the systematic uncertaintieswhich MZ′ >680 GeV/c2 at 95% C.L. (D0 370 pb−1) could delay the discovery by a few months. On the other hand, they do not account for the possible improvements in analysis techniques. Including a safety C. Search for W’ margin, we conclude that the evidence of single top production should be observed by the end of 2006 or AleptophobicW-likebosonwouldshowupasares- 2007 at both CDF and DØ. onance producing single top. To search for this kind ofparticle,DØemploysthesametechniquesasforthe SM single top: in the lepton+jets channel an ANN is VII. SEARCH FOR NEW PARTICLES built to discriminate against both the tt¯background and the the W+jets background. The production of A. Heavy top: t’ single top would be signed as a peak in the distribu- tionofthereconstructedinvariantmass,√sˆ,ofthese- If a 4th generationheavy quark exists, it may show lectedevents. Asnoexcessisobservedinthe230pb−1 ′ up just like the top quark, but with different kine- ofanalyzeddata,limitsaresetonthemassoftheW . maticproperties. Namelytheeventswillbeproduced 200258 GeV/c2 at 95% C.L. tionoftanβ, the ratio ofthe vacuumexpected values THUPL05

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