Top physics at the Tevatron Collider

a r X i v :0710.3617v 1 [h e p -e x ] 19 O c t 2007

2Fabrizio Margaroli

2Top quark properties

Thanks to high statistics and high purity,semileptonic tˉt events are the best candidates to test SM predictions and non-SM particle production in the top sector:

Pair production cross section

The measurement of the tˉt production cross section provides a test of QCD calculation and any discrepancy from the theoretical expectation could hint to production or decay mechanisms not predicted by the SM.The most re-cent measurement comes from the D?experiment[4]and is performed in the semileptonic channel using L=1fb?1,counting events passing selection cuts and requiring at least one jet to be tagged as a b-quark jet;the measured

cross section corresponds toσtˉt=8.3+0.6

?0.5(stat.)+0.9

?1.0

(syst.)±0.5(lumi.)pb.

The measurements performed by CDF and D?in the complementary sam-ples give compatible results.

Production mechanism

SM predicts the top pairs to be produced through quark-antiquark annihila-tion85%of the time,and the rest15%through gluon-gluon fusion.Taking advantage of the fact that the average number of low-P T tracks is proportional to the gluon content of a sample,CDF deploys a template method to?t a gluon-rich and a gluon-deprived track multiplicity distribution to the data,to measure[5]the fraction of events produced through gluon-gluon fusion to be σ(gg→tˉt)/σ(pˉp→tˉt)=0.07±0.14(stat.)±0.07(syst.).

Decay mechanism

According to the SM the W boson is produced70%of the time with longitu-dinal helicity,and the rest with left-handed helicity.;right handed helicity is forbidden by the theory.A template method is used here,the template vari-able being cosθ?,the cosine of the decay angle between the momentum of the charged lepton in the W boson rest frame and the W momentum in the top quark rest frame,which is highly sensitive to the W helicity.CDF measures[6]

F0=0.59±0.12(stat.)+0.07

?0.06(syst.)and F+=?0.03±0.06(stat.)+0.04

?0.03

(syst.).

New physics with top quarks?

The top quark can be seen as an hadronic probe to very high mass scales. CDF scans the tˉt invariant mass distribution to look for possible peaks due to resonant Z′production in the mass range450-900GeV/c2.Limits can be set to the product of the cross section times the branching ratio to top pairs. This limit amounts[7]toσ×BR(Z′→tˉt)<0.8pb at95%Con?dence Level (CL)for a Z′mass greater than600GeV/c2.

Overall,the measurements performed by the two experiments are in good agreement with each other and with the theoretical prediction.

Top physics at the Tevatron collider3 3The top quark mass

The top quark is the only quark that decays before hadronizing.Its mass, which is a free parameter in the SM,can thus be direcly measured.Moreover, due to the top quark and W contribution to radiative corrections,the mea-surements of their masses provide a powerful constraint on the Higgs boson mass.The top quark mass has traditionally been measured in each channel;

a major boost in precision has been achieved by exploiting the presence of hadronically decaying W whose daughter jets can be used to constrain the biggest source of systematic uncertainty,the Jet Energy Scale(JES).For this reason,the most precise results now come from the analysis of the semilep-tonic and the all-hadronic samples.There are two main classes of methods to extract the mass:the Template Method and the Matrix Element method.The former consists in choosing a variable which is strongly correlated with the observable one wants to measure,and in building templates of this variable for simulated signal and background events.The variable used to measure the M top is a tri-jet reconstructed invariant mass;the light quark dijet mass is cho-sen to simultaneously measure the JES.The Matrix Element technique aims to use all the available informations to calculate a probability for the event to come from signal or background according to the theory predictions for the ?nal state kinematics.Transfer functions are needed in order to convert re-constructed objects into kinematical tree-level quantities.For both techniques a likelihood will compare the data to the signal and background and its maxi-mization will provide us the measured values.The most precise measurements are performed using the matrix element technique in the semileptonic sam-ple to simultaneously measure M top and JES.The most recent D?measure-ment[8]amount to M top=170.5±1.8(stat.)±1.6(JES)±1.2(syst.)GeV/c2. CDF alone explores the all-hadronic channel,where the latest analysis em-ploys a cut based selection to improve the signal-to-(mostly QCD)background ratio from～1/1000to～1/1.This analysis uses a mixed technique to ex-tract the mass:a template is built out of the probability given by the ma-trix element computation,and a dijet mass is used to measure the JES; this result[9]is now the most precise in this channel and corresponds to M top=171.1±2.8(stat.)±2.4(JES)±2.1(syst.)GeV/c2.The best measure-ment in each channel is then combined to give the very precise Tevatron aver-age value[10]of M top=170.9±1.1(stat.)±1.5(syst.)=170.9±1.8GeV/c2. With such a1%precision achieved,the M top measurement will likely be a long-standing legacy of the Tevatron collider.

4Single top production

The SM allows the electroweak production of single top quarks with the the-oretical cross section at NLO[11]of1.98pb in the t-channel and0.88pb in the s-channel(assuming M top=175GeV/c2).Single top quark events can

4Fabrizio Margaroli

Fig.

is

be tb

the CKM matrix without assuming only three generation of quarks.CDF and D?restrict their searches to events where the W decays leptonically; the signature is thus characterized by missing energy from the neutrino,one high-Pt lepton and,a b-jet from the top decay,which is required to be tagged to further reduce the background.Additionally we expect a light quark jet in the t-channel or one more b-jet in the s-channel.After the event selection we are left with a S/B of about1/20.Both CDF and D?experiments use di?erent advanced techniques to better isolate the signal from the large back-ground.The best D?measurement uses a machine-learning technique that applies cuts iteratively to classify events,namely a boosted decision tree.It produces an output variable distribution which ranges from0to1,with the background peaking close to0and the signal close to1.A binned likelihood ?t is used to extract the cross section,that D?measures[12]to beσ(s+t

channel)=4.3+1.8

?1.4pb,3.4σaway from the background only hypothesis,and

in agreement with the SM expectation;D?also measures the element V tb of the CKM matrix to be0.68<|V tb|<1at95%CL.CDF’s best result comes from using the event matrix element to build a probability for the event to come from signal or background.An event probability discriminant is then built and a likelihood?t extracts the signal and background relative normal-izations.CDF measures[13]an excess of2.3σand extract a cross section for the s+t channel to be2.7+1.5

?1.3

pb.

Top physics at the Tevatron collider5

Fig.2.On left,fraction of background-only pseudo-experiments giving a cross sec-tion higher than the observed.On the right,event probability discriminant used by CDF to extract the cross-section.

5Conclusions

The measurement presented here con?rm the SM expectation for top quark production and decay within the theoretical uncertainty,and provide high precision on the most important top property,the top quark mass,that it will take years for the LHC to achieve it.The?rst evidence of single top production and?rst direct measurement of the V tb parameter constitute another major Tevatron success.However,most analysis are statistically limited and with 2fb?1already recorded,and between6to8fb?1expected,uncertainties will be reduced and smaller deviation from the SM investigated.I would like to thank here the conference organizers and my CDF and D?collaborators for the hard work and e?ort spent in achieving the results presented above. References

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