Top Quark Physics At TeVatron and LHC

Overview A Lightning Review of the Standard Model Introducing the Top Quark tt* Pair Production Single Top Production Decay Measuring the Top Quark Mass Charge and Spin Rare Decays Yukawa Couplings Summary

A Lightning Review of the Standard Model H Higgs W Vector Bosons Z g    e   e Fundamental Fermions bsd tcu I Quarks II III Leptons

The Particle Masses The top quark is the most massive elementary particle known!

Characteristics of the Top Quark (I) Third generation quark +2/3 the proton charge Weak isospin partner of the b quark Produced in pairs via the strong interaction Produced singly via the weak interaction Decays via the weak interaction to b, s or d quarks

Characteristics of the Top Quark (II) m t = ± 2.9 GeV Most massive known elementary particle Lifetime s Width  t  = 1.4 GeV  Lifetime is very short in comparison to the hadronization time 1/  QCD ~ s  Decays as a bare quark before hadronizing  Lifetime is so short that the strong force does not depolarize its spin before decay occurs

Why is the Top Quark Interesting? (I) Measurement of pair production cross section may validate the Standard Model prediction. The masses of W boson, top quark, and Higgs boson are interrelated.  M t introduces significant radiative corrections to m W.  An accurate measurement of m t will help to constrain m H.

Why is the top Quark Interesting? (II) May provide a probe of the mechanism responsible for fermion mass generation. m t is on the electroweak scale:  t may provide a probe for electroweak symmetry breaking. May indicate the existence of other massive particles. Will provide dominant background in many future searches for new physics at the TeV scale. Provides an important means of calibrating the ATLAS detector.

Top Quark Experiments LEP Tevatron  Proton-antiproton collider  Center-of-Mass energy 1.96 TeV  Pair production cross section  ~7 pb  Detectors CDF and D0 LHC  Proton-proton collider  Scheduled to begin operation in 2007  CM energy 14 TeV  Pair production cross section  ~834 pb  Detectors ATLAS, CMS

Pair Production via Strong Interaction Top-antitop pairs are produced by collisions between valence quarks, Top-antitop pairs are produced via gluon fusion,

The Pair Production Cross Section (I) Any deviation from the cross section predicted by the Standard Model could be an indication of new physics.  A heavy resonance which decays to a top-antitop pair might enhance the cross section.  The Standard Model Higgs may decay to a top-antitop pair If the decay is kinematically allowed Pair production cross section at Tevatron is predicted to be  ~ 7 pb Pair production cross section at LHC is predicted to be  ~ 834 pb

Electroweak Single Top Production Single top quarks can be produced I. Via fusion of a W boson and a gluon II. Via production of a virtual W* boson III. In conjunction with a W boson These processes offer means of measuring the CKM matrix element V tb. Cross sections will be checked at LHC.

I) W-gluon Fusion Standard Model prediction for cross section at Tevatron is  ~ 2.4 pb At LHC  ~ 250 pb  Will be largest source of single top production at LHC  Will create a background for other processes W-gluon fusion would be sensitive to Flavor Changing Neutral Currents

II) The W* Process Proceeds via production of virtual W boson W boson is significantly off its mass shell  m W = 80.4 GeV, m t = GeV Tevatron  ~ 0.9 pb LHC  ~ 10 pb W* process would be sensitive to the existence of a new, heavy W´ boson

III) Wt Production Cross section at Tevatron  ~ 0.12 pb LHC  ~ 60 pb

Decay to W Boson and b Quark (I) In the Standard Model t decays almost exclusively to a W boson and a b quark The W boson then decays to produce  Hadron jet S  A light lepton (electron or muon) and its neutrino  A tau lepton and its neutrino

Decay to W Boson and b Quark (II) Topology of the top-antitop decay depends on the decay modes of the two W bosons: Hadronic 44.4% Tau 21.1% Semileptonic 29.6% Dileptonic 4.9% Hadronic Tau Semileptonic

Tau Decays Why must decays resulting in a tau lepton be discarded?  Tau undergoes three body decay to produce a light lepton, the corresponding antineutrino, and a tau neutrino.  Tau undergoes two-body decay to produce   and a tau neutrino. The additional neutrinos produced in tau decay escape undetected, making it impossible to reconstruct the top quark.

I) Semileptonic Decays One W decays hadronically while the other decays to produce a light lepton and an antineutrino. Isolated lepton of large transverse momentum provides an efficient trigger. Complete final state can be reconstructed up to a quadratic ambiguity.

II) Dileptonic Decays Both W‘s decay to produce light leptons. Two isolated, high p T leptons allow efficient triggering. Final state neutrinos evade detection.  Neither top quark can be fully reconstructed.

III) Hadronic Decays Both W bosons decay to produce hadron jets Largest sample of top-antitop decays Suffer from a large QCD background

Measurement of the Top Quark Mass Most recent value of top quark mass from CDF and D0 experiments is  m t =172.7 ± 2.9 GeV m t is calculated using several different data samples and methods:  Semileptonic decays  Dileptonic decays  Hadronic decays

Uncertainty in m t (I) How precise does m t need to be?  Radiative corrections relate m t to m H and m W.  If  W ~ 20 MeV, then  t ~ 2 GeV is desirable.  Supersymmetric GUT‘s would benefit from  t ~ 1 GeV 8 million top-antitop pairs per year are expected at the LHC for low luminosity years. Statistical error will be minimized; uncertainty will be dominated by systematic error.

Measurement of m t Using Semileptonic Decays Important tool in selection of top-antitop events is the ability to identify b quarks. At LHC selection of semileptonic event requires:  Isolated lepton with p T > 20GeV  Missing transverse energy E T > 20 GeV  At least four jets, each with p T > 20GeV  Including two b jets W is reconstructed by combining the two jets not tagged as b-jets. W candidate is then combined with one b jet to reconstruct t quark. A combinatorial ambiguity remains.

Measurement of m t Using High p T Semileptonic Events Top and antitop will emerge back to back. Decay products will appear in two distinct hemispheres of detector. Requirements for selection include:  One b jet in same hemisphere as lepton  Three jets in hemisphere opposite lepton  Including one b jet Ambiguity in t reconstruction is greatly lessened. Background is negligible.

Measuring m t with J/  Events At least one b-quark decays to J/   Uniquely identifies b-jet J/  has extremely long lifetime  produces distinctive experimental signature Background free

Measuring m t With Hadronic Decays Identification will require  Six or more jets with p T >15GeV  At least two b-jets Reconstruction includes  Identifying two b-jets  Grouping remaining four jets into pairs  Assigning each pair to W boson  Combining W candidates and b candidates to reconstruct top and antitop

Systematic Error in m t Systematic errors in m t dominated by  Final state radiation  Jet energy scale At LHC final state radiation is expected to result in 1-2 GeV systematic errors. Jet energy calibration depends on  Nonlinearities in calorimeter response  Energy losses due to gluon radiation  Energy losses due to detector effects Energy of light-quark jets can be calibrated by assuming m jets = m W.. This provides an essential calibration tool for the ATLAS calorimetry system.

Uncertainty in m t (II) Within first year at LHC statistical error on m t is expected to be below 0.1 GeV Systematic error of less than 2 GeV will be possible using semileptonic events  if a good understanding of the jet energy scales can be obtained. Complementary measurements will be performed using dileptonic and hadronic samples.

Determining the Top Quark Charge t is expected to have charge Q t = +2/3 There remains the exotic possibility of charge Q t = - 4/3 The top antitop decay would then be  Q t will be measured at LHC  Via radiative process pp*  tt*    Q t 2

Top-antitop Spin Correlations Substantial top-antitop spin correlations are predicted in pair production.  At LHC 80% of top-antitop pairs will have two quarks with the same helicity. t decays before the strong interaction acts to depolarize its spin. Quark spin orientation will be preserved during weak decay. A measurement of this spin correlation could set an upper limit on the top quark lifetime. CP violation in production or decay could alter the predicted spin correlations.

Rare Decays and Branching Ratios Within the Standard Model t decays to W boson and b quark with branching ratio  Wb =99.9%. The predicted branching ratio for decay to W and s quark is  Ws =0.1%. The predicted branching ratio for decay to W and d quark is  Wd =0.01%. Experimental verification of branching ratios would provide a good test of the Standard Model.

Possible Production of Higgs Boson A significant branching ratio for the decay t  H + b is possible  If a charged Higgs boson H + exists If the decay is kinematically permitted If H+     Unexpectedly high rate of tau lepton production If H+  cs*  Unexpected jet production

Radiative Top Quark Decay m t is very close to decay threshhold for t  WbZ. Measurement of the branching ratio could provide strong constraint on top quark mass. The decay t  WbH might also be possible  Assuming a light, neutral Higgs boson  Experimental observation will not be possible since H  bb* suffers from large QCD background

Flavor Changing Neutral Currents (I) Events where a quark emits a neutral vector boson and changes to quark of different generation, same charge. t  Zq, t   q, t  gq where q is u or c quark. Within the Standard Model FCNC‘s are highly suppressed.

Flavor Changing Neutral Currents (II) The production of like charged top pairs would indicate an observation of FCNC‘s. Stringent experimental limits on FCNC decays of light quarks already exist. CDF has established upper limits on branching ratios for FCNC top quark decays.   (t  Zq) < 33%   (t   q) < 3.3%

Yukawa Couplings In the Standard Model the masses of the fundamental fermions are attributed to the strength of their Yukawa couplings to the Higgs boson. The measured value of m t implies that top has a Yukawa coupling of Y t ~ 1. An independent measurement of Y t might provide important insight into the mechanism of fermion mass generation.

Measuring Y t Search for gg  tt*H  Significant cross section only for a light Higgs boson H would be detected via H  bb*

Summary Characteristics of the top quark:  Pair production via strong interaction  Produced singly via weak interaction  Decays via weak interaction t  Wb  Current mass measurement m t = ± 2.9 GeV Uncertainty of  t ~1 GeV is desireable  Lifetime s  Decays before hadronization  Decays before spin is depolarized