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

Qua

rks

II III

Lept

ons

The Particle Masses

0.0001

0.001

0.01

0.1

1

10

100

1000

electron u d s muon c taon b W Z H t

Mas

s (G

eV)

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)

mt = 172.7 ± 2.9 GeV Most massive known elementary particle Lifetime 10-24 s Width t= 1.4 GeV

Lifetime is very short in comparison to the hadronization time 1/QCD ~ 10

-23 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. Mt introduces significant radiative corrections to mW. An accurate measurement of mt will help to constrain

mH.

Why is the top Quark Interesting? (II)

May provide a probe of the mechanism responsible for fermion mass generation.

mt 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 producedI. Via fusion of a W boson and a gluonII. Via production of a virtual W* bosonIII. In conjunction with a W boson

These processes offer means of measuring the CKM matrix element Vtb.

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

mW = 80.4 GeV, mt = 174.3 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 jetS 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:

Hadronic44.4%

Tau21.1%

Semileptonic29.6%

Dileptonic4.9%

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 pT 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 mt=172.7± 2.9 GeV

mt is calculated using several different data samples and methods:Semileptonic decaysDileptonic decaysHadronic decays

Uncertainty in mt (I)

How precise does mt need to be? Radiative corrections relate mt to mH and mW. 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 mt 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 pT > 20GeV Missing transverse energy ET > 20 GeV At least four jets, each with pT > 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 mt Using High pT 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 mt 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 mt With Hadronic Decays

Identification will require Six or more jets with pT>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 mt

Systematic errors in mt 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 mjets = mW.. This provides an essential calibration tool for the ATLAS calorimetry system.

Uncertainty in mt (II)

Within first year at LHC statistical error on mt 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 Qt = +2/3 There remains the exotic possibility of charge Qt= - 4/3 The top antitop decay would then be

Qt will be measured at LHC Via radiative process pp*tt* Qt

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 tH+ 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 mt is very close to decay threshhold for tWbZ. Measurement of the branching ratio could

provide strong constraint on top quark mass. The decay tWbH might also be possible

Assuming a light, neutral Higgs boson Experimental observation will not be possible since Hbb* 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.

tZq, tq, tgq 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. (tZq) < 33% (tq) < 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 mt implies that top has a Yukawa coupling of Yt ~ 1.

An independent measurement of Yt might provide important insight into the mechanism of fermion mass generation.

Measuring Yt

Search for ggtt*HSignificant cross section only for a light Higgs

boson H would be detected via Hbb*

Summary

Characteristics of the top quark: Pair production via strong interaction Produced singly via weak interaction Decays via weak interaction tWb Current mass measurement mt = 172.7± 2.9 GeV

Uncertainty of t~1 GeV is desireable Lifetime 10-24s Decays before hadronization Decays before spin is depolarized