An Update on Top Quark An Update on Top Quark PhysicsPhysics

Robert RoserFermilab

PASCOS06Sept 10th 2006

(the day after “the game”)

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Physics of the Top Quark

Top physics is still one of the more sexy things to study at the Tevatron…

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# of Physicists for Particle Discovery

Tevatron ~ 800

Year Discovered

Nu

mb

er

of

Ph

ysic

ists

LHC

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Top Quark Discovery: 1995The search for top lasted almost two

decades. It’s unexpectedly heavy mass delayed discovery.

April 1994: “Evidence for top production at the Tevatron” CDF– PRD 50, 2966 (1994).... lum =

19 pb-1

• 150 pages ! ..... 2.8 s excess

• Mtop = 174 (16) GeV & s(tt) = 14 (6) pb

• March 2nd, 1995: CDF and D0 announce simultaneously the discovery of the Top Quark !!!

• CDF: PRL 74, 2626 (1995) .... 67 pb-1

• D0: PRL 74, 2632 (1995).... 50 pb-1 Top Physics

begins…

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For You Bean Counters…• Run 1 duration:

– ~ 2 years of data taking• Number of collisions:

– ~ 5 x 1012 !• Event size:

– 180 Kbytes (DST)... 32 Kbytes (PAD)• Number of events on tape:

– ~ 50 millions• Number of tapes:

– ~ 1800• Number of people/expt:

– ~ 400 physicists• Number of events with a high Pt, isolated lepton:

• approx 200,000 e & µ

• Number of top event candidates– ~50 !

Cost is (if 1/2 billion $ spent): ~ 1 million dollars/evt. Generated ~ 5 million dollars/evt. detected

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Tevatron Physics

Trigger is key in reducing the hugeamount of interactions producing“something” in the final state(trigger efficiency for high pT leptons~90% for both CDF and D0)One top pair each 1010 inelastic collisions at s = 1.96 TeV

pb7.6)175@( GeVMttpp top

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Why Study Top?

Top pairs: (tt) ~7 pb

•Top production rate•Mass of top•W helicity in top events•QCD tests•New physics in X tt•Anomalous couplings, new particles

Single top: (tb) ~3 pb

•|Vtb|•QCD tests•New physics?

Great Place to test S.M. and Search for…

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Why Is Top So Interesting?

Well, top physics is different!

•Top quark lifetime is short: decays before hadronizing

No spectroscopy like other heavy flavor

Top momentum and spin transferred

to decay products

• Probes physics at higher scales than other known fermions

Top (or heavy top) very hip in many

EWSB models: Higgs, Top Color,

Little Higgs, SUSY mirror models

Is it only Top?

top ~ 10-24 s , 1 1.5 GeV 1 << QCD

-1 ~ (200 MeV)-1

top ~ 10-24 s , 1 1.5 GeV 1 << QCD

-1 ~ (200 MeV)-1

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Experimental Landscape: The Tevatron Collider

– Beam energy =980 GeV

– 36x36 bunches, 396 ns coll. sep.

– Recycler and e-cooling in use

– Pbar “stashes” >350e9 in recycler

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Tevatron LuminosityTevatron Luminosity

• Peak luminosity record: 2.2 x1032 cm-2 s-1

• Integrated luminosity– Weekly record: 33 pb-1 /week/expt – Total delivered: 1.7 fb-1 /expt. Total recorded: 1.5 fb-1 /expt

• Doubling time: ~1 year • Future: ~2 fb-1 by 2006, ~4 fb-1 by 2007, ~6-8 fb-1 by 2009

Today’s Presentation:300 pb-1 ~ 1 fb-1

Peak Luminosity Integ. Lum. (delivered) / Experiment2002 2003 2004 2005 2002 2003 2004 2005

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What Can We Expect?Total Luminosity (fb-1)

0

1

2

3

4

5

6

7

8

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10/1/03 9/30/04 9/30/05 9/30/06 9/30/07 9/29/08 9/29/09

Design

Base

We Are Here

Date

Ldt

/ex

pt

We Can Do It!!!

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The CDF Detector

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The DØ Detector

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Producing Top at Hadron Colliders

Run 1 Run 2 LHC

ppbar ppbar pp

ECM 1.8 TeV 1.96TeV 14 TeV

qq 90% 85% 5%

gg 10% 15% 95%

tt 5.0 pb 6.7 pb 830 pb

• Top produced strongly in pairs • Typical S/BG 0.5 at TeV, 2.5 at

LHC

85%

15%

•σ (tt)LHC~100xσ(tt)TEV

•Fraction of qq vs gg is inverted

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How Else is Top Produced?

Standard Model Tevatron Single Top Production

pb3)175@( GeVMXtpp top

p

t

t

p

XResonance Production?

Top Color-Assisted TechnicolorOR

?????

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How Does Top Decay?

Main “usable” top event topologies:• tt llbb di-lepton 5% e+• tt lqqbb lepton+jets 30% e+• tt qqqqbb all hadronic 45%

Standard Model:tWb ~ 100%

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What do we look for in top events?

Electrons Muons Neutrinos Quarks (Jets) b Quark Jets

“Lepton + Jets Channel”

W l , W qq ~30%

“Di-Lepton Channel”

W l W l ~5%

“All Hadronic Channel”

W qq, W qq ~45%

Identifying Top Quarks

=> Signature-Based Analyses!

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Particle Signatures• Electrons - deposit all their energy in electromagnetic

calorimeter which can be matched to a track • Photons - no track

• Muons: Match signal in muon chambers to track

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Particle Signatures (2)• Quarks - fragment into many particles to form a jet

– Leave energy in both calorimeters

• Neutrinos - pass through all material– Measured indirectly by imbalance

of transverse energy in calorimeters

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CDF Double-Tagged Event

b-jet taggers provide clean samples of single and double b-tagged events, useful for single top, top properties,

and searches such as for WH.

Impactparameterresolutionfor high-pT

tracks ~18m

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Lepton+Jets Cross Section•This is the golden channel for its high yield and relative purity (after b-tag)!

•Used in top property measurements, single top and Higgs searches.

has single bestresults in 750 pb-1

has results up to 370 pb-1

with 1 and 2 btags

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Dilepton Cross Section

b1.3(syst)p(stat)8.6)tσ(t 1.91.7

Signal to background already good enough with l+MET+>=2 jets

Even more purity with b-tagging

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All Hadronic Cross Section• Start from a sample >=6

jets (special trigger). Still overwhelmed by QCD multijets background.

• Combine topological selection and b-tag

• Lot’s of data to model background !

(360 pb-1) requires 2 btags and fits the dijet (no btag) and trijet (1 btag) distribution

(310 pb-1) requires at least1 btag and anti-MET cut.

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Cross Section Summary

• Different channels and techniques all in agreement

• Precision at 14%, No combined result as of yet

Tevatron goal: 10% uncertainty/experiment with 2 fb-1

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Top Mass• Fundamental Parameter of the Standard Model• This summer’s Tevatron Combination yields

• Based on that, EWK fit results

• Run II TDR Predictions– W Mass Width of 25 MeV and top mass uncertainty of 2 GeV– 35% constraint on

the Higgs Mass

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Introduction to the Top Mass

• It is the same for top events with the following complication1.t1 Wb jet – jet – jet

2.t2 Wb lep – – jet

• Each event has 2 top quarks– Two chances to measure its mass in each event– We don’t know which decay products belong to

which object

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Appreciating the Top Mass Measurement

Why is it so hard??– Life ain’t pretty…– Combinatorics of assigning jets to W/t– ISR/FSR jets

W+

W-

t t

b-jet

b-jet

jet

jet

X

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Template MethodEvaluate event-by-event best “reconstructed mass”, Mrec, by using observed kinematics of ttbar event (e.g.: c2 fitter)

Create “templates”, i.e. MCpredictions for Mrec usingdifferent true masses , Mtop.

DATA

Top Template

Bckg Template

Measure top mass withlikelihood fit of data Mrec tosignal + background template.

Improved jet energy scale by simultaneous fit in 4 samples to top mass and jet energy scale using Wjj decays

1

2

3

4

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Jet Energy Scale• JES uncertainties are the largest

source of systematics:

• Fit simultaneously for MW->jj and Mbjj using 2D templates of true Mtop and σJES

(680 pb-1) achieves world single best measurement and improves JES systematics by 40% by using in-situ

2top GeV/c (syst.) 1.3JES)2.5(stat.173.4M

best measurement (370 pb-1) uses a ME method with simultaneous JES fit

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Top Mass in DileptonUnder constrained system: two neutrinosbut only one MET measurement.

Both experiments use Matrix Element Technique

(Calculate event-by-event signalprobability curve (rather than single Mrec) using decay matrix element and transfer

functions.

1 fb-1

0.37fb-1

78 Evts

27.8 bckg

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Summary of Top Mass Results

0 5 10 15 20 25 30 35 40 45 50

CDF-I all-j

D0-II l+j

CDF-II (l+j)eCDF-II (l+j)i

D0-I l+j

CDF-I l+j

D0-II di-lCDF-II di-l

D0-I di-l

CDF-I di-l

Weight (%)

2top 2.3GeV/c172.5M

Combined Average:

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Top Charge and LifetimeTop quark in SM has charge 2/3e. Some models propose an exotic 4th quark with Q=4/3e

(365 pb-1) in l +4jets (2 btag) use a jet charge algorithm to discriminate between b and bbar.

When paired to l , top charge is inferred.

Data excludesQ = 4/3e @94% C.L.

Top in SM has very short lifetime (SM c ~ 3x10-10 m) ~10-24 seconds

(320 pb-1) look for anomalous lifetime by fitting impact parameter of lepton in l+jets events

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Search for Single TopSingle top is produced via weak

interaction at a rate ~1/3 that of top. Allows direct measurement of Vtb.

• Kinematically wedged between non-top and top signal, plus high backgrounds (S/B~1/20) require very sophisticated analysis techniques.

• Use l +MET+2jet (>=1 btag) events: same signature as

• s and t-channel searched jointly and separately (have different sensitivity to new physics).

s-channel production (W*)

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Single Top Limits(695 pb-1) has 2 analysis:1. Neural Network2. Multivariate Likelihood

function

95% observed (expected) exclusion limit getting close to SM expectations!

Projections(ignoring syst):•2.4 s excess with 1 fb-1•3 s excess around 1.5 fb-1

Based on SM single top XS

Stat error only

370 pb-1Uses a likelihood discriminant

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Does something New Decay to ttbar?

Resonance production

Look for bumps in the ttbar invariant mass spectrum

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ttbar Resonance searchlooks for generic spin 1 resonance (X0)• Assume ГX0 = 1.2%´MX0

• Test masses between 450 GeV and 900 GeV in 50 GeV increments.

(680 pb-1)

Set 95% confidence level limit for σX0 ateach mass.Exclude leptophobic Z’ with Mz’ < 725 GeV.

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Another Search in Top Sample

search for something with “top-like” signature, t’,

fit HT vs MWq

(Ht is the total energy in the event)

(760 pb-1)

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The Future of Top• TeV Measurements

will continue to improve

• Ultimate systematics at TeV or LHC will likely be energy-scale related

• Will help calibrate LHC detectors

<1 week of data taking

at 1033 cm-2 s-1

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Summary•The Tevatron Top program is well underway.

•We will exceed initial expectations on what we can achieve in measuring both the top mass and cross section.

•We may soon discover whether the top quark plays a special role in the Standard Model.

•The top sector will continue to be a hunting ground for hints of new physics at the Tevatron, and at the LHC

•With Increasing Luminosity, the program of top properties measurements will soon tell us how “top” like this particle really is.

•It is an exciting time to be a physicist

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History: Top Mass Publications