Julio 2009 Jesús Vizán 1

Jesús Vizán (on behalf of CMS collaboration)

Física del quark top en CMS

Universidad de OviedoUniversidad de Oviedo

Departamento de FísicaDepartamento de Física

Outline.Outline. Introduction to Top Quark Physics at LHC First measurements at CMS

Top rediscovery and σ in tt dilepton channelo Detailed example

Top rediscovery and σ in tt semileptonic channel

Other top-quark properties and signatures Single top (t-channel), τ channels Mass, spin correlations, rare decays, tt resonnances

Top as a detector calibration tool B-tagging Jet Energy Scale

Summary

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What makes top-quark special?What makes top-quark special?

Top quark mass is a fundamental parameter of the EW theory. In SM, mIn SM, mtop top and mand mWW constrain Higgs mass constrain Higgs mass

Large mass and short life time makes top unique. It decays before fragmenting observe “naked” quark“naked” quark

Top quark in searches beyond the SM searches beyond the SM at CMS A decay product of new particles thanks to higher s Major background to many searches

Due to distinct experimental signatures and final state topologies, tt events will also constitute one of the main benchmark sample in detector commissioningbenchmark sample in detector commissioning, useful from the very early data taking period understanding of most physics objects required jet energy scale determination measurements of performance of b-tagging

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From Tevatron to LHCFrom Tevatron to LHC

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Discovered at Tevatron (1995) We know much about top

already from CDF and D0 Mass, spin, QCD coupling, EW

coupling, constraints on its mixing helicity in decays.

Except for mass, precision for most of the measurements is statistically limited

LHC opens up new era of precision measurements in the Top quark sector: ~8M top pairs & ~2M single top events/year expected at the low luminosity at s=14 TeV

In this talkIn this talk

Special emphasis on latest results approved by CMS: obtained considering 10 TeV collissions and 10 TeV collissions and focused on low luminosityfocused on low luminosity

More detailed description of the top-antitop dilepton cross-section measurement. Example of the kind of objects used for low luminosity

analyses, data-driven methods considered, study of systematic uncertainties etc

Participation of Spanish groups (U. Oviedo)

Early analysis are being repeated now considering 7 7 TeV collisionsTeV collisions. Not results approved yet but first comparisons and preliminary analysis ready.

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Top Rediscovery at CMSTop Rediscovery at CMS Measurement of top pair production cross section is one of top pair production cross section is one of

early physics goalsearly physics goals: Test the theoretical predictions at the LHC energy. Top quark

signal During the commissioning phase, the top quark signal will play an

important role in understanding the detector performance Extensive and robust analyses to extract the top signal. In

the beginning, focus on channels with leptonic W decay(s) leptonic W decay(s) without using b-tagging information and even missing Ewithout using b-tagging information and even missing ETT dilepton : simple counting experiment

o 3 independent analysis mergedo 6 institution: 3 USA, 3 Europe (including U. Oviedo); 27peopleo 2004 (CMS PTDR-2): 2 institution (U. Oviedo & Aachen)

lepton + jets: reconstruct top from 3-jet combination with highest vector sum pT. Further enhance the signal by finding one of the dijet combinations with mass closer to mW

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CMS PAS TOP-09-002CMS PAS TOP-09-002

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tt dilepton channel: signaturett dilepton channel: signature Relatively clean final state It represents a small fraction of

tt sample Signature

Two opposite signed isolated high PT leptons

µ+/µ- (1/81) Less fakes e/µ (2/81) Clearest Signal e+/e- (1/81) Completeness Events with leptonic tau decays

also considered as signal(1/45)

High Missing ET coming from the two neutrinos

Two b-tagged high ET jets

Advantages 2 charged leptons

o Good energy resolution

o Reduce backgrounds

Fewer jetso Reduce

dependence on JES

Disadvantages 2 neutrinos

o Loss of information

No hadronic Wo Can’t do in situ

calibration of JES

tt dilepton channel: Event Selectiontt dilepton channel: Event Selection Triggers

Used triggers depends final state

o µµ : Single Muon trigger (9 GeV)

o ee: Single Electron (15 GeV)

o eµ: OR of previous

Efficiency per lepton ~95%~95%

Efficiency per dilepton ~99%~99%

Leptons

At least two isol. leptons PPTT> 20 GeV, |> 20 GeV, |ηη|<2.4|<2.4

Isolation: Separate tracker and calorimeter cutsSeparate tracker and calorimeter cuts

Electrons faking muons: ΔR(e, µ’s) > 0.1

DY removal ee, µµ: |M-91| < 15 GeV|M-91| < 15 GeV

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CMS PAS TOP-09-002CMS PAS TOP-09-002 Jets

2 (SIScone) jets with EETT>30GeV, |>30GeV, |ηη|<2.4|<2.4

Njet = 0,1 cross-check sub-sample

MET ee, µµ > 30 GeV> 30 GeV: εε~86%~86%;reject ~70% DY~70% DY

eµ > 20 GeV> 20 GeV: εε~93% ~93% ;reject ~50% QCD~50% QCD

tt dilepton channel: expected event yieldtt dilepton channel: expected event yield

36 (e36 (eµ) + 25 (ee, µµ) signal eventsµ) + 25 (ee, µµ) signal events

eµ cleanest final state

DY main background in ee, µµ

15% stat uncertainty15% stat uncertainty

Fake leptons most visible in Njet=0,1

DY and fake leptons to be estimated using data-driven methods

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Ldt = 10 pb-Ldt = 10 pb-1 1 @ 10 TeV@ 10 TeV

tt dilepton channel: data-driven methodstt dilepton channel: data-driven methods

Estimate DY contribution Event count near Z-peak |M-91|<15GeV in data (Nin) used to estimate what’s

(outside) passing the Z-veto (Nout): NNout (est)out (est)DY DY = N= Ninin

DY DATADY DATARRMCMCout/inout/in

Use DY MC to predict RRMCMCout/inout/in=N=Noutout

DY MCDY MC/N/NininDYMCDYMC

30% systematics30% systematics: from variations in RMCout/in wrt MET, generators, conditions

Fake leptons Use fake-dominated events with loose leptons failing full cuts

Main variable ratio of fake leptons after full cuts wrt looser cuts

o FR=N(fakes | pass full cuts)/ N(fakes | pass loose ID&iso cuts) Main test: use FR from QCD and apply to Wjets events (with MC truth match

to real lepton) and compare to observed count in Wjetso Agreement within 15%, precision limited by MC statistics

50% systematics 50% systematics from FR, signal leptons not passing full cuts but passing loose cuts in fake-dominated sample, double fakes

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Dileptons: SystematicsDileptons: Systematics

Lepton ID and isolation to be obtained from tag and probe

JES: estimated from scaling all jets by 10% up/down

Theory: comparison with Pythia and MC@NLO

Residual backgrounds (tW, part of VV, DY→ττ) assigned 50% syst

Luminosity normalization uncertainty is treated separately

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Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi)

tt semileptonic (e+jets)tt semileptonic (e+jets) 1 isolated electron1 isolated electron: pT30 GeV, ||

2.5 reject events containing ’s 4 jets4 jets with pT30 GeV, ||<2.4 Loose electron veto to reduce Z+jets tightening to barrel-region of ||

<1.442 to reduce fake electrons from QCD

No b-tagging or MET No b-tagging or MET cut

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Ldt = 20 pb-1 Ldt = 20 pb-1

@ 10 TeV@ 10 TeV

CMS PAS TOP-09-004CMS PAS TOP-09-004

signal 1721

Bgd 108 10.3

W+Jets 57 2

Z+Jets 12 1

QCD 31 10

Single Top 8 0

To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3

M3: invariant mass of 3-jet combination giving M3: invariant mass of 3-jet combination giving

highest vector sum of jet pT’shighest vector sum of jet pT’s

Δσ/σ=23%(stat) ± 20% (syst) ±10%(lumi)

tt semileptonic (µ+jets)tt semileptonic (µ+jets) select exactly 1 isolated 1 isolated : pT20

GeV, ||2.1

veto events with 1 to reduce contamination from ttbar, Z+jets and diboson events

reject events with an isolated electron with pT>30 GeV

4 jets 4 jets with pT30 GeV, ||<2.4

No b-tagging and cut No b-tagging and cut on MET

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Ldt = 20 pb-1 Ldt = 20 pb-1

@ 10 TeV@ 10 TeV

CMS PAS TOP-09-003CMS PAS TOP-09-003

To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3

M3: invariant mass of 3-jet combination giving M3: invariant mass of 3-jet combination giving

highest vector sum of jet pT’shighest vector sum of jet pT’s

Δσ/σ=20%(stat) ± 25% (syst) ±10%(lumi)

signal 320

Bgd 171

W+Jets 140

Z+Jets 10

QCD 7

Single Top 14

Other signatures(t-channel)Other signatures(t-channel) Single top (t-channel)

template-fit method: takes advantage of spin correlations of decay products

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Cos angle(Cos angle(µ,µ, untagged jet) (top untagged jet) (top

rest frame)rest frame)

CMS PAS TOP-09-005 @10 TeVCMS PAS TOP-09-005 @10 TeV

Channels with hadronic τ (tt dileptons) S/B up to ~0.4 S/B up to ~0.4 (1-prong τ) using sequential cut

procedure (@ 14 TeV)

CMS PAS TOP-08-004 @14 TeVCMS PAS TOP-08-004 @14 TeV

Δσ/σ (200 pb-1)=35%(stat) ± 14%(syst) ± 10% (lumi)

Measure top-quark propertiesMeasure top-quark properties Large mass, large width → unique to top quark properties: tests of the V-A structure

of top decays; top spin; |Vtb|; couplings Top Quark Mass

o Measurement in the main decay modes (dilepton. semilepton) competitive wrt Tevatron (~ few GeV)

o Need good understanding of systematic uncertainties

Spin Correlations in tt decays accesible via an asymmetry measurement using semileptonic W decays

o Semileptonic W decay → ψ=ψ=angle(lepton/Wangle(lepton/W(restframe)(restframe),Wtop,Wtop(restframe)(restframe)) distr.) distr.

o ΔA ~ ± 0.05 ΔA ~ ± 0.05 (dominated

systematyc errors)

Measure R = |Vtb|2

o Contrary to Tevatron case measurement dominated by systematic uncertainties (~10%~10%) (250 pb-

1)

o Main uncertainty due to b-tagging

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(10 fb-1 @14 TeV P-TDR2)

CMS PAS TOP-09-001CMS PAS TOP-09-001

Anomalous top production and rare top decaysAnomalous top production and rare top decays

Large Yukawa coupling (~1) => Significant potential to discover new physics (top resonances, Z’,Kaluza-Klein modes, Susy)

FCNC rare decays (t->(Z,γ,g)q) can be investigated Smallest 5σ observation BR(t→Zq) = 14.9 x10-4 10 fb10 fb-1-1 @14TeV (PTDR-2) @14TeV (PTDR-2)

Smallest 5σ observation BR(t→γq) = 8.4 x10-4 10 fb10 fb-1-1 @14TeV (PTDR-2) @14TeV (PTDR-2)

Resonance Z’→ tt→ lνqqbb

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Expected limits on the Expected limits on the σσZ’ × Br(Z’Z’ × Br(Z’→→ tt) at tt) at 95% C.L. (100pb95% C.L. (100pb-1-1 @10TeV) @10TeV)

CMS PAS TOP-09-009CMS PAS TOP-09-009

Top as a calibration tool: b-taggingTop as a calibration tool: b-tagging tt events used to isolate a highly enriched b-jet sample Exploit it to calibrate jet algorithm and extract b-tagging effficiency εb

for energ From an enriched sample (topological/kinematicselection) , εb= (Ftag-

εb(1-Pb))/Pb , Ftag= measured fraction of jet tagged, Pb= b purity

Get εb versus ET and η of the jet

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Δεb /εb (1 fb-1)= 6 (10)% for barrel (endcap)

Main systematic ISR/FSR, event selection and purity

Top as a calibration tool: JESTop as a calibration tool: JES

Selection of tt→ µνbjjb final states and identification of hadronic top system

Use of b-tagging

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Obtain residual JES corrections residual JES corrections for light and heavy quarks for light and heavy quarks using world average values for Mtop and MW by means of least square kinematic fit.

~1%1% on b-JES and light-JES with 100pb-1

CMS PAS TOP-07-004 @14 TeVCMS PAS TOP-07-004 @14 TeV

SummarySummary Top events are essential in CMS. Important role to test the standard test the standard

model, search for new physics and calibrate detector performancemodel, search for new physics and calibrate detector performance Top events rediscovery already possible at pretty low luminosity

(robust methods). For 10 TeV collisions: Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi) (dilepton @10 TeV) Δσ/σ=23%(stat) ± 20% (syst) ± 10%(lumi) (semi e @10 TeV) Δσ/σ=20%(stat) ± 25% (syst) ± 10%(lumi) (semi µ @10 TeV)

Other challenging top signatures will be detected: Δσ/σ (200 pb-1)=35%(stat) ± 14%(syst) ± 10% (lumi) (Single top t-channel @10 TeV)

Precision measurements of top-quark properties Limits on the σZ’ × Br(Z’→ tt) at 95% CL<15 pb in the range [0.75,2] TeV (100 pb-1

@10 TeV) Spin correlations ΔA ~ ±0.05 (high lumi: 10 fb-1 @ 14 TeV)

Top events will be used to calibrate detector performance Mtop and Mw constrains to calibrate residual JES. Uncertainties as low as 1%

Estimate b-tagging efficiency [6-10]% uncertainty (1 fb-1)

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BACK-UP SLIDESBACK-UP SLIDES

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Top Quark ProductionTop Quark Production

Single topProcess Tevatron LHC

tt pair 7 pb 833 pb

t-channel 1.98 pb 246 pb

s-channel 0.88 pb 11 pb

Wt channel 0.25 pb 66 pb

In high energy proton colliders top quark is mainly produced in tt pairs: At LHC

o gg ~ 90%o qq ~ 10%

At Tevatrono gg ~ 15%o qq ~ 85%

Important increment for top-pair at LHC Also for t-channel and W+t channel (not

yet evidence at Tevatron)

Wt-channel s-channel

t-channel

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Top Quark Physiscs: DecayTop Quark Physiscs: Decay Final state

Energetic Jets b-jets Leptons Missing ET

All subdetectors in play Vital tool to validate detector

performance SM: Almost 100% to Wb tt pair classification depending

on W’s decay Fully hadronic Semileptonic Dilepton channel

tt semileptonic channel