Julio 2009 Jesús Vizán 1
Jesús Vizán
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
Julio 2009 Jesús Vizán 2
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
Julio 2009 Jesús Vizán 3
From Tevatron to LHCFrom Tevatron to LHC
Julio 2009 Jesús Vizán 4
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-IFCA)
Early analysis are being repeated now considering 7 7 TeV collisionsTeV collisions. Not results approved yet but first comparisons and preliminary analysis ready.
Julio 2009 Jesús Vizán 5
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. 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 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
dilepton : simple counting experimento 3 independent analysis mergedo 6 institution: 3 USA, 3 Europe (including U. Oviedo-IFCA)o 27peopleo 2006 (CMS PTDR-2): 2 institution (U. Oviedo-IFCA & Aachen)
Julio 2009 Jesús Vizán 6
CMS PAS TOP-09-002CMS PAS TOP-09-002
Julio 2009 Jesús Vizán 7
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
Julio 2009 Jesús Vizán 8
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
Julio 2009 Jesús Vizán 9
Ldt = 10 pb-Ldt = 10 pb-1 1 @ 10 TeV@ 10 TeV
tt dileptons: data-driven methodstt dileptons: 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
Substract from NinDYDATA non peak bck contribution using eµ channel
30% systematics30% systematics: from variations in RMCout/in wrt MET, generators, conditions
Fake leptons Main variable ratio of fake leptons after full cuts wrt looser cuts (QCD)
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
Final estimate: N(1lepton full cuts + 1 lepton loose but not full cuts) *FR/(1-FR)o Good agreement wrt MC prediction
50% systematics 50% systematics from FR, signal leptons not passing full cuts but passing loose cuts in fake-dominated sample, double fakes
Julio 2009 Jesús Vizán 10
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
Julio 2009 Jesús Vizán 11
Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi)
Dileptons: Spanish ContributionDileptons: Spanish Contribution IFCA U. Oviedo contribution to dilepton channel since 2006 (PTDR)
tt dilepton σ for 14 TeV and
tt mass in dilepton channel
tt Lepton + hadron τ
σ Measurement for low luminsoity 14 TeV and 10 TeV analysis approved in 2008 and 2009
7 TeV in process.
Julio 2009 Jesús Vizán 12
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
Julio 2009 Jesús Vizán 13
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
Julio 2009 Jesús Vizán 14
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
Julio 2009 Jesús Vizán 15
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)
Top-quark properties: MassTop-quark properties: Mass
Dilepton Build observable sensitive to top mass using as much information as possible of
the final state
With ~ 1fb-1 possible ~4.2%4.2% uncertainty (JES main uncertaintyJES main uncertainty)
Julio 2009 Jesús Vizán 16
Hadronic top-quark reconstruction: Pair of anti b-tagged jets combined with b-tagged jets according to largest likelyhood: Purity of ~82%Purity of ~82%
Studied different estimators: Simple gaussian fit or Simple gaussian fit or event by event ideogram or likelyhood function to event by event ideogram or likelyhood function to reflect compatibility P(Mreflect compatibility P(Mtt|M|MTT
truetrue))
ΔMt(10fb-1) = 0.32 (stat) + 1.15 (sys) GeV
Main uncertainties: pile-up, b-tagging, JESJES
Studied mainly in the dilepton and semileptonic channel
Semileptonic
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 Daughter particles carry spin information
o ΔA ~ ± 0.05 ΔA ~ ± 0.05 (dominated
systematyc errors)
Measure R = |Vtb|2
o Measurement dominated by systematic uncertainties (~10%~10%) (250 pb-1)
o Main uncertainty due to b-tagging
Julio 2009 Jesús Vizán 17
(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
Julio 2009 Jesús Vizán 18
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
energetic jets From an enriched sample (topological/kinematicselection),
εεbb=[F=[Ftagtag-- ε ε00(1-P(1-Pbb)]/P)]/Pbb, Ftag= measured fraction of jet tagged; Pb= b-purity and ε0=mistag rate from simulation
Get εb versus ET and η of the jet
Julio 2009 Jesús Vizán 19
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
Julio 2009 Jesús Vizán 20
Obtain residual JES residual JES corrections for light and heavy corrections for light and heavy quarks 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 100 pb-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 ΔMt ~ 1.2 GeV with 10 fb-1
Limits on the σZ’ × Br(Z’→ tt) at 95% CL<16 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)
Julio 2009 Jesús Vizán 21
Julio 2009 Jesús Vizán 22
BACK-UP SLIDESBACK-UP SLIDES
Julio 2009 Jesús Vizán 23
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
Julio 2009 Jesús Vizán 24
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