Jesús Vizán (on behalf of CMS collaboration)
description
Transcript of Jesús Vizán (on behalf of CMS collaboration)
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) Spin correlations Rare decays
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 at LHC 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 and lepton ID
tools
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 put 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, 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.
Julio 2009 Jesús Vizán 5
Top Rediscovery at CMSTop Rediscovery at CMS Measurement of top pair production cross section is one of
early 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 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
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
Julio 2009 Jesús Vizán 8
CMS PAS TOP-09-002CMS PAS TOP-09-002 Jets
2 (SIScone) jets with ET>30GeV, |η|<2.4
Njet = 0,1 cross-check sub-sample
MET ee, µµ > 30 GeV: ε~86%; reject ~70% DY
eµ > 20 GeV: ε~93%; reject ~50% QCD
tt dilepton channel: expected event yieldtt dilepton channel: expected event yield
36 (eµ) + 25 (ee, µµ) signal events
eµ cleanest final state
DY main background in ee, µµ
15% 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 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)
Use DY MC to predict 30%30% systematics: from variations in Rout/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 variable ratio of fake leptons after full cuts wrt looser 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%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)
tt semileptonic (e+jets)tt semileptonic (e+jets) 1 isolated electron: pT30 GeV, ||
2.5 reject events containing ’s 4 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 cut
Julio 2009 Jesús Vizán 12
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 : 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 with pT30 GeV, ||<2.4
No b-tagging and cut on MET
Julio 2009 Jesús Vizán 13
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
Single top (t-channel)Single top (t-channel) σ(t)~300pb~1/3 σ(tt) important
back template-fit method is proposed,
that takes advantage of the spin correlations of the decay products in signal events
Julio 2009 Jesús Vizán 14
CMS PAS TOP-09-005CMS PAS TOP-09-005
Δσ/σ (200 pb-1)=35%(stat) ± 14% (syst) ± 10% (lumi)
Cosine of the angle between charged muon and Cosine of the angle between charged muon and
untagged jet, in the reconstructed topuntagged jet, in the reconstructed top
rest frame after the full event selectionrest frame after the full event selection
Top propertiesTop properties Large mass, large width => unique to top quark properties:tests of the
V-A structure of top decays; top spin; |Vtb|; charge; couplings; rare decays
SemileptonicW decay -> distribution of ψ= angle(lepton/W-restframe, Wtop-restframe)
Julio 2009 Jesús Vizán 15
Precision ~O(20%) (10 fb-1)
Top spin correlations in tt decays: accessible via an asymmetry measurement:
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
Resonance Z’→ tt→ lνqqbb
Julio 2009 Jesús Vizán 16
Expected limits on the Expected limits on the σσZ’ × Br(Z’Z’ × Br(Z’→→ tt) at 95% C.L. for an integrated luminosity tt) at 95% C.L. for an integrated luminosity
of 100pb-1 with and wihtout systematic uncertaintiesof 100pb-1 with and wihtout systematic uncertainties
@ 10 TeV@ 10 TeV
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 jet
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
Julio 2009 Jesús Vizán 17
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-> lνbjjbfinal states and identification of
hadronictop system {jjb} Use of the B-tagging
Julio 2009 Jesús Vizán 18
Rescale each jet with relative shifts {ΔE(light-jet),ΔE(b-jet)}, remake/refit W had. Mass and had. top (bW) mass spectra, solve the equation M(top,W;{ΔE})=M(top,W)PDG-> best estimate of {ΔE}
~1%1% on b-JES and light-JES with 100pb-1
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. For
10 TeV collisions: Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi) (dilepton) Δσ/σ=23%(stat) ± 20% (syst) ± 10%(lumi) (semi e) Δσ/σ=20%(stat) ± 25% (syst) ± 10%(lumi) (semi µ)
CMS will improve many measurements of top-quark properties Limits on the σZ’ × Br(Z’→ tt) at 95% C.L <15 pb in the range [0,75,2] TeV
Julio 2009 Jesús Vizán 19
Julio 2009 Jesús Vizán 20
BACK-UP SLIDESBACK-UP SLIDES
Julio 2009 Jesús Vizán 21
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 22
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