Top quark physics at CMS From Tevatron to LHC Reconstruction of the top quark events Precision measurements : m t, tT, t, W helicity, spin,

Top quark physics at CMS From Tevatron to LHC Reconstruction of the top quark events Precision measurements : mt, tT, t, W helicity, spin, ... Discovery potential

Standard Model : single-top, couplings, ... Beyond Standard Model : resonances, FCNC, ...

Calibration sample : jet energy scale, b-tag efficiency, ...

Jorgen D’Hondt (Vrije Universiteit Brussel)

Roberto Tenchini (Pisa)on behalf of the CMS Collaboration

CTEQ workshop, Michigan (US), 14-15 May 2007

CTEQ, Michigan, May 2007 Jorgen D'Hondt (Vrije Universiteit Brussel) 2

From Tevatron/LEP to LHC

Obtaining orders in magnitude in both the integrated luminosity and the energy, we will collect a huge amount of Standard Model benchmarks channels.

~109 events/10fb-1 W (200 per second)~108 events/10fb-1 Z (50 per second)~107 events/10fb-1 tt (1 per second)

These can be used as control/calibration samples for searches beyond the Standard Model, but can also be used to scrutinize even further the Standard Model.

pile-up becomes an important issue

(10 fb-1 = 1 year of LHC running at low luminosity 1033, hence by end 2009)

rates@1033


Top Quark Production at the LHC

~87 %

NLO cross-section NLO = 833 pb ~8M events/10fb-1

Some references (not a complete list!): (top pairs) N.Nason et al. Nucl.Phys. B303 (1988) 607, S.Catani et al. Nucl.Phys. B478 (1996) 273, M.Beneke et al. hep-ph/0003033, N.Kidonakis and R.Vogt, Phys.Rev. D68 (2003) 114014, W.Bernreuther et al. Nucl.Phys. B690 (2004) 81-137 (single-top) T.Stelzer et al. Phys.Rev. D56 (1997) 5919, M.C.Smith and S.Willenbrock Phys.Rev. D54 (1996) 6696, T.M.Tait Phys.Rev. D61 (2000) 034001

10 tt pairs per day @ Tevatron 1 tt pair per second @ LHCqq →tt : 85% gg→tt : 87%

?? single-top @ Tevatron 30 single-tops per minute @ LHC

p p

t

t

p p

t

X NLO = 6.6 pb NLO = 153 pb NLO = 60 pb

top & anti-top not equal

NLO = 4.1 pb NLO = 90 pb NLO = 60 pb top production anti-top production

s-channel

t-channel

associated tW

NLO(total) = 373 pb ~3.7M events/10fb-1

top pairs

single-top


Topics in top quark physics at the LHC

TOP

≈ 0.4 10-24 s

BOTTOM

W

L

PRODUCTIONCross sectionSpin-correlationsResonances XttFourth generation t’New physics (SUSY)Flavour physics (FCNC)

PROPERTIESMass (matter vs. anti-matter)ChargeLife-time and widthSpin

DECAYCharged HiggsW helicityAnomalous couplingsCKM matrix elementsCalibration sample !!

kinematic fit (mW)

jetsb-tagging

trigger

missing energy

This data will extend the Tevatron precision reach and allow new possible topics.


Current status of Simulationand Reconstruction

Still some time before real data… hence all based on Monte Carlo simulation Main generators used : PYTHIA 6.2 or AlpGen (Leading-Order)

does not include many features present in dedicated generators Next-to-Leading-Order applied where needed (eg. single-top production)

Simulation: studies based on both fast and full simulation ‘fast’ simulation: generated objects smeared to mimic the detector ‘full’ simulation: detector responds simulated with GEANT-4 large Data-Challenge efforts have been made to provide dedicated GEANT-4 simulation (created ~100M simulated events, ~1Mb/event) pile-up collisions added in low-luminosity settings (2.1033 cm-2s-1)

Reconstruction based on documented advanced tools Results illustrate what we can learn from CMS data upon arrival References: talk based on notes/information available on

CMS http://cms.cern.ch/iCMS (‘full’ simulation results from P-TDR)


Top Quark Pair Selection

Fully hadronic channel : 3.7Mevnt/10fb-1

• QCD multijet (2→2 parton processes) → apply also topological cuts

• at least 6-jets pT>30-40GeV, b-tags≥2 : S/B~1/9, ~2.7%

Lepton + jets channel (lepton = e/) : 2.5Mevnt/10fb-1

• W+jets→lv+jets, Z+jets→ll+jets, WW→lv+jets, WZ→lv+jets, ZZ→ll+jets

• before selection we have S/B ~ 10-5

• pTlepton>20GeV, kinematic fit, pT

jet>30GeV, b-tags≥2 : S/B ~ 27, ~6.3%

Di-lepton channel : 0.4Mevnt/10fb-1

• Drell-Yan processes, Z+jets→ll+jets, WW+jets, bb → Z-mass cuts

• pT>20GeV, ETmiss>40GeV, pT

jets>20GeV, b-tags=2 : S/B~5.5, ~5%

main backgrounds in lepton+jet and di-lepton channel from ttbar other decays


Top Quark Pair Production Cross-Section

Shape analysis (identical á la D0 in Phys.Lett. B626 (2005) 45) combine the topological/kinematic information of several event observables

not as powerful as at the Tevatron (CMS Note 2006/064 & ATLAS Eur.Phys.J.C39(2005)63)

Simple counting experiment possible due to high S/N after selection di-lepton : (e/) 0.9 (stat) ± 11 (syst) ± 3 (lumi) % (10fb-1)

() 1.3 (stat) ± 16 (syst) ± 3 (lumi) % (10fb-1) lepton+jet : (e/) 0.4 (stat) ± 9.2 (syst) ± 3 (lumi) % (10fb-1) fully hadronic : 3 (stat) ± 20 (syst) ± 3 (lumi) % (10fb-1)

AlpGen (W+jets)

PYTHIA(tt)

CMS Note 2006/064

norm

aliz

ed

Phys.Lett B626 (2005) 45



Breakdown of total uncertainty (CMS Note 2006/064) example in the semi-leptonic muon channel

dominated by uncertainty on b-tagging efficiency which is conservative when assuming 2% (rather than 5%) total uncertainty ~ 7% (10fb-1) 2-3 GeV uncertainty on mt is feasible via cross section measurement

conservative

→ Tevatron ~2% conservative


Properties: Top Quark Mass

TOP

≈ 0.4 10-24 s

BOTTOM

W

L

Most important parameter is the top quark mass (mt), to be

estimated with an accuracy of around mt ~ 1 GeV/c2.

fully hadronic channel: background rejection semi-leptonic channel: kinematic fits di-leptonic channel: neutrinos


‘golden channel’

CMS Note 2006/066

Three top quark mass estimators are investigated:1. Gaussian fit on reconstructed mass spectrum → mt

Simple

2. convolution with Gaussian parametrized ideogram → mtParamIdeo

3. convolution with full scanned ideogram → mtFullIdeo

A likelihood variable is constructed reflecting the probability for signal → Psign

Jet combinations are ordered according to a likelihood variable → Pcomb

A kinematic fit is applied (CMS Note 2006/023) forcing the W boson mass

The Ideogram is convoluted with a theoretical template

Maximum likelihood gives the estimated top quark mass 4th estimator: mt

FullIdeo but IterCone, MidPoint & kT should give same jet direction

Breit-Wigner Monte Carlo parametrized

can be fixed in kinematic fit



CMS Note 2006/066

Properties of the estimators :

spectrum with kinematic fit



CMS Note 2006/066 Main uncertainy arises from heavy quark jet energy scale.

(1.5%)

(2%)

(5% On-Off)

• Pile-Up: could be reduced to 10% and overlaps with JES, hence 5% taken of On-Off• JES: could reach 1.5% in well understood detector range (barrel) + energy flow (to be seen as a benchmark for the jet calibration methods) • B-tagging: 2% could be reached according to Tevatron experience

1 GeV uncertainty reachable with a good detector understanding


Top quark mass: other methods

From t l + J/ + X decays :

→ 100 fb-1 gives after selection ~ 1,000 signal events (S/B > 100) the large mass of the J/ induces a strong correlation with the top mass easier to identify (extremely clean sample) BR(overall in tt) ~ 5.3 x 10-5

hep-ph/9912320

1GeV @ 20fb-1

CMS Note 2006/058‘full’ simulation

First ‘full’ simulation study


Top quark mass: other methods

CMS Note2006/058 no jet related systematics

2 GeV uncertainty after 2 years of LHC running conservative estimate as most of the systematic uncertainties are related to the theoretical modeling of the events after a better understanding of these phenomena a 1 GeV uncertainty is feasible

TH

EO

RETIC

AL

EX

PER

IMEN

TA

L


CMS notes [1] ATLAS Eur.Phys.J. C39 (2005) 63-90 [2]LHC Yellow Report on Standard Model Physics [3]

Single-lepton channel (Full-Analysis) [1,2] 100 ~1000 High pT single-lepton sample (Jet-Analysis) [3] 250 ~1000 High pT single-lepton sample (Cluster-Analysis) [3] 150 ~1500 Single-lepton channel (Continuous jet algorithm) [2] 100 ~1000

Di-lepton channel (Jet-Analysis with mlb) [3] 900 ~1300 Di-lepton channel (Energy-Analysis) [3] 400 ~2000 Di-lepton channel (Tri-lepton events) [3] 1000 ~1500 Di-lepton channel (Full-reconstruction) [1,2] 300 ~1300

From t l + J/ + X decays [1,2,3] 1000 <1000

High pT fully-hadronic channel [1,2] 180 >3500

Stat.Unc.MeV

Syst.Unc.MeV

combining all those results could lead to a more precise measurementcombining all those results could lead to a more precise measurement (correlations to be estimated !!)(correlations to be estimated !!) systematic effects do not necessary overlap between analysessystematic effects do not necessary overlap between analyses

Expectation :Expectation : top mass determination better than 1 GeV after understanding the detector top mass determination better than 1 GeV after understanding the detector

Top quark mass: combination !?


Top versus anti-top

Measuring spin correlations is something different compared to observing spin correlations. The second one could be easier.

The top quark does not loose its spin properties before it is decaying. Hence in the decay of top quark pairs, angular correlations should be present.

results (CMS, 10fb-1) :good agreement with TopRex input

Main systematics are related to the jet energy scale and the b-tagging

CMS Note 2006/111


Single-top productionNever observed ?

Each channel sensitive to different signals

• heavy W’ → s-channel• FCNC → t-channel• H± → Wt-channel

Also directly related to |Vtb| to percent level(s-channel preferred, t-channel dominated by PDF scale uncertainties of ~10%)


Single-top production: t-channel

Production channel with largest cross section Optimized event selection (NLO ~ 243pb)

MET > 40 GeV b-jet: pT>35GeV, ||<2.5 light-jet: pT>40GeV, ||>2.5 (forward) topological cuts: mrec(top), mT(W)

Expected number of events (10fb-1) signal = 2389 tt = 1188 W+jets = 597 (CompHEP, TopReX, MadGraph) QCD = negligible (using factorisation of )

Resulting S/B~1.34 Estimate of cross section (10fb-1)

CMS Note 2006/084

3 0%

theory, JES, b-tagging


Single-top production: Wt-channel

Two production channels (CMS Note submitted) di-lepton → pT

/e>20GeV, pTj1>60GeV & pT

j2>20GeV (veto others), MET>20GeV

semi-lepton → pTe>30GeV & pT

>20GeV, 1 strong b-tag & 2 non-b-jets pT>35GeV (veto others), MET>40GeV .... jet pairing via likelihood variable

Reject jets with a bad reconstruction quality (fakes) → likelihood variable10fb-1 di-lepton semi-lepton

tW di-lepton 562 -

tW semi-lepton - 1699

tt 1437 7624

WW 65 -

t-channel <20 351

s-channel - 14

Wbb - 10

W2j - 130

W3j - 539

W4j - 80

multi-jet <10 508

S/B 0.37 0.18

Control regions for background reduce tt bck uncertainty 60%→negligible

Results (10fb-1) (di-lepton) 8.8 (stat) ± 23.9 (syst) ± 9.9 (MC stat) % (semi-lepton) 7.4 (stat) ± 17.7 (syst) ± 15.2 (MC stat) %

Main uncertainty from jet energy scale and b-tagging (will improve with better detector understanding)

CMS Note 2006/086


Top quarks for calibration: Jet Energy Scale

Rescale each jet with relative energy shift C (rescaling |p| to keep jetmass invariant) Remake/refit the obtained W mass spectrum → mW(C) from fit Solve the simple equation mW(C|data) = MW

PDG → best estimate for C

Compare this measured shift with the true shift from Monte Carlo information (for well matched jet-parton couples (R<0.2) one can determine the average C)

Result : Cmeas = -14.96 ± 0.26 % (Ctrue = -14.53 %) with 5.41fb-1

mW

C(%) Erec/Egen

true shift Ctrue

measured shift Cmeas

scaling to 1fb-1 incl. electron channel : 0.4%

used space-angle match(~1% effects with R)

L = 0.33fb-1

Pile-up (On/Off) = 3%

CMS Note 2006/025


Top quarks for calibration: b-tag efficiency

The b-tagging efficiency can be measured to 4% in barrel and 5% in endcaps. Crucial for a correct estimation of the selection efficiency of Higgs decays to bb.

Optimize the jet pairing efficiency via mass constraints in kinematic fits. Select a very pure b-jet sample on which the b-tag algorithms can be applied and the efficiency estimated. CMS Note

2006/013


Comparing the direct and indirect values of mH

Ultimate test of the Standard Model

To give mt and mW equal weight :mW = 0.7 10-2 mt

Goal of LHC experiments :mt < 1 GeVmW < 15 MeV

mH/mH < 25 %

After a discovery one can use EW measurements to differentiate

between SM or MSSM Higgs bosons

W boson mass references:CMS Note 2006/061ATLAS ATL-PHYS-PUB-2006-007


Conclusions and outlook

Potential of top quark physics with CMS has been demonstrated CMS published a Physics-TDR vol.1 on detector & reconstruction tools and simulation results from CMS (Physics-TDR vol.2) based on dedicated reconstruction & analysis tools (incl. trigger) applied in realistic state-of-the-art settings of the detector talk is based on hundreds of pages of public (or almost public) notes

From Tevatron to LHC cross sections of signal and background are favourable for many topics LHC has the potential to go significantly beyond Tevatron

Outlook... prepare for data taking top quarks are crucial for calibration and commissioning at start-up and beyond jet calibration and b-tagging performance can be derived from data via top quarks over the next year prepare to extract top quark out of this first data

thanks to all CMS collaborators who contributed to these Top Quark studies


Extra’s

Some extra informative slides :

lepton reconstruction: resolution jet reconstruction b-tagging performance missing transverse energy kinematic fit performance trigger dependency of cross section on top quark mass some distributions of event selection observables semi-leptonic: other top mass estimators di-lepton events: other top mass estimators top mass and W boson mass breakdown uncertainty W polarization more possibilities beyond the Standard Model


Event reconstruction: lepton reconstruction

Good resolution is needed for triggering (cfr. turn-on curve) good performance of tracking, calorimeter and muon systems muon pT resolution ~1.5% (CMS P-TDR vol.1) electron pT resolution ~1% (CMS P-TDR vol.1)

Resolution checked with tt events lepton+jet (CMS Note 2006/024)

Lepton isolation (several lepton candidates per event) combine several observables per lepton candidate likelihood ratio variable correct identification of tlb lepton in 99% of the selected events

effect of magnetic field

() ~ ||

lepton+jet (tt)

CMS Note 2006/024


Event reconstruction: jets

Jet clustering : mostly Iterative Cone R=0.5 (optimization eg. Les Houches ’05)

calorimeter towers above E & ET thresholds (added to online zero suppression)

JES calibration : via method which will be applied on data (to the % level) +jet and Z+jet events (CMS Note 2006/042, ATLAS P-TDR): based on pT balans Wjj events (CMS Note 2006/025, ATLAS P-TDR): from well known W boson mass

Pile-up and underlying event treatments (¤) reject jets not attached to primary vertex (CMS Note submitted) underlying event measurements (CMS Note 2006/067)

#jet

s pe

r ev

ent

(¤) after primary vertex constraint

lepton+jet (tt)

CMS Note 2006/066

CMS P-TDR (vol.1)


Event reconstruction: b-tagging

Combined secondary vertex b-tagging algorithm (CMS Note 2006/014) based on the combination of several topological and kinematical observables

CMS Note 2006/064

60%

uds

c

g

10%

2%<1%

CMS P-TDR (vol.1)

General performance plot QCD jets 50<ET<80 GeV in barrel ||<1.4

distribution of discriminator in lepton+jet events

lepton+jet (tt)


Event reconstruction: missing ET

Missing transverse energy challenging due to multiple pile-up collisions also magnet field will influence resolution (photons versus pions) but very good hermiticity and granularity

Missing ET performs well at the LHC but rather hard cuts are needed

Resolution of 20 GeVfor ET

miss ~ 40GeV

CMS P-TDR (vol.1)

inclusive tt

before jet correction

after jet correction

single-top & tt

CMS Note 2006/035

40 GeV

QCD multi-jet


Event reconstruction: kinematic fit

~70% purity

To obtain the same precision without the fit, one needs 5 times more data (the bias wrt 175GeV is reduced)

Using kinematic fit techniques (applying Least-Square methods with Lagrange multipliers) mass constraints can be enforced to the reconstructed event topology. in the t→Wb decay constrain the W boson mass to its precisely measured value

CMS Note 2006/023 CMS Note 2006/023


Triggering

CMS L1 & HLT triggers applied on full simulation CMS Collaboration ‘DAQ and High-Level Trigger’ TDR

fully hadronic: inclusive jet trigger ~17% (QCD reduced to 23 Hz, S/B~1/300) b-jet stream improvement of 15% on efficiency

semi-leptonic: single-lepton triggers (pTe>26GeV, ||<2.4 and pT

>19GeV, ||<2.1) ()~62% and (e)~48%

di-leptonic: also di-lepton triggers (pTe> 12/12 GeV and pT

> 7/7 GeV) ()~88% and (ee)~77%

single-top: .OR. of single-lepton triggers and e+jet (pTe>19GeV and pT

jet>45GeV) (W→lv) in s-channel: 24% 25% 7% 38% (W→lv) in t-channel: 25% 25% 7% 43%

single- single-e e+jet combined

Thresholds and rates to be updated in Physics-TDR vol.2→ changes in algorithms and improved understanding of the background→ single-lepton are the dominant streams (rate(e)~23.5Hz and rate()~25.8Hz) (numbers for L = 2.1033 cm-2s-1)



Sensitive to top massSensitive to top mass : ~ 5 mt/mt 5% on gives 2 GeV on mt

Cross-section sensitive to renormalisation and factorisation scale, and to the choice of PDF (Parton Density Function)

systematics dominated by the uncertainty on the luminosity

ATLAS ATL-PHYS-PUB-2005-024


Top Quark Pair Selection: distributions

Fully hadronic channel:

Neural Network output combining the information of several topological obervables (scaled to effective cross section)

CMS Note 2006/077

CMS Note 2006/077

Di-leptonic channel:

Mass window around the Z boson mass rejects basically all Z+jet events


Flavour Physics : like-sign top quark pairs

at 30fb-1 a pptt cross section of 1pb becomes visible as a 5 effect on the

ratio R

Di-lepton top quark pairs have a clear topology 2 b-jet and 2 isolated leptons with a different charge, selected with a large S/N exploit the performance of the lepton isolation criteria (CMS Note 2006/024)

Motivation for this search FCNC (in SM suppressed, Z’ bosons in Topcolor assisted Technicolor (TC2)) from top- and techni-pion in TC2 models in MSSM from for example gluino pairs

Variable to be measured is R = N++,- - / N+-

ratio of events with a pair of leptons with same and different electric charge most of the systematics cancel in the ratio

CMS Note 2006/065

Documents

Top quark physics at CMS From Tevatron to LHC Reconstruction of the top quark events Precision measurements : m t, tT, t, W helicity, spin,