Measurement of the ttbar cross section in the e t and mt dilepton chan nels at the LHC

18 December 2008 Nuno Almeida

Measurement of the ttbar cross section in theMeasurement of the ttbar cross section in theee and and dilepton chan dilepton channels at the LHCnels at the LHC

(With the CMS detector)(With the CMS detector)

N. AlmeidaOn behalf of LIP CMS Group

PASC Winter School,17-19 December

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OutlineOutline

IntroductionIntroduction

Event Selection & Event YieldsEvent Selection & Event Yields

Data driven method for estimating the backgroundData driven method for estimating the background

ttbar cross sectionttbar cross section

Probing new physics with the tau dilepton channel Probing new physics with the tau dilepton channel

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IntroductionIntroduction

Why studying top quark ?Why studying top quark ? The LHC is a top factory! The LHC is a top factory! 1 ttbar pair/s @ low luminosity (L~101 ttbar pair/s @ low luminosity (L~103333 cm cm-2-2ss-1-1))Produced mostly by gluon fusion(~90%) and quark annihilation (~10%)Produced mostly by gluon fusion(~90%) and quark annihilation (~10%)

It will allow the re-establishment of the top quark measurements and may provide It will allow the re-establishment of the top quark measurements and may provide indication of new physicsindication of new physics

~ 840 pb~ 840 pb

Tau dilepton channel with hadronic tau decays :Tau dilepton channel with hadronic tau decays :

Provides a clean signature and ~5% of all ttbar decaysProvides a clean signature and ~5% of all ttbar decays

Significant improvement is expected over Significant improvement is expected over

Tevatron measurements (limited by statistics)Tevatron measurements (limited by statistics)

Tau lepton may preferentially couple with Higgs BosonTau lepton may preferentially couple with Higgs Boson

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Event SelectionEvent Selection

e/e/ Trigger Trigger

≥ ≥ 1 isolated lepton (e/1 isolated lepton (e/): p): pTT>20 GeV/c>20 GeV/c

≥ ≥ 2 jets E2 jets ETT>30 GeV in |>30 GeV in ||<2.4|<2.4

≥ ≥ 1 tau (tau ID + e/μ vetos)1 tau (tau ID + e/μ vetos)[See[See Marcelo’s talk :Tau Rec. ID. in CMS for ttbar events” Marcelo’s talk :Tau Rec. ID. in CMS for ttbar events”]]

MET>60 GeVMET>60 GeV

Opposite Sign (OS) of leading lepton (e/Opposite Sign (OS) of leading lepton (e/))

and leading track associated with the and leading track associated with the

Possible to apply b-taggingPossible to apply b-tagging

2 classes of events : 2 classes of events : 1 [BR ~50%] and 3 prongs [BR~14%]1 [BR ~50%] and 3 prongs [BR~14%]

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Jet multiplicity for signal and main backgrounds

signature : (e/μ) +2 b-jets, MET

≥2jets


MET cut more effective in Z+jet bkg suppression

MET discrimination power higher than HT

ttbar events characterized by large HT (large top quark mass)

HT versus METHT versus MET

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MET > 60 GeV65% of signal and 30% of bkg

MET > 60 GeV

(Before OS cut)(Before OS cut)


Event YieldsEvent Yields

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After OS cut ~ 60 signal events selected and S /B ~0.7

B-tagging improves S/B ratio by a factor of ~2

S/B ratios and event yields larger for 1 prong selection ( 3 prongs : signal: ~12 events, S/B ~0.2 after OS)

Analysis performed with full simulation and reconstruction of CMS detector (100 pb-1 and s1/2 ~14 TeV )

1 prong selection


Background estimation (I)Background estimation (I)

Tau fakes computed from data :Tau fakes computed from data :

Main background coming from “ W + jets” like events with 3 or more jets where Main background coming from “ W + jets” like events with 3 or more jets where one jet is misidentified as a tauone jet is misidentified as a tau

Background computed by applying the tau fake probability to the inclusive jet Background computed by applying the tau fake probability to the inclusive jet distribution after “W+jet” like selection plus one “tau” jetdistribution after “W+jet” like selection plus one “tau” jet

““W+jet”selection : ≥ 1 isolated lepton (e/W+jet”selection : ≥ 1 isolated lepton (e/) p) pTT>20 GeV/c>20 GeV/c in | in ||<2.4, ≥ 2 jets E|<2.4, ≥ 2 jets ETT>30 GeV in |>30 GeV in ||<2.4 MET>60GeV|<2.4 MET>60GeV

Fakes probability can be evaluated from QCD multi-jet/Fakes probability can be evaluated from QCD multi-jet/+jet samples, +jet samples,

by computing the ratio between the jets passing the tau-ID and the inclusive by computing the ratio between the jets passing the tau-ID and the inclusive jet distributionjet distribution

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Background estimation (II)Background estimation (II)

Fake rate constant over pT and Fake rate constant over pT and the barrel region |the barrel region |<1.5<1.5

Fake rates applied to the “w+jet” Fake rates applied to the “w+jet” like inclusive distribution and the like inclusive distribution and the extra “tau” jetextra “tau” jet

Fake rates computed Fake rates computed for different jet categories :for different jet categories :

All jetsAll jets Leading jetsLeading jets Next to leading jetsNext to leading jets Back to back jetsBack to back jets

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Fake rate projections on pT (all jet category)

2D Fake rate matrix (all jet category)

Tau fake rate O(10-3)


Background estimation (III)Background estimation (III)

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Fake rate obtained from +jet is higher than the fake rate obtained from QCD multi-jets. Difference comes from jet flavor composition which has a larger quark-jet contribution in +jet samples.

Combining results from both samples (QCD multi-jet and +jet) for each jet category yields a good agreement (~10%) between fakes computed from “data” and Monte Carlo expectations.


Other Background sourcesOther Background sources

Lepton pairs with additional jet production and mis-reconstructed missing Lepton pairs with additional jet production and mis-reconstructed missing

ET can mimic ttbar dilepton events :ET can mimic ttbar dilepton events :

Drell YanDrell Yan Z/Z/*→ee, *→ee, , , + jets + jets

Background can be estimated from MBackground can be estimated from M evaluation evaluation

Other backgrounds like Wbb, Wcc are found to be negligible from Other backgrounds like Wbb, Wcc are found to be negligible from simulation simulation

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Cross Section MeasurementCross Section Measurement

ttbar cross section:ttbar cross section:

Computed from the number of selected candidates (Nobserved), the estimated Computed from the number of selected candidates (Nobserved), the estimated background (Nbackground), the integrated luminosity (L) and the total Acceptance background (Nbackground), the integrated luminosity (L) and the total Acceptance (Atot) :(Atot) :

Results for 1 prong and after opposite sign cut :Results for 1 prong and after opposite sign cut :

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Systematics included due to tau-ID (variation of 10% from default values) , estimation of -fakes , Other source of systematics :

miscalibration and misalignement, luminosity, ISR/FSR/fragmentation/PDF, b-tagging, trigger efficiency,…


Probing new physics with Probing new physics with (e/(e/) channel) channel

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SummarySummary

The potential for measuring The potential for measuring tt tt in dilepton events with one e/in dilepton events with one e/lepton and lepton and one hadronically decaying tau was presentedone hadronically decaying tau was presented

S/B ~ 0.7 is found after full event selection for 1-prong tau decays. S/B ~ 0.7 is found after full event selection for 1-prong tau decays. B-tagging can improve results by a factor of two.B-tagging can improve results by a factor of two.

Analysis performed after full simulation and CMS detector reconstruction Analysis performed after full simulation and CMS detector reconstruction (100 pb(100 pb-1-1 and s and s1/21/2 ~14 TeV ) ~14 TeV )

Most important background source comes from W+jets like events, a Most important background source comes from W+jets like events, a method to derive this background from data is used and good agreement method to derive this background from data is used and good agreement is found with MC expectations.is found with MC expectations.

The ratio R =The ratio R =ll//eecan be used to investigate possible excess of tau can be used to investigate possible excess of tau production and probe new physics.production and probe new physics.

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Measurement of the ttbar cross section in the e t and mt dilepton chan nels at the LHC

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