Tau lepton in ttbar decays at the LHC

Tau leptonsin ttbar decays at the LHCNuno AlmeidaOn behalf of LIP CMS GroupPASC Winter SchoolSesimbra-Portugal, 19-21 December 2009

OutlineIntroductionTau IdentificationTau ID in CMSLepton/Jet rejectionttbar cross section in the tau dilepton channelEvent SelectionEvent Yields@200 pb -1 and s 1/2 =10 TeVBackground estimation from dataConclusionsNuno Almeida 2

IntroductionThe LHC is a top factory! 1 ttbar pair/s prod.@low luminosity (L~10 33 cm -2 s -1 )Gluon fusion(~90%) Quark annihilation (~10%)Studying the top quark will allow the re-establishment of the top quarkmeasurements and may provide indication of new physics !Tau dilepton channel in ttbar decaysttbar decaysProvides a clean signature~5% of all ttbar decaysSignificant improvement is expected over TevatronCan be used to test lepton universality σ lτ/ σ eµ= 1Tau may preferentially couple with Higgs Boson(see talk from Leonardo P. “Search for the Charged Higgs in CMS at the LHC” )Nuno Almeida 3

Tau is the SM heaviest lepton (1.777 GeV/c 2 )Tau Decays :1 lepton (e/u) ( BR ~ 36% )1 charged hadron (BR ~50%)3 charged hadrons (BR~14%)Tau LeptonsHadronic decaying taus are well isolated and produce narrow jetsTau ID in CMSAlgorithm seeks leading track (p T>20Gev/c) within Jet axis (ΔR ~0.1)Signal cone (ΔR ~0.07) and isolation cone (ΔR 0.5) are definedaround the leading track1 or 3 tracks (p T>1GeV/c) are required to be found in the signal coneand no additional tracks on the isolation coneAdditional calorimeter information is used to reject electrons, muonsand jets contamination in tau reconstructed objectsTau Particle Flow ID also available and is based on gatheringinformation from all detectors giving an ID to jet constituentsNuno Almeida 4

Electron/Muon vetosElectrons may fake taus...Electrons have high pt tracks pointing atelectromagnetic clustersCharacterized by high energy deposits in theECAL and low energy deposits in HCALhadronic tausElectromagnetic fraction < 0.9 removes ~98%of the electronsMuons may fake taus...Muons MIP particules and do not leave muchenergy in calorimetersThe ratio E ( jet ) / p( lead track) >0.5 removes~99% of the muonsNuno Almeida 5

QCD Jet RejectionTracker IsolationQCD jets are wider and have highercharged hadron multiplicityNo tracks ( p T> 1GeV/c ) in isolationcone supresses ~ 90% of jets whilekeeping tau efficiency highCalorimeter IsolationQCD Jets deposit larger amounts ofenergy in ECALTausJetsΣE TECAL(Iso)< 3 GeV supresses ~ 60% ofjets, while tau efficiency remains above95%Nuno Almeida 6

Event Selection in the ttbar tau dilepton channelSingle lepton trigger (e/µ triggers )“W+jet” like selection≥ 1 isolated lepton (e/µ) with p T>20GeV/c,|η| 0.9Calorimeter iso : E calo/(E calo+calo iso) > 0.8 (e) or 0.9 (µ)≥ 2 jets with p T>30GeV/c,|η|40 GeVSignature : (e/µ) + 2 bjets + tau+ MET≥ 1 τ p T>20GeV/cOpposite sign ( leading e/µ and τ )( B-tagging )Nuno Almeida 7

Event yields @ 200/pb, 10 TeV1 prong tausAfter Opposite Sign : ~100 signal events (eτ/µτ) , with S/B ~ 0.7Background dominated by lqq in ttB-tagging may improve S/B by a factor of ~2Nuno Almeida 8

Estimating background from dataMain bkg coming from fake tau jets in eventsthat pass the “W+jet” like selection and that havean extra jetfake probability ~O(-3)Bkg can be computed by applying a tau fakeprobability to this inclusive jet (p T,η) distributionTau fake prob. can be evaluated from data :QCD multijetGamma+jetsOther main bkg comes from Drell-Yan events in particular accompanied by additionaljet production may appear as top events, when one tau decays leptonically and the otherhadronically, this background can be estimated by investigating the Mττ .Nuno Almeida 9

Estimating background from dataConvolution between “W+jet” like inclusive distribution and the tau fake rateDifferent jet (all jets and leading jet) categories have been considered for the tau fake ratesGood agreement (~10% using multijet-QCD samples) between yields from fake ratemethod and event selectionNuno Almeida 10

Cross section yieldsGood agreement obtained for 1 prong taus and after Opposite Sign cutThe ratio of cross sections (R) allows to test discrepancies from the SM predictionsIn R most of the systematics uncertainties cancel out, remaining the systematicsassociated with tau ID and and tau-fakeseµ is chosen as the normalization because of the smaller bkg and better S/BNuno Almeida 11

ConclusionsA study of the measurement of ttbar cross section with the tau dilepton channel (200 pb -1of data and with s 1/2 ~10 TeV ) was shown~ 100 eτ/µτ signal events are expected from simulationData driven technique for estimating the background was studied yielding goodagreement (~10% ) against expectationsMeasuring the ratio of tau dilepton cross sections will allows us to test SM predictions=Plans for 2010Improvement of tau id algorithmMeasurement of tau fake rates from dataFirst measurements on cross sections, and RNuno Almeida 12