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

1. Measurement of the ttbar cross section in the etandmtdileptonchannels at the LHC (With the CMS detector) N. Almeida On behalf of LIP CMS Group PASC Winter School,17-19 December

2. Outline • Introduction • Event Selection & Event Yields • Data driven method for estimating the background • ttbar cross section • Probing new physics with the tau dilepton channel

3. Introduction • Why studying top quark ? • The LHC is a top factory! 1 ttbar pair/s @ low luminosity (L~1033 cm-2s-1) Produced mostly by gluon fusion(~90%) and quark annihilation (~10%) • It will allow the re-establishment of the top quark measurements and may provide indication of new physics ~ 840 pb • Tau dilepton channel with hadronic tau decays : • Provides a clean signature and ~5% of all ttbar decays • Significant improvement is expected over Tevatron measurements (limited by statistics) • Tau lepton may preferentially couple with Higgs Boson

4. Event Selection • e/m Trigger • ≥ 1 isolated lepton (e/m): pT>20 GeV/c • ≥ 2 jets ET>30 GeV in |h|<2.4 • ≥ 1 tau (tau ID + e/μ vetos) [See Marcelo’s talk :Tau Rec. ID. in CMS for ttbar events”] • MET>60 GeV • Opposite Sign (OS) of leading lepton (e/m) and leading track associated with the t • Possible to apply b-tagging • 2 classes of events : • 1 [BR ~50%] and 3 prongs [BR~14%] signature : (e/μ)t +2 b-jets, MET ≥2jets Jet multiplicity for signal and main backgrounds

5. HT versus MET ttbar events characterized by large HT (large top quark mass) MET discrimination power higher than HT MET cut more effective in Z+jet bkg suppression MET > 60 GeV MET > 60 GeV 65% of signal and 30% of bkg (Before OS cut) (Before OS cut)

6. Event Yields • Analysis performed with full simulation and reconstruction of CMS detector • (100 pb-1 and s1/2 ~14 TeV ) 1 prong selection • 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)

7. Background estimation (I) Tau fakes computed from data : • Main background coming from “ W + jets” like events with 3 or more jets where one jet is misidentified as a tau • Background computed by applying the tau fake probability to the inclusive jet distribution after “W+jet” like selection plus one “tau” jet “W+jet”selection : ≥ 1 isolated lepton (e/m) pT>20 GeV/c in |h|<2.4, ≥ 2 jets ET>30 GeV in |h|<2.4 MET>60GeV • Fakes probability can be evaluated from QCD multi-jet/g+jet samples, by computing the ratio between the jets passing the tau-ID and the inclusive jet distribution

8. Background estimation (II) • Fake rate constant over pT and the barrel region |h| <1.5 • Fake rates applied to the “w+jet” like inclusive distribution and the extra “tau” jet • Fake rates computed for different jet categories : • All jets • Leading jets • Next to leading jets • Back to back jets Fake rate projections on pT (all jet category) Tau fake rate O(10-3) 2D Fake rate matrix (all jet category)

9. Background estimation (III) • Fake rate obtained from g+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 g+jet samples. • Combining results from both samples (QCD multi-jet and g+jet) for each jet category yields a good agreement (~10%) between fakes computed from “data” and Monte Carlo expectations.

10. Other Background sources Lepton pairs with additional jet production and mis-reconstructed missing ET can mimic ttbardilepton events : • Drell Yan Z/g*→ee, mm, tt + jets Background can be estimated from Mtt evaluation • Other backgrounds like Wbb, Wcc are found to be negligible from simulation

11. Cross Section Measurement • ttbar cross section: • Computed from the number of selected candidates (Nobserved), the estimated background (Nbackground), the integrated luminosity (L) and the total Acceptance (Atot) : • Results for 1 prong and after opposite sign cut : • Systematics included due to tau-ID (variation of 10% from default values) , estimation of t-fakes , • Other source of systematics : • miscalibration and misalignement, • luminosity, • ISR/FSR/fragmentation/PDF, • b-tagging, trigger efficiency,…

12. Probing new physics with t(e/m) channel • Ratio between cross sections can be used to test SM predictions R = slt/ sem~ 1 • Ratio allows the cancelation of most of the systematics, while systematics associated with tau-ID and tau-fakes estimation remains • sem used as normalization because of the smaller background and better S/B ratio • Computed from the number of selected candidates (Nobserved), the estimated background (Nbackground), the integrated luminosity (L) and the total Acceptance (Atot) : • Results for 1 prong and after opposite sign cut :

13. Summary • The potential for measuring sttin dilepton events with one e/m lepton and one hadronically decaying tau was presented • 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. • Analysis performed after full simulation and CMS detector reconstruction (100 pb-1 and s1/2 ~14 TeV ) • Most important background source comes from W+jets like events, a method to derive this background from data is used and good agreement is found with MC expectations. • The ratio R =stl/semcan be used to investigate possible excess of tau production and probe new physics.