ATLAS results on inclusive top quark pair production

ATLAS results on inclusive top quark pair production cross section in dilepton channel Frédéric Derue, LPNHE Paris Rencontres françaises sur la physique des hautes énergies au LHC, LHC France 2013, 2 -6 avril 2013, Annecy Object reconstruction dilepton (e, - event selection - background estimation - result dilepton (e/ Analysis of 2011 data at s = 7 Te. V LHC France 2013, 3 rd April 2013 1

Object reconstruction To study top quark it implies good understanding of many different objects reconstructed in all different ATLAS subdetectors Electrons matched track and EM cluster tight identification using shower shape variables, ID central : | |<2. 5, p. T>25 Ge. V isolated in ID and ECAL Triggers based on single lepton high p. T Muons combined fitted tracks tight identification central : | |<2. 5, p. T>20 Ge. V isolated in ID and ECAL Jets anti-k. T-algorithm (R=0. 4) central | |<2. 5 p. T>25 Ge. V b-tagging miss ET vector sum of energy in calorimeter corrected for identified objects LHC France 2013, 3 rd April 2013 Neural Network based algorithm with average b-tagging efficiency ~70 % and light jet rejection factor ~140 2

Event selection Signature : miss 2 isolated e/ + ET + jets (1 b) Trigger : 1 single isolated lepton Offline : njet 2, opposite sign leptons + ETmis , ET(e ), mll (Z veto) s Channels : 8 analyses are performed ee, , e with and without b-tagging e. TL and TL without b-tagging where TL = Track lepton = Good quality isolated track Analysis Strategy : counting experiment data driven estimation of Z+jets, W+jets and QCD backgrounds JHEP 1205 (2012) 059 LHC France 2013, 3 rd April 2013 3

Z+jets background Z/ * + jets events contaminating signal region miss large ET possible from mismeasurement difficult to model in Monte Carlo simulation Define control region miss |mll-m. Z|<10 Ge. V and ET same jet requirements >30 Ge. V Determine scale factors after subtracting other backgrounds JHEP 1205 (2012) 059 LHC France 2013, 3 rd April 2013 4

Fake lepton background Backgrounds with fake leptons W+jets (mainly), « QCD » jets, tt single lepton, single top. . . Determination with 2 -dimension matrix method define Loose and Tight leptons (isolation) which give 4 observable states Loose-Loose, Loose-Tight, Tight-Loose, Tight-Tight also four classes of background Fake-Fake, Fake-Real, Real-Fake, Real-Real Fractions f and r from control samples, then solve equation for NRF/NFR, NFF fraction f measured from Z(→ ll)+jets events fraction r measured from sample with single loose lepton JHEP 1205 (2012) 059 LHC France 2013, 3 rd April 2013 5

Number of signal and background events 0. 70 fb-1 JHEP 1205 (2012) 059 LHC France 2013, 3 rd April 2013 6

Cross section measurement JHEP 1205 (2012) 059 0. 70 fb-1 tt = 176 5 +14 (stat) -11 (syst) 8 (lumi) pb overall precision ~9%, limited by systematic uncertainties Systematic uncertainties : mis Jet/Es. T (~4 %), generator (~5%), lepton (~2. 5%), fake lepton (~2%) LHC France 2013, 3 rd April 2013 7

Other cross section measurement Ph. D of Timothée Theveneaux-Pelzer (LPNHE Paris): Etudes sur la reconstruction des électrons et mesure de la section efficace de production de paires de quarks top dans les canaux en dileptons dans l’expérience Atlas, UPMC, 3 juillet 2012 CERN-THESIS-2012 -114 similar Analysis as the published one with 0. 7 fb-1 but based on 4. 7 fb-1 - ee, , e channels - no b-tagging - dedicated work on fake leptons - re-estimation of all systematics 4. 70 fb-1 overall precision ~7%, limited by systematic uncertainties +8. 9 tt = 178. 8 2. 3 (stat) -8. 4 (syst) 8 (lumi) pb LHC France 2013, 3 rd April 2013 8

pair production with e/ + + jets PLB 717 (2012) 89 BR could be enhanced by the existence of H± Signature : miss 1 isolated e/ + + ET + jets (1 b) Trigger : 1 single isolated lepton Offline : opposite sign lepton + lepton mis ETs >30 Ge. V, ET>200 Ge. V, 2 jets at least one of them is b-tagged Analysis Strategy : perform template fit of BDT - background distribution is different with jet flavor - to reduce # of templates, SS events are subtracted to remove b, gluon originated τ candidates (charge symmetric) 2. 05 fb-1 tt( + ) = 186 15 (stat) 20 (syst) 7 (lumi) pb tt(e+ ) = 187 18 (stat) 20 (syst) 7 (lumi) pb tt = 186 13 (stat) 20 (syst) 7 (lumi) pb overall precision ~14%, limited by systematic uncertainties Systematics : b-tag (~9 pb), -ID (~4 pb) LHC France 2013, 3 rd April 2013 9

Summary ttbar production cross section measured accuracy for combined measurement < theoretical one tt is measured in alternative channels ( ), showing SM is applicable at LHC ATLAS-CONF-2012 -024 NB: combination takes results in dilepton from ee, , e without b-tagging LHC France 2013, 3 rd April 2013 - total precision ~6%, half of theory uncertainty - agreement of channels within uncertainties 10