1. Ijaz AhmedNational Centre for Physics, Islamabad Top Anti-Top pair Production, Decays and Branching ratios at LHC

2. Ijaz AhmedNational Centre for Physics, Islamabad outlines Introduction Why top quark Top production and Decay diagrams Decay modes of Top quark Branching Ratios + Events Selection Background Processes Spin Correlation

3. Ijaz AhmedNational Centre for Physics, Islamabad Introduction Top quark---  the heaviest quark Discovery 1995 at Tevatron (ppbar collider), CDF+D0 Pole mass--  (174 ± 5.1) GeV Decay width ---  1.4 GeV Life time ~ 10 -25 s <<< time for depolarization t--  Wb BR(~100 %) Spin and parity----  J P (SM) = 1/2 + Weak iso-spin eigen values =I 3 = +1/2 ttbar Production cross-section at LHC = 830 pb (NLO) Integrated luminosity = 10 fb -1 ttbar events to be generated = 10 x 10 15 x 830 x 10 -12 = 8.3 M/year

4. Ijaz AhmedNational Centre for Physics, Islamabad Need of Top Quark Cancellation of triangle anomaly Probe the t--->Wb vertex (V tb ) Contribution to W and Higgs boson propagator Partner required for b quark

5. Ijaz AhmedNational Centre for Physics, Islamabad Leading order ttbar pair production channel Quark annihilation Gluon fusion

6. Ijaz AhmedNational Centre for Physics, Islamabad Top quark yields  Top quark in pairs  Exclusively decay into W and b  Events depend on W decay modes  Leptons plus jets: one W decays to jets and other into leptons (30%)  Hadronic Decay: both W’s decay to jets (65%)  Di-leptons: both W’s decay into leptons plus neutrinos (~ 5%)

7. Ijaz AhmedNational Centre for Physics, Islamabad Top Anti-Top Decay modes

8. Ijaz AhmedNational Centre for Physics, Islamabad Top Decay modes l  l = e,  Decay modeBranching ratio tt→W + W - bb→bbqqqq 36/81 tt→W + Wbb→bbqq'e 12/81 tt→W + Wbb→bbqq‘  12/81 tt→W + Wbb→bbqq‘  12/81 tt→W + Wbb→e  bb 2/81 tt→W + Wbb→e  bb 2/81 tt→W + Wbb→  bb 2/81 tt→W+Wbb→e e bb 1/81 tt→W+Wbb→  bb 1/81 tt→W+Wbb→  bb 1/81 In Standard Model : t W + b N ttbar = Lumi x  ttbar N ttbar = X events => Lumi x (  ttbar x BR) ud, us, ub cd, cs, cb td, ts, tb

9. Ijaz AhmedNational Centre for Physics, Islamabad Branching ratio of W decays W + /W - Decay modes W + →cs,ud (6/9) W + →e  e (1/9) W + →    (1/9) W + →    (1/9) W - →cs,ud (6/9) 36/816/81 W - →e - (1/9) 6/811/81 W - →     6/811/81 W - →    (1/9) 6/811/81

10. Ijaz AhmedNational Centre for Physics, Islamabad 1. Semi leptonic decay mode Branching ratio = 12/81 = 14.8 % (1.21 M/year) 2. PP→tt→bbqq'  .......(4 jets+  +E miss ) Branching ratio = 12/81 = 14.8% (1.21 M/year)..................................................................................................... Total Branching ratios of Semi Leptonic decays = 29.6 % Total semi-leptonic events to be generated = 8.2x29.6/100(M) = 2.5 M/year t t Main Reaction (tt→W+W-bb→bb(qq')(l ) Medium sized branching ratio with the managable background 1.PP→tt→bbqq'e e.......(4 jets+e+E miss ) jets W b jets l + jets q q jets

11. Ijaz AhmedNational Centre for Physics, Islamabad Some useful relations Rapidity: Y = 1/2*log[(E W + P W )/(E W  P W )] Pseudorapidty:  = -log(tan  /2) Tan  = P y / P x P t = sqrt (P x 2 + P y 2 ) Absolute Momentum = sqrt(P x 2 + P y 2 + P z 2 ) Invariant mass = m W 2 = sqrt [(E l + E    P l + P     P 1 P 2 (1  cos  12 ) Jet cone radius = sqrt [(     Angular distribution = cos  P z / P t

12. Ijaz AhmedNational Centre for Physics, Islamabad Event Selection Criteria Three methods to measure the mass of top quark Three jets invariant mass of the hadronic top decay The entire ttbar system is fully exploited to determine the top quark mass from a kinematics fit. Using kinematics fit, but jets are reconstructed using a continuous algorithms  At least 1 lepton with |  | < 2.4  Exactly 1 lepton with P t (l) > 20 GeV  Missing E t > 20 GeV  Lepton isolation  R = 0.3 < 0.1  At least 4 jets reconstructed with a cone size (R cone = 0.4)  4 jets with E t > 40 GeV  At least two jets to be tagged as b jets  Total E t > 450 GeV  Exactly 2 b jets with E t > 50 GeV  60 < reconst (M W ) < 100 GeV  Transverse mass m t (W lep ) < 100 GeV  Rec. top mass difference |m t - m t | < 25 GeV   R = 0.7  P t (jjb > 250 GeV  After selection cuts S/B ~ 78 (87,000 events)

13. Ijaz AhmedNational Centre for Physics, Islamabad Background Processes  ttbar-  l +jets = 2.2x10 6 pb W+jets→l +jets Dominant Background S/B = 18.6 => 7.8x10 3 pb (1658/year) Z+jets→l + l - +jets = 1.2x10 3 pb (232/year) WW→l +jets = 17.1 pb (10/year) WZ→l +jets = 3.41 pb (8/year) ZZ→l + l - +jets = 9.21 pb (14/year)...................................................................................... Total BG events (1922/year) At production level S/B = 10 -5

14. Ijaz AhmedNational Centre for Physics, Islamabad 2. Purely Leptonic Decays Main Reaction (tt--->W + W - bb--->bb(l )(l ) pp--->tt--->bbe e e e.......(4jets+2e+E miss ) Branching Ratio = 1/81 =1.23 % (100,000/year) pp--->tt--->bb   e e.......(4jets+2e+E miss ) Branching Ratio = 2/81 = 2.46 % (200000/year) pp--->tt--->bb    .......(4jets+2  +E miss ) Branching Ratio = 1/81 = 1.23 % (100,000/year)................................................................................................................. Total BR for Leptonic Decay = 4.9 % (400,000/year)

15. Ijaz AhmedNational Centre for Physics, Islamabad Detection of leptons and signatures Two opposite sign leptons with |h| < 2.5 P t (l 1 ) > 35 GeV P t (l 2 ) > 25 GeV E miss > 40 GeV |M ll -M Z | > 10 GeV M 2 W = (l 1 + 1 ) 2, M 2 W = (l 2 +  ) 2 M 2 top = (l 1 + b 1 + 1 ) 2, M 2 top = (l 2 + b 2 +  ) 2 Two b-jets with P t > 25 GeV (S/B = 10), |  | < 2.4,  R cons = 0.4 For two neutrinos "neutrino weighting" technique is used Neutrino rapidities, top mass, charged lepton and b-quark momenta, system can be solved for transverse and longitudinal momentum components of neutrino After event selection 80000 signal events are left t t W + b W - b l + l - jets

16. Ijaz AhmedNational Centre for Physics, Islamabad b-tagging How to distinguish a b jet from a lighter quark jet: – can contain low-p  leptons from b  c l (BR=10% per lepton) B hadrons have lifetimes long enough so that they can travel several mm before decaying  b-jet particles come from a displaced vertex

17. Ijaz AhmedNational Centre for Physics, Islamabad Background Processes Dilepton decays have low statistics bbbar→l +jets WW+jets→(2l)(2 )+jets Background is small mainly dominated by Z decays to leptons Leptons misidentification increases Drell-Yan processes associated with jets Z→ t + t - (associated with jets) WW + jets Backgrounds easier to eliminate than in all-hadronic mode because of lepton tag. Most promising decay mode for search

18. Ijaz AhmedNational Centre for Physics, Islamabad 3. Purly Hadronic Decays Main Reaction (tt→W + W - bb→bb(jj)(jj)(370 pb) pp→tt→bbudud............(2 bjets+4 quark jets) Branching Ratio = 9/81(911,000/year) pp→tt→bbusus.............(2 bjets+4 quark jets) Branching Ratio = 9/81(911,000/year) pp→tt→bbubub...........(2 bjets+4 quark jets) Branching Ratio = 9/81(911,000/year) pp→tt→bbcdcd............(2 bjets+4 quark jets) Branching Ratio = 9/81(911,000/year) pp→tt→bbcscs.............(2 bjets+4 quark jets) Branching Ratio = 9/81(911,000/year) pp→tt→bbcbcb...........(2 bjets+4 quark jets) Branching Ratio =9/81(911,000/year).......................................................................................................... Total BR of purely Hadronic decays = 9/81*6 = 66 % (5.41M/year)

19. Ijaz AhmedNational Centre for Physics, Islamabad Event Selection Criteria Multi jet trigger threshold ~ 4 jets Events are selected by requiring at least six or more jets with P t = 40 GeV, and at least two of them are tagged as b-jets Jets are required to satisfy |  | < 3 (|  | < 2.5 for b-jet candidates) Jets are reconstructed using a fixed cone algorithm with  R = 0.4 Sum of the transverse momenta of the jets is required to be greater than 200 GeV At least one b-tagging is required using secondary vertices Tagging required efficiency 60% with at least 100 rejection against prompt jets ttbar signal efficiency for these cuts should be 19.3 % Only 0.29 % of QCD multi-jets events should be survived For QCD multi-jet cross-section of 1.4 *10 -3 mb and P t > 100 GeV, S/B ~ 1/57

20. Ijaz AhmedNational Centre for Physics, Islamabad Background Processes (q i q j ---> q i q j, q i g ---> q i g, gg ---> gg, qqbar---> gg, gg ---> qqbar, q i q i bar---> q j q j bar)  (pp  6 jets)  10 3  (pp  t t  b b + 4 light-quark jets) Difficult to distinguish between light jets and b jets Kinematics cut technique High statistics required Least interesting decay mode Large QCD multi-jet events

21. Ijaz AhmedNational Centre for Physics, Islamabad Comment All jets in the final state create difficulty in triggering. QCD background is generated with a pt cut on the hard scattering process above 100 GeV, resulting cross-section 1.73 mb. The requirement of having at lest two b-tagged jets in the final stat helps in rejecting a large part of the physical background, but also reduces considerable the signal sample. The fraction of signal events with at least two b- tagged jets is three times smaller than the fraction with at least one b-tagged jet. Requiring only one b-tagged jet would decrease the S/B ratio from 78 to 28, which would be still acceptable.

22. Ijaz AhmedNational Centre for Physics, Islamabad Overview of Branching ratios

23. Ijaz AhmedNational Centre for Physics, Islamabad Systematic Uncertainties in Top Mass Main contributions Jet energy scale ISR and FSR MC generator Method for mass fitting Model for background

24. Ijaz AhmedNational Centre for Physics, Islamabad Theoretical uncertainties in Top Mass Renormalization scale < 10 MeV (30-150 GeV) Strong coupling constant < 75 MeV MS bar ~ ± 12 MeV Which implies that at LHC accuracy in top mass will be of the order of 1 GeV

25. Ijaz AhmedNational Centre for Physics, Islamabad Spin Correlation in ttbar production Top decay width =  t = 1.4 GeV QCD Hadronization scale =  qcd = 0.22 GeV. Time scale for depolarization of top spin = m t /  2 qcd >> 1/  t ~ 10 -24 s Spin correlation in decay products of tt systems is interesting for several reasons. It provides probe of a quark that is at least free of confinement of effects. Since life time of top quark is proportional to CKM matrix element |V tb | 2, so observation of spin correlation would yield, information about lower limit of |V tb | with out assuming that there are three generation of quarks. Charged leptons +weak isospin quarks are sensitive to the initial polarization  *d 2  /d(cos    d(cos    cos    cos   

26. Ijaz AhmedNational Centre for Physics, Islamabad Top decay diagram W+W+ W-W- b bbar ttbar e-e- e+e+  Jet1 Jet2 Strong coupling CKM Weak coupling 

27. Ijaz AhmedNational Centre for Physics, Islamabad PYTHIA (Some results of ttbar pair production) 1000 events of ttbar pairs produced Sigma (gluon fusion) = 435 pb ( Sigma (quark annihilation) = 80 pb Total cross-section = 515 pb 10,000 events of ttbar pairs produced Sigma (gluon fusion) = 450 pb Sigma (quark annihilation) = 67 pb Total cross-section = 518 pb