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Top quark physics

Top quark physics. Anne-Isabelle ETIENVRE. Outline. Introduction Top quark discovery: a long search! Top-antitop production at hadron colliders Single top production Sensitivity to physics beyond Standard Model. Introduction (1/2). Identity card (a peculiar quark):

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Top quark physics

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  1. Top quark physics Anne-Isabelle ETIENVRE

  2. Outline • Introduction • Top quark discovery: a long search! • Top-antitop production at hadron colliders • Single top production • Sensitivity to physics beyond Standard Model A.-I.Etienvre-FAPPS 2008

  3. Introduction (1/2) • Identity card (a peculiar quark): • SU(2)L partner of the bottom • Q = 2/3, T3=1/2 • Heaviest quark (gold atom!): 40 * m(bottom quark), • Mtop = 172.5 ± 1.2 GeV/c2 (Tevatron) • Produced predominantly (in hadron-hadron collisions) by strong interaction • Width Gtop = 1-2 GeV/c2 (increases with top mass) • Corresponding lifetime short = 0.5 x 10-24 s • Top decays before hadronisation keeping its properties A.-I.Etienvre-FAPPS 2008

  4. Introduction (2/2) • Identity card: • Yukawa coupling ytop = 1 (mtop = yt v2) • Decays almost exclusively through t Wb: • In the Standard Model, 99.9% • i.e. CKM matrix |Vtb|1 • For each measurement, we will see: • Why it is interesting to be performed • Where do we stand • What we should learn with LHC A.-I.Etienvre-FAPPS 2008

  5. Motivations for top quark physics studies • Top quark exists and will be produced abundantly • In Standard Model (SM): top- and W-mass constrain Higgs mass • through radiative corrections • Scrutinize SM by precise determination of the top quark mass • Beyond SM: new physics? • Many heavy particles decay in tt • Handle on new physics by detailed properties of top • Experiment: top quark useful to calibrate the detector • Commissioning (jet energy scale, b-tagging,..) • Beyond top quark: • Top quarks will be a major source of background for many searchs (Higgs, SUSY, exotics,…) A.-I.Etienvre-FAPPS 2008

  6. Top quark discovery(1975  1995) A.-I.Etienvre-FAPPS 2008

  7. Top quark discovery • A third family • 1974 : 2 quarks and leptons families • 1975: tdiscovery (SLAC) • a third family is needed! • 1977 : b quark discovery (Y resonance) μ+μ- spectrum • b quark should have an electroweak partner : the top quark A.-I.Etienvre-FAPPS 2008

  8. Top quark discovery • Direct searches: • Search for a toponium (bounded state t t) around 27 GeV/c2 • top massexpected around 15 GeV/c2 (mass s/c/b : 0.5/1.5/4.5 -> mtop~15 GeV/c2) • Search at the e+e- colliders: • DESY-PETRA (1980) :  s = 12 -36 GeV  mtop > 30 GeV/c2 • LEP 1 (1989), study of Z t t , with  s = 91 GeV  mtop > 45.8 GeV/c2 • Search at the p p̅ colliders: • UA1 and UA2 (1981 -> 1990) ,  s = 640 GeV  mtop > 69 GeV/c2 • « discovery » by UA1 in 1984 (mtop= 40 GeV/c2): 9 signal evts / 0.2 background evts (background was underestimated) • Tevatron (1990  1992),  s = 1.8 TeV  mtop > 91 GeV/c2 A.-I.Etienvre-FAPPS 2008

  9. Top quark discovery • Discovery : Tevatron (1995) mtop = 176 ± 8 (stat.) ± 10 (syst.) GeV/c2 (CDF) mtop = 199 ± 19(stat.) ± 22 (syst.) GeV/c2 (D0) • Indirect searches: • Precise electroweak measurements correlated to the radiative corrections: Dr = K mtop2 , Drrésiduel= f(ln(mH2)) A.-I.Etienvre-FAPPS 2008

  10. Top quark discovery • Top mass evolution Tevatron discovery Current measurement A.-I.Etienvre-FAPPS 2008

  11. Colliders Luminosity: 2.7 fb-1 summer 200820 pb-1 december 2008 ( s = 10 TeV) 7-8 fb-1 2009 1 fb-1 2009 ( s = 14 TeV) expected! Tevatron (p p̅ ),  s = 1.96 TeV LHC (pp): s = 14 TeV A.-I.Etienvre-FAPPS 2008

  12. Colliders Cross section comparison Tevatron/LHC A.-I.Etienvre-FAPPS 2008

  13. Colliders • Comparison Tevatron/LHC A.-I.Etienvre-FAPPS 2008

  14. Opposite @ Tevatron Top pair production • Production (strong interaction): • Cross section • LHC NLO(tt) = 834 ± 100 pb • Tevatron: 7 pb • Comparison to other production processes @ LHC: ~90% gg~10% qq LHC is (also) a top factory! A.-I.Etienvre-FAPPS 2008

  15. Top quark decay • Standard Model: • Br(tWb)  100% • Studies % products of W decay • Branching ratio / main backgrounds In tt events: Background: QCD (bb̅) Large combinatorial background Clean but low BR Many unknowns Bkgd:Z+jets, W+ jets, Z+jets, WW+jets, QCD (bb̅) Golden channel: Good B.R. Clean sample (background = W+jets, Z+jets, diboson, QCD) A.-I.Etienvre-FAPPS 2008

  16. Systematic errors in top quark studies • Jet energy scale (light jets / b-jets) • LHC aim : jet energy knowledge better than 1 % • light jet energy scale contribution: • can be strongly reduced using an in-situ calibration based on the W mass constraint (see later on) • B jet energy scale: • Dominant jet energy scale • Tevatron : estimated from Monte Carlo (global rescaling factor) • LHC : could be estimated from data A.-I.Etienvre-FAPPS 2008

  17. Systematic errors in top quark studies • Initial and final state radiations (ISR, FSR) • ISR • Enhancement of the combinatorial background • Bias on the top quark mass (over-estimation of the jet energies in the final state) • FSR : • Enhancement of the combinatorial background • Bias on the top quark mass (under-estimation of the jet energies in the final state) • Estimated on Monte Carlo at the Tevatron • Could be estimated on data at LHC • b-quark fragmentation • error estimated changing the Peterson parameter (-0.006 ) within its theoretical uncertainty (0.0025) • combinatorial background • error estimated varying the background shape and size in the fitting procedure A.-I.Etienvre-FAPPS 2008

  18. Top-antitop production • Mass measurement • top-antitop cross section measurement A.-I.Etienvre-FAPPS 2008

  19. Top quark mass measurement • Why do we need a precise measurement of mtop? • The uncertainties on mtop, and mW are the dominating ones in the electroweak fit • Precise measurement of mtop, mW • one can get information on the missing parameter mHiggs • one can test the validity of the Standard Model Dr = K mtop2 , Drrésiduel= f(ln(mH2)) A.-I.Etienvre-FAPPS 2008

  20. Top quark mass measurement • Present measurement (Tevatron, July 2008): Mtop= 172.4 ± 0.7 (stat.) ± 1.0 (syst.) Most accurate measurement in the l+jets channel A.-I.Etienvre-FAPPS 2008

  21. + = 33 m 76 - H 24 Top quark mass measurement • Electroweak fit: • The blue band plot • Incidence of precision: • On Da: • On mtop: mtop (2007) = 170.9 ± 1.8 GeV(2007) • On mW: if with the same central values , A.-I.Etienvre-FAPPS 2008

  22. Top quark mass measurement • Electroweak fit: direct mtop and mw indirect mtop and mw A.-I.Etienvre-FAPPS 2008

  23. Leptonic side Hadronic side Top quark mass measurement • At LHC : Lepton+jets channel • Event selection: • Direct hadronic top reconstruction: • Pairing of the 2 light jets < hadronic W • Association hadronic W  b-jet • Typical selection efficiency: ~5-10%: • Isolated lepton PT>20 GeV • ETmiss>20 GeV • ≥ 2 light jets and 2 b-jets with pT>40 GeV • S/B: 10-4 30 for a generated top mass = 175 GeV/c2 : M(top) = 175.0 ± 0.3 GeV/c2 s(top) = 11.8 ± 0.3 GeV/c2 A.-I.Etienvre-FAPPS 2008

  24. Top quark mass measurement • Lepton + jets channel (cont.) • Systematic uncertainties: • Jet energy scale (JES): • light jet energy scale constrained by an in-situ rescaling based on the W mass • b jet energy scale: dominant source of uncertainty Statistical uncertainty will be quickly negligible; Error on the top mass = 1 to 3.5 GeV for a JES = 1 to 5 % A.-I.Etienvre-FAPPS 2008

  25. Top quark mass measurement • In-situ light jet energy scale (LHC) • Light jet energy scale using W constraint: • Template histograms of the invariant mass mjj have been generated, from W  qq PYTHIA for several values of the energy scale a c2 (template – data) minimum • a • All jets are calibrated with a • a can beevaluated % energy, and h • Ratio E(light jet) / E(b jets) estimated on Monte Carlo • 1% on JES is achievable with 1 fb-1 A.-I.Etienvre-FAPPS 2008

  26. Top quark mass measurement • Alternative measurements: • Di-leptons A.-I.Etienvre-FAPPS 2008

  27. Top quark mass measurement • Top quark mass measurement expected for 10 fb-1  Huge effort to be performed for JES in order to reach < 1 GeV A.-I.Etienvre-FAPPS 2008

  28. Cross section measurement • Motivations for a precise measurement of s(tt): • Sensitivity to new physics: • Search for resonances • Beyond Standard Model top decay • Not seen up to now (Tevatron) • Indirect top mass measurement • First step = reconstruction of the tt final state (in common with top mass) A.-I.Etienvre-FAPPS 2008

  29. Cross section measurement • Cross section estimation: • Counting method: A.-I.Etienvre-FAPPS 2008

  30. Cross section measurement • Present tt cross section measurement (Tevatron): Stat. syst. Lumi Stat. syst. Lumi. A.-I.Etienvre-FAPPS 2008

  31. Cross section measurement • Studies at LHC • Early measurement (100 pb-1) • Counting method • Di-lepton channel:(in % of the cross-section): • Systematic uncertainties dominated by JES, ISR and FSR, luminosity • L+jets channel (in % of the cross-section): • Systematic uncertainties dominated by JES and ISR/FSR, luminosity ATLAS ATLAS A.-I.Etienvre-FAPPS 2008

  32. Top mass from tt cross section • Assuming that tt production is governed by Standard Model, mtop can be extracted from s(tt) • If Ds/s(theo.)  5%, and • Ds/s(exp.)  5 %, • Dmtop  2.6 GeV • Not so far from direct measurements at the beginning of LHC A.-I.Etienvre-FAPPS 2008

  33. Single top production A.-I.Etienvre-FAPPS 2008

  34. Single top production Electroweak production of the top quark: 3 channels • s channel: • Cross section Tevatron = 0.88 ± 0.14 pb • Cross section LHC : 10 ± 0.8 pb • t channel : • Cross section Tevatron = 1.98 ± 0.30 pb • Cross section LHC: 245 ± 30 pb • Wt channel : • Cross sectionTevatron = 0.21 ± 0.03 pb Not reachable @ Tevatron • Cross section LHC : 60 ± 15 pb A.-I.Etienvre-FAPPS 2008

  35. Single top production • Interest of the measurement: • Cross section proportional to |Vtb|2 : single top is a way to measure |Vtb| • Irreducible background for many processes (Higgs, SUSY) • Sensitivity to new physics • Each of the processes have different systematic errors for Vtb and are sensitive to different new physics A.-I.Etienvre-FAPPS 2008

  36. Single top production • Tevatron study: • Challenge • Low Cross sections • Large background (W+ 2 jets) • S and t channels can be observed, • Wt not reachable • Evidence for single top process at Tevatron in 2006 for the first time (both s and t channels) • July 2008 results • D0 will update its results with more statistic A.-I.Etienvre-FAPPS 2008

  37. Single top production • At LHC: • With 1 fb-1, an accurate measurement is foreseen in t and Wt channels; • s channel could be seen with 10 fb-1, but difficult (background) • The Wt channel should be observed for the first time • A limit on |Vtb| will be extracted from this measurement A.-I.Etienvre-FAPPS 2008

  38. Physics beyond Standard Model • In top – antitop production • In single top production No evidence seen at Tevatron At LHC? A.-I.Etienvre-FAPPS 2008

  39. Physics beyond Standard Model • In top pair production • Search for heavy resonance in M(tt) • Non Standard Model distribution of M(tt) would be • a signal of heavy particle X  tt • Interference from non SM process • Would also appear as a deviation in ds(tt)/dm(tt) • Example: Z’ search • Z’ is an hypothetical massive boson (spin 1) predicted by several extensions of SM A.-I.Etienvre-FAPPS 2008

  40. Physics beyond Standard Model • In top pair production • Z’ search LHC: For mZ’ = 700 GeV/c2, Can be discovered if s > 11 pbfor an inclusive decay -- >tt • Likelihood analysis: • No excess • M(Z’) > 760 GeV @ 95% C.L. A.-I.Etienvre-FAPPS 2008

  41. Physics beyond Standard Model • Sensitivity to charged Higgs: • Context: • In Minimal extension of Standard Model (MSSM): • 2 Higgs doublet  5 Higgs boson (h,H,A,H+,H-) • Charged Higgs could modify top decay (BR(t Wb)≠ 1) • Can be searched in single top or in tt production A.-I.Etienvre-FAPPS 2008

  42. Physics beyond Standard Model • Sensitivity to charged Higgs • Example in tt production: mH+ < mtop A.-I.Etienvre-FAPPS 2008

  43. Physics beyond Standard Model • Sensitivity to charged Higgs • search in single top production: for H+ mass > mtop • Leads to same final state as s-channel A.-I.Etienvre-FAPPS 2008

  44. Physics beyond Standard Model • In single top production: • Modification of the coupling (Flavour Changing Neutral Current : « FCNC »): • tcZ, tcg, tcg,… • Strongly suppressed in SM (BR  10-13 – 10-11) • Less suppressed in MSSM (10-6 – 10-4) A.-I.Etienvre-FAPPS 2008

  45. Physics beyond Standard Model • In single top production • FCNC @Tevatron • BR(tqg) < 0.03 @ 95% C.L. • BR(tqZ) < 0.04 @ 95% C.L. • BR(tqg) < 0.01 @ 95% C.L. • FCNC @ LHC (1 fb-1) B.R. t->Zq A.-I.Etienvre-FAPPS 2008

  46. e,m n B-jet b-jet jet jet MET Top quark electric charge • Never measured: should be 2/3, but -4/3 exist in non SM • Tevatron (l+jets channel): measurement via b-jet charge measurement • LHC : measurement should be achieved with 1 fb-1 Qt= - 4/3 e excluded @ 94% C.L. A.-I.Etienvre-FAPPS 2008

  47. Left-handed W (lW=-1 ) Longitudinal W (lW=0 ) Right-handed W (lW=+1 ) b b W W t t t t W W b b W polarisation • Goal: tt decay (l+jets) used for W polarisation study • SM: 2 states of helicity: F-= 0.297, F0 = 0.703 (LO) • 2 discriminant distributions: • pT(lepton) • angle(lepton, W) A.-I.Etienvre-FAPPS 2008

  48. W polarisation • Results • D0: • CDF:  no evidence for physics beyond SM • LHC: • Polarisation measurement @1-2 % with 10 fb-1 A.-I.Etienvre-FAPPS 2008

  49. Conclusion • Tevatron and LHC are complementary A.-I.Etienvre-FAPPS 2008

  50. Conclusion • What could we do with the first data taken @LHC? • This year: • s = 10 TeV  cross section top-antitop divided by 2 • Many tops should be produced • And used as a tool for commissioning (JES, b-tagging) • But also first cross section measurement • Next year: • s = 14 TeV • With 1 fb-1, several measurements should be achieved • Their precision relies strongly on how well we will understand our detector A.-I.Etienvre-FAPPS 2008

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