1 / 50

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):

moisesm
Télécharger la présentation

Top quark physics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related