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PART 2

PART 2. Precise measurements of: m W , m top. Electromagnetic constant measured in atomic transitions, e + e - machines, etc. Fermi constant measured in muon decay. radiative corrections r ~ f (m top 2 , log m H ) r  3%. Weinberg angle measured at LEP/SLC.

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PART 2

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  1. PART 2

  2. Precise measurements of: mW , mtop

  3. Electromagnetic constant measured in atomic transitions, e+e- machines, etc. Fermi constant measured in muon decay radiative corrections r ~ f (mtop2, log mH) r  3% Weinberg angle measured at LEP/SLC Motivation: W mass and top mass are fundamental parameters of the Standard Model:  since GF, aEM, sinW are known with high precision, precise measurements of mtop and mW constrain radiative corrections and Higgs mass (weakly because of logarithmic dependence) So far : W mass measured at LEP2 and Tevatron top mass measured at the Tevatron

  4. Direct measurements mH dependence in SM through radiative corrections mW (from LEP2 + Tevatron) = 80.451  0.033 GeV mtop (from Tevatron) = 174.3  5.1 GeV light Higgs is favoured

  5. Year 2007: DmW  25 MeV (0.3 ‰)from LEP/Tevatron Dmtop  2.5 GeV (1.5 %)from Tevatron Can LHC do better ? YES : thanks to large statistics

  6. Measurement of W mass Method used at hadron collidersdifferent from e+e- colliders • W  jet jet : cannot be extracted from QCD • jet-jet production  cannot be used • W  tn : since t  n + X , too many undetected • neutrinos  cannot be used onlyW  en and W mndecays are used to measure mW at hadron colliders

  7. q  W n s (pp  W + X)  30 nb e, mn ~ 300  106 events produced ~ 60  106 events selected after analysis cuts one year at low L, per experiment W production at LHC : Ex. q’ ~ 50 times larger statistics than at Tevatron ~ 6000 times larger statistics than WW at LEP

  8. Since not known (only can be measured through ETmiss), measure transverse mass, i.e. invariant mass of  in plane perpendicular to the beam :  ETmiss

  9. W en events (data) from CDF experiment at the Tevatron

  10. mW= 79.8 GeV mW= 80.3 GeV mTW (GeV) mTW distribution is sensitive to mW mTW distribution expected in ATLAS  fit experimentaldistributions with Monte Carlo samples with different values of mW  find mW which best fits data

  11. CDF data : W   transverse mass From fit to transverse mass distribution: mW = 80.465  0.100 GeV

  12. Uncertainties on mW • Statistical error negligible  dominated by • systematics (mainly Monte Carlo reliability • to reproduce real life): • detector performance: lepton energy resolution, • lepton energy scale, recoil modeling, etc. • physics: pTW, W, GW, structures functions, • background, etc. Constrained in situ by using mainly Z   decays (1 Hz at low L per ) : e.g. calibrate the electron energy scale in the EM calorimeter requiring mee= mZ Dominant error (today at Tevatron,also at LHC): knowledge of lepton energy scale of the detector: if lepton energy scale wrong by 1%, then measured mW wrong by 1%  to achieve DmW  20 MeV (~ 0.2‰) need to know lepton scale to  0.2 ‰  most serious experimental challenge

  13. e- beam CALO E = 100 GeV Emeasured Calibration of detector energy scale Example : EM calorimeter • if Emeasured = 100.000 GeV calorimeter is • perfectly calibrated • if Emeasured = 99, 101 GeV  energy scale • known to 1% • to measure mW to ~ 20 MeV need to • know energy scale to0.2 ‰, i.e. • if E electron = 100 GeV then • 99.98 GeV < Emeasured < 100.02 GeV  one of most serious experimental challenges

  14. in in cell out • calorimeter modules calibrated with test beams • of known energy set the energy scale • inside LHC detectors: calorimeter sits behind • inner detector  electrons lose energy in • material of inner detector  need a final • calibration “ in situ ” by using physics samples: • e.g. Z  e+ e- decays 1/sec at low L • constrain mee = mZ known to  10-5 from LEP reconstructed Calibration strategy: • detectors equipped with calibration systems • which inject known pulses:  check that all cells give same response: if not  correct

  15. Source of uncertainty DmW Statistical error << 2 MeV Physics uncertainties ~ 15 MeV (pTW, W, GW, …) Detector performance < 10 MeV (energy resolution, lepton identification, etc,) Energy scale 15 MeV Total ~ 25 MeV (per experiment, per channel) Expected precision on mW at LHC Combining both channels (en, mn) and both experiments (ATLAS, CMS), DmW  15 MeV should be achieved. However: very difficult measurement

  16. tt  b bjj events -- u c t d s b Dm (t-b)  170 GeV  radiative corrections S+B B Measurement of mtop • Top is most intriguing fermion: • -- mtop  174 GeV  clues about origin of particle masses ? • -- Gtop 1.8 GeV  decays before hadronising • Discovered in ‘94 at Tevatron precise • measurements of mass, couplings, etc. • just started Top mass spectrum from CDF

  17. t g t q g t t q s (pp  + X)  800 pb 107 pairs produced in one year at low L production is the main background to new physics (SUSY, Higgs) Top production at LHC: e.g. ~ 102 times more than at Tevatron measure mtop, stt, BR, Vtb, single top, rare decays (e.g. t  Zc), resonances, etc.

  18. W t b Top decays: BR  100% in SM -- hadronic channel: both W  jj  6 jet finalstates. BR  50 % but large QCD multijet background. -- leptonic channel: both W    2 jets + 2 + ETmiss final states. BR  10 %. Little kinematic constraints to reconstruct mass. -- semileptonic channel: one W  jj , oneW    4 jets + 1 + ETmiss final states. BR  40 %. If  = e, m : gold-plated channel for mass measurement at hadron colliders. In all cases two jets are b-jets  displaced vertices in the inner detector

  19. Example from CDF data : tt  Wb Wb  b bjj event W- , m = 79 GeV (b) W+ e+ Jet 4 (b)  Secondary vertices  (b-hadrons) ~ 1.5 ps  decay at few mm from primary vertex Detected with high-granularity Si detectors (b-tagging)

  20. Selection of  bW bW  b  bjj Require: -- two b-tagged jets -- one lepton pT > 20 GeV -- ETmiss > 20 GeV -- two more jets ATLAS W jj ATLAS t  bjj Then require: -- |mjj-mW| < 20 GeV -- combine jj with b-jets. Choose combination which gives highest pT top Note : W  jj can be used to calibrate jet energy scale

  21. Source of uncertainty Dmtop Statistical error << 100 MeV Physics uncertainties ~ 1.3 GeV (background, FSR, ISR, fragmentation, etc. ) Jet scale (b-jets, light-quark jets) ~ 0.8 GeV Total ~ 1.5 GeV (per experiment, per channel) Expected precision on mtop at LHC Uncertainty dominated by the knowledge of physics and not of detector.

  22. Searches for the Standard Model Higgs boson

  23. Higgs production at LHC gg fusion WW/ZZ fusion associated WH, ZH associated Cross-section for pp  H + X

  24. H f ~ mf Higgs decays Decay branching ratios(BR) • mH < 120 GeV: H  dominates • 130 GeV < mH < 2 mZ : H  WW(*), ZZ(*) dominate • mH > 2 mZ : 1/3 H  ZZ • 2/3 H  WW • important rare decays : H   • N. B.:GH ~ mH3 GH ~ MeV (100 GeV) mH ~100 (600) GeV

  25. Search strategy Fully hadronic final states dominate but cannot be extracted from large QCD background look for final states with leptons and photons (despite smaller BR). Main channels: • Low mass region (mH < 150 GeV): • -- H  : BR ~ 100%  s  20 pb • however: huge QCD background (NS/NB< 10-5) •  can only be used with additional leptons: • W H  n , H  nXassociated • production • (s  1 pb) • --H  gg:BR ~ 10-3 s  50 fb • however: clean channel (NS/NB  10-2)

  26. High mass region ( mH > 2 mZ): • -- H  ZZ    • gold-plated channel (~ no background) ! • -- H  ZZ   nn, n jet jet • -- H  WW  n jet jet • Intermediate mass region (120 GeV  mH  2 mZ): • -- H  WW*  n n • -- H  ZZ*    • ~ only two channels which can be extracted • from background larger BR  increase rate for mH > 500 GeV This mass region is disfavoured by EW data (SM internal consistency if Higgs is so heavy ?) Only two examples discussed here : H  gg H  4

  27. g H W* W* W* g mH 150 GeV H  gg s BR  50 fb mH  100 GeV • Select events with two photons in the detector • with pT ~ 50 GeV • Measure energy and direction of each photon • Measure invariant mass of photon pair • Plot distribution of mgg Higgs should appear • as a peak at mH Most challenging channel for LHC electromagnetic calorimeters

  28. q g g q g g g g q g ~ 108 g g (s) q p0 Main backgrounds: • gg production: irreducible (i.e. same final • state as signal) e.g. :  60 mgg ~ 100 GeV • g jet + jet jet productionwhere • one/two jets fake photons: reducible e.g. :

  29. How can one fight these backgrounds ? • Reducible gjet, jet-jet:need excellent g/jet • separation (in particular g/p0 separation) to reject • jets faking photons • Rjet 103needed for eg  80% ATLAS and CMS have calorimeters with good granularity to separate single g from jets or from p0 gg. Simulation of ATLAS calorimeter With this performance : (gjet + jet-jet)  30% gg  small

  30. energy resolution of EM calorimeter resolution of the measurement of the g angle  • Irreducible gg :cannot be reduced. But signal • can be extracted from background if mass • resolution good enough GH < 10 MeV for mH ~ 100 GeV

  31. vertex spread ~ 5.6 cm • ATLAS EM calorimeter: • liquid-argon/lead sampling calorimeter • longitudinal segmentation •  can measure g direction sm 1.3 GeV mH ~100 GeV e 30% • CMS EM calorimeter: • homogeneous crystal calorimeter • no longitudinal segmentation  vertex measured using • secondary tracks from spectator partons  difficult • at high L  often pick up the wrong vertex e 20% sm 0.7 GeV mH ~100 GeV

  32. CMS crystal calorimeter

  33. ~ 1000 events in the peak mH (GeV) 100 120 150 Significance4.46.54.3 ATLAS, 100 fb-1 Expected performance ATLAS : 100 fb-1

  34. CMS:significance is ~ 10% better thanks to better EM calorimeter resolution 100 fb-1

  35. e, m Z(*) H e, m Z mZ e, m H  ZZ(*)  4  120  mH < 700 GeV e, m • “Gold-plated”channel for Higgs discovery • at LHC • Select events with 4 high-pT leptons (t excluded): • e+e- e+e-, m+m- m+m-, e+e-m+m- • Require at least one lepton pair consistent with • Z mass • Plot 4 invariant mass distribution :  Higgs signal should appear as peak in the mass distribution

  36. W t , t n b  g b  Z  g  b Backgrounds: -- irreducible : pp  ZZ (*)  4 sm (H  4) 1-1.5 GeV ATLAS, CMS For mH > 300 GeV GH > sm --reducible (s ~ 100 fb) :   Both rejected by asking: -- m ~ mZ -- leptons are isolated -- leptons come from interaction vertex ( leptons from B produced at  1 mm from vertex)

  37. H  ZZ*  4 ATLAS, 30 fb-1 Expected performance • Significance : 3-25(depending on mass) • for 30 fb-1 • Observation possibleup to mH 700 GeV • For larger masses: • --s (pp  H) decreases • --GH > 100 GeV

  38. in CMS 20 fb-1 100 fb-1

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