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PART 2

PART 2. The LHC physics programme. Search for Standard Model Higgs boson over ~ 120 < m H < 1000 GeV. Search for Supersymmetry and other physics beyond the SM (q/  compositness, leptoquarks, W’/Z’, heavy q/ , unpredicted ? ….) up to masses of ~ 5 TeV

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PART 2

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  1. PART 2

  2. The LHC physics programme • Search for Standard Model Higgs boson • over ~ 120 < mH < 1000 GeV. • Search for Supersymmetry and other physics • beyond the SM (q/ compositness, leptoquarks, • W’/Z’, heavy q/, unpredicted ? ….) up to • masses of ~ 5 TeV • Precise measurements : • -- W mass • -- WW, WWZ Triple Gauge Couplings • -- topmass, couplings and decay properties • -- Higgs mass, spin, couplings (if Higgs found) • -- B-physics: CP violation, rare decays, B0 • oscillations (ATLAS, CMS, LHCb) • -- QCD jet cross-section and as • -- etc. …. • Study of phase transition at high density from • hadronic matter to plasma of deconfined quarks • and gluons. Transition plasma  hadronic matter • happened in universe ~ 10-5 s after Big Bang • (ALICE)

  3. Process Events/sEvents/yearOther machines (total statistics) W e 15 108 104 LEP / 107 Tev. Z ee 1.5107107 LEP 0.8107104 Tevatron 1051012108 Belle/BaBar 0.001104 (m=1 TeV) H 0.001104 (m=0.8 TeV) QCD jets 102 109107 pT > 200 GeV Keyword: large event statistics Expected event rates in ATLAS/CMS for representative (known and new) physics processes at low luminosity (L=1033 cm-2 s-1) High L : statistics 10 times larger  LHC is a B-factory,top factory, W/Z factory Higgs factory, SUSY factory, etc….

  4. Precise measurements of: mW , mtop

  5. 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

  6. 4•10-4 Direct measurements mH dependence in SM through radiative corrections 3% mW (from LEP2 + Tevatron) = 80.450  0.034 GeV mtop (from Tevatron) = 174.3  5.1 GeV light Higgs is favoured

  7. Year 2007: DmW  25 MeV (0.3 ‰)from LEP/Tevatron Dmtop  2 GeV (1%)from Tevatron Can LHC do better ? YES : thanks to large statistics

  8. 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

  9. 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

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

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

  12. mW= 79.8 GeV mW= 80.3 GeV mTW (GeV) mTW distribution is sensitive to mW mTW distribution expected in ATLAS  fit experimental distributions with SM prediction (Monte Carlo simulation) for different values of mW  find mW which best fits data

  13. Uncertainties on mW • Come mainly from capability of Monte Carlo • prediction to reproduce real life, that is: • detector performance: energy resolution, • energy scale, etc. • physics: pTW, W, GW, backgrounds, etc. Dominant error (today at Tevatron,most likely also at LHC): knowledge of lepton energy scale of the detector: if measurement of lepton energy wrong by 1%, then measured mW wrong by 1%

  14. 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

  15. 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

  16. 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

  17. 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 ? • Discovered in ‘94 at Tevatron precise • measurements of mass, couplings, etc. • just started Top mass spectrum from CDF

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

  19. 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

  20. Example from CDF : tt  Wb Wb  b bjj event W- , m = 79 GeV (b) W+ ATLAS e+ Jet 4 (b)  t  bjj Selection : -- two b-tagged jets -- one lepton -- ETmiss > 20 GeV -- two more jets with |mjj-mW| < 20 GeV l1=4.5 mm l2=2.2mm • Secondary vertices • (b-hadrons) ~ 1.5 ps  decay at few mm from primary vertex detected with high-granularity Si detector (b-tagging)

  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. • By combining both experiments and all channels : • Dmtop ~ 1 GeV at LHC FromDmtop ~ 1 GeV, DmW ~ 15 MeV  indirect measurement DmH/mH ~ 25% (today ~ 50%) If / when Higgs discovered, comparison of measured mH with indirect measurement will be essential consistency checks of EWSB

  22. Searches for the Standard Model Higgs boson Where is the Higgs ?

  23. Best fit of SM to data (minimum 2) found for mH= 81+52GeV -33 What do we know today about it ? • Needed in SM to generate particle masses • Mass not predicted by theory except that mH < 1000 GeV • mH > 114.4 GeV from direct searches at LEP • Indirect limits from fit of SM to: • -- LEP1/SLD precise measurements at s = mZ • -- mW measurement LEP2/Tevatron • -- mtop measurement at Tevatron mH < 193 GeV 95% C.L.  Higgs could be around the corner ! •  1.7  excess from LEP for mH ~ 115 GeV • Higgs production and decays versus mH predicted

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

  25. H f ~ mf Higgs decays Decay branching ratios(BR) • mH < 130 GeV: H  dominates • mH  130 GeV : H  WW(*), ZZ(*) dominate • importantrare decays : H   • Dominant fully hadronic final states (inclusive H bb, • H WW  4jets, H ZZ  4jets) cannot be extracted • from QCD background  look for H , H  ZZ  4, • HWW , etc.

  26. peak width due to detector resolution mgg NS= number of signal events NB= number of background events in peak region How can one claim a discovery ? Suppose a new narrow particleX  gg is produced: Signal significance : NB error on number of background events S > 5 : signal is larger than 5 times error on background. Probability that background fluctuates up by more than 5s : 10-7 discovery

  27.  NB increases by ~ 2 (assuming background flat) NS unchanged  S =NS/NB decreases by 2 Two critical parameters to maximise S: • detector resolution: • if sm increases by e.g. two, then need to enlarge • peak region by two.  S 1 /sm detector with better resolution has larger probability to find a signal Note: only valid if GH << sm. If Higgs is broad detector resolution is not relevant. GH ~ mH3 GH ~ MeV (~100 GeV) mH =100 (600) GeV • integrated luminosity : NS ~ L NB ~ L  S ~ L

  28. 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

  29. 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. :

  30. Reducible gjet, jet-jet background : need excellent g/jet separation (in particular g/p0 separation) to reject jet faking  ATLAS EM calorimeter : 4 mm strips in first compartment Rjet 103 achieved for eg  80% Adequate to reject this background well below  irreducible backgund

  31. 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

  32. 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 • similar significance of both experiments within ~ 10% S ~ 1/m S ~ 

  33. CMS crystal calorimeter

  34. mH (GeV) 120 150 Significance ~ 6.5 ~ 4.5 per experiment with 100 fb-1 Expected performance 100 fb-1 CMS

  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 ( B lifetime : ~ 1.5 ps  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. Summary of Standard Model Higgs Expected significance for one experiment over mass range  1 TeV • LHC can discover SM Higgs over full mass • region (S > 5) after  2 years of operation • in most regions more than one channel is available • detector performance (coverage, energy/momentum • resolution, particle identification, etc.) crucial in • most cases • if Higgs found, then mass can be measured • to 1‰ for mH < 600 GeV

  39. L is per experiment 5s LEP2 -- SM Higgs boson can be discovered at  5  with 10 fb-1/ experiment (nominally one year at 1033 cm-2 s-1) for mH  130 GeV -- Discovery faster for larger masses -- Whole mass range can be excluded at 95% CL after ~1 month of running at 1033 cm-2 s-1. However, it will take time to operate, understand, calibrate ATLAS and CMS

  40. What about Tevatron ? see lectures by F.Bedeschi • For mH ~ 115 GeV Tevatron needs (optimistic scenario): • ~ 2 fb-1 for 95% C.L. exclusion  2003-2004 ? • ~ 5 fb-1 for 3 obervation  2004-2005 ? • ~ 15 fb-1 for 5 discovery  end 2007 – beg 2008? • Discovery possible up to mH ~120 GeV • 95% C.L. exclusion possible up to mH ~185 GeV

  41. LHCversus TEVATRON Higgs cross-section ~10-100 higherS/B ~ 5 higher Conservative estimatesLess conservative predictions (cross-sections, cut analyis, etc.) (e.g. Neural Network analysis) mH=115 GeV 10 fb-1 S/B  4.7mH=115 GeV 10 fb-1 S/B  5.3 4.7  7 using Tevatron approach Will take lot of time to understandHas lot of time to understand detectors and physicsdetectors and physics Ready in 2007 ?15 fb-1 by 2008? Need 3 * For mH > 120 GeV the LHC has no competition 2008 Both machines (Tevatron, LHC) could achieve 5 discovery if mH  115 GeV. Who will find it first ?

  42. End of Part 2 Summary of Part 2 • Examples of precision physics at LHC: W mass can be • measured to ~15 MeV, top mass to ~ 1 GeV •  Higgs mass constrained indirectly to ~ 25% • Standard Model Higgs bosoncan be discovered over • the full mass region upto 1 TeV in ~ 1 year of operation. •  final word about SM Higgs mechanism • Excellent detector performance required: •  Higgs searches have driven the LHC detector design. • Among main channels : H  gg, H  4 • If SM Higgs not found before/at LHC, then alternative methods • for Electroweak Symmetry Breaking will have to be found

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