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Physics Program of the experiments at L arge H adron C ollider

Physics Program of the experiments at L arge H adron C ollider. Lecture 3. The LHC physics programme. Search for Standard Model Higgs boson over ~ 115 < m H < 1000 GeV.

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Physics Program of the experiments at L arge H adron C ollider

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  1. Physics Program of the experiments at Large HadronCollider Lecture 3

  2. The LHC physics programme • Search for Standard Model Higgs boson over ~ 115 < mH < 1000 GeV. • Search for Supersymmetry and other physics beyond the SM (q/l compositness, leptoquarks, W’/Z’, heavy q/l, 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. Outline of this lecture Physics programme:  precise measurement of the W and top-quark mass  detector comissioning with top physics

  4. 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 QCD jets 102 109107 Tevatron pT > 200 GeV Keyword:large event statistics Expected event rates in ATLAS/CMS for representative known physics processes at low luminosity (L=1033 cm-2 s-1)  LHC is a B-factory, top factory, W/Z factory,.... also possible Higgs factory, SUSY factory, ….

  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 Precise measurements: 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. Precision of direct measurements Tevatron, Run II data : CDF: 79 MeV D0: 84 MeV preliminary results from winter/spring conferences integrated luminosity ~ 200 fb-1 • 1983: UA1 5 GeV • 1989: UA1 2.9 GeV • 1990: UA2 900 MeV • 1995: CDF 180 MeV • 2000: DO 91 MeV • combined Run I 59 MeV • ( L =120pb-1) • 2004: LEP 42 MeV (in June 2005 Tevatron declared collected 1fb-1)

  7. Why we need precise measurement? -> test for the Standard Model consistency -> prediction for the Higgs mass For equal contribution to the Higgs mass uncertaintity DMw ~ 0.7 10-2Dmt LHC will measure top mass with 1 GeV precision  to match it the 7 MeV error on the wish list ?!

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

  10. W boson production at LHC

  11. W boson decay

  12. W boson production at Tevatron

  13. Transverse momentum of the charged lepton

  14. Transverse momentum of the charged lepton

  15. Transverse mass of the W boson (mT)

  16. Transverse mass of the W boson (mT)

  17. W mass measurement at Tevatron

  18. 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%

  19. e- beam CALO E = 100 GeV Emeasured Calibration of detector energy scale Example : EM • 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

  20. in in cell out Calibration strategy • detectors equipped with calibration systems which inject • known pulses:  check that all cells give same response: if not  correct • 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 Lconstrain mee = mZ known to  10-5 from LEP reconstructed

  21. Use Z events to calibrate recoil in W events

  22. Expected precision on mW at LHC 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 (per experiment, per channel) ~ 25 MeV Combining both channels (en, mn) and both experiments (ATLAS, CMS), DmW 15 MeVshould be achieved. However: very difficult measurement

  23. Recent (summer 2005) results from CDF

  24. tt  bl 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

  25. Why is top quark so important

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

  27. 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  l  2 jets + 2l + ETmiss final states. BR  10 %. Little kinematic constraints to reconstruct mass. -- semileptonic channel: one W  jj , one W  l  4 jets + 1l + ETmiss final states. BR  40 %. If l = 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

  28. Tagging a b-quark

  29. Example from D0: tt  Wb Wb  bm bjj event

  30. Expected precision on mtop at LHC 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) • 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

  31. Detector commissioning with top physics • Top one of ‘easier’ bread and butter • Cross section 830±100 pb • Used as calibration tool • Rich in ‘precision and new physics’ • Top mass Mt, cross section σt • What are we going to do with the first month of data? • Many detector-level checks (tracking, calorimetry etc) • Try to see large cross section known physics signals • But to ultimately get to interesting physics, also need to calibrate many higher level reconstruction concepts such as jet energy scales, b-tagging and missing energy

  32. t t Learning from ttbar production • Abundant clean source of b jets • 2 out of 4 jets in event are b jets  O(50%) a priori purity(need to be careful with ISR and jet reconstruction) • Remaining 2 jets can be kinematicallyidentified (should form W mass) possibility for further purification

  33. t t Learning from ttbar production • Abundant source of W decays into light jets • Invariant mass of jets should add up to well known W mass • Suitable for light jet energy scale calibration (target prec. 1%) • Caveat: should not use W mass in jetassignment for calibration purposeto avoid bias • If (limited) b-tagging is available,W jet assignment combinatoricsgreatly reduced

  34. t t Learning from ttbar production • Known amount of missing energy • 4-momentum of single neutrino in eachevent can be constrained from eventkinematics • Inputs in calculation: m(top) from Tevatron, b-jet energy scale and lepton energy scale • Calibration of missingenergy relevant forall SUSY and mostexotics!

  35. t t Learning from ttbar production • Two ways to reconstruct the top mass • Initially mostly useful in event selection,as energy scale calibrations must be understood before quality measurementcan be made • Ultimately determine m(top)from kinematic fit to complete event • Needs understanding of bias and resolutions of all quantities • Not a day 1 topic

  36. Various scenarios under study • pp collisions • What variations in predictions of t-tbar – which generator to use? • Underlying event parameterization • Background estimation from MC • Aim to be as independent from MC as possible. • Detector pessimistic scenarios • Partly or non-working b-tagging at startup • Dead regions in the LArg • Jet energy scale • Get good ‘feel’ for important systematic uncertainties – use data to check data • Estimate realistic potential for top physics during the first few months of running

  37. UE / min-bias determination Mtop for various UE models <nch> at h = 0 LHC? Top peak for standard analysis using 2 b-tags Difference can be as large as 5 GeV • Extremely difficult to predict the magnitude of the UE at LHC • Will have to learn much more from Tevatron before startup • Energy dependence of dN/dh ? • Really need data to check data on UE – Only few thousand events required

  38. e-,p0 QCD Multijet Background • Not possible to realistically generate this background • Crucially depends on Atlas’ capabilities to minimize mis-identification and increase e/ separation of 10-5 • This background has to be obtained from data itself • E.g. method developed by CDF during run-1: Use missing ET vs lepton isolation to define 4 regions: A. Low lepton quality and small missing ET Mostly non-W events (i.e. QCD background) B. High lepton quality and small missing ET Observation of reduction in QCD background by isolation cut C. Low lepton quality and high missing ET W enriched sample with a fraction of QCD background D. High lepton quality and high missing ET W enriched sample • The QCD reduction factor B/A can be applied to the “W enriched sample “ (region C and D). The non-W candidate in D will therefore be (B/A)xC. Therefore, the fraction of non-W events in the region D will be: (B.C)/(A.D)

  39. Top Mass: physics TDR reconstruction B-tagging essential, JES dominate • Gold-plated channel : single lepton • pT (lep) > 20 GeV • pTmiss > 20 GeV • ≥ 4 jets with pT > 40 GeV • ≥ 2 b-tagged jets • | mjjb-<mjjb>|< 20 GeV Uncertainty on light jet scale:Hadronic 1%  Mt < 0.7 GeV 10%  M = 3 GeV Uncertainty On b-jet scale:Hadronic 1%  Mt = 0.7 GeV 5%  Mt = 3.5 GeV 10%  Mt = 7.0 GeV

  40. Pessimistic scenario: LArg dead Regions • Argon gap (width ~ 4 mm) is split in two half gaps by the electrode • ~ 33 / 1024 sectors where we may be unable to set the HV on one half gap  multiply energy by 2 to recover • Simulated 100 000 tt events (~ 1.5 days at LHC at low L) • If 33 weak HV sectors die (very pessimistic), effects on the top mass measurement, after a crude recalibration, are: • Loss of signal: < 8 % • Displacement of the peak of the mass distribution: -0.2 GeV • (Increase in background not studied) EM clusters ATLAS Preliminary Jets mtop(without ) – mtop(with)

  41. Pessimistic scenarios: b-tagging & JES • Algorithms benefiting from early top-sample for calibration • B-tagging • Identify jets originating from b quarks from their topology • Select a pure t-tbar sample with tight kinematical cuts • Compare 0 vs 1 vs 2 b-tagged jets in top events • Can expect the b-tagging efficiency different in data from MC • Jet energy scale calibration • Relate energy of reconstructed jet to energy of parton • Dependent of flavor of initial quark  need to measure separately for b jets • Observation of hadronic W for calibrating JES • In most pessimistic scenario b-tag is absent at start Can we observe the top without b-tagging?

  42. Top analysis w/o b-tag • First apply selection cuts • Assign jets to W, top decays Missing ET > 20 GeV Selection efficiency = 5.3% 1 lepton PT > 20 GeV 4 jets(R=0.4) PT > 40 GeV W CANDIDATE 1 Hadronic top: Three jets with highest vector-sum pT as the decay products of the top TOP CANDIDATE 2 W boson: Two jets in hadronic top with highest momentum in reconstructed jjj C.M. frame.

  43. W CANDIDATE TOP CANDIDATE Signal-only distributions • Clear top, W mass peaks visible • Background due to mis-assignment of jets • Easier to get top assignment right than to get W assignment right • Masses shifted somewhat low • Effect of (imperfect) energy calibration m(tophad) m(Whad) MW = 78.1±0.8 GeV mtop = 162.7±0.8 GeV L=300 pb-1 (~1 week of running) Jet energy scalecalibration possible fromshift in m(W) S B S/B = 1.20 S/B = 0.5

  44. Consistency checks m(t) Subset of events where chosen 3-jet combination does not line up with top quark Empirical background shape describes combinatoric background well under peak

  45. W CANDIDATE TOP CANDIDATE Signal + Wjets background • Plots now include W+jets background • Background level roughly triples • Signal still well visible • Caveat: bkg. cross section quite uncertain m(tophad) m(Whad) Jet energy scalecalibration possible fromshift in m(W) S L=300 pb-1 (~1 week of running) B S/B = 0.27 S/B = 0.45

  46. W CANDIDATE TOP CANDIDATE Signal + Wjets background • Now also exploit correlation between m(tophad) and m(Whad) • Show m(tophad) only for events with |m(jj)-m(W)|<10 GeV m(tophad) m(tophad) L=300 pb-1 (~1 week of running) m(Whad) S S/B = 1.77 B S/B = 0.45

  47. Signal + Wjets background • Can also clean up sample by with requirement on m(jln) [semi-leptonic top] • NB: There are two m(top) solutions for each candidate due to ambiguity in reconstruction of pZ of neutrino • Also clean signal quite a bit • m(W) cut not applied here TOP CANDIDATE SEMI LEPTONIC TOP CANDIDATE m(tophad) m(tophad) L=300 pb-1 (~1 week of running) S |m(jln)-mt|<30 GeV B S/B = 1.11 S/B = 0.45

  48. Exploiting ttbar for JES calibration • Use the W mass constraint to set the JES • Rescale jet E and angles to parton energy = Eparton / Ejet • Takes into account variation of rescaling parameter with energy and correlation between energies and opening angle before after

  49. W CANDIDATE TOP CANDIDATE Exploiting ttbar as b-jet sample • Simple demonstration use of ttbar sample to provide b enriched jet sample • Cut on m(Whad) and m(tophad) masses • Look at b-jet prob for 4th jet (must be b-jet if all assignments are correct) W+jets (background) ‘random jet’, no b enhancement expected ttbar (signal) ‘always b jet if all jet assignment are OK’ b enrichment expected and observed Clear enhancementobserved!

  50. Use b-tag of 4th jet Use b-tag of 4th jet to clean up hadronic top Standard analysis (for comparison) Cut on b signal probability > 0.90 on 4th jet ttbar ttbar W+jets W+jets

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