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Exploring Physics at Large Hadron Collider: Lecture on LHC Physics Program

This lecture provides an overview of the experiments at the Large Hadron Collider, including the search for the Standard Model Higgs boson and physics beyond the Standard Model. It covers precise measurements of various parameters such as W mass, top mass, and Higgs mass, as well as the study of phase transitions in high-density matter. The importance of these measurements in testing the Standard Model consistency and predicting the Higgs mass is discussed. The methodology used for measuring the W boson mass at hadron colliders is explained, highlighting the challenges and uncertainties involved.

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Exploring Physics at Large Hadron Collider: Lecture on LHC Physics Program

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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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