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Higgs Boson at CMS: gearing up for discovery

Higgs Boson at CMS: gearing up for discovery. Andrey Korytov. Outline. Introductory remarks: what we already know LHC, CMS SM Higgs frontrunners: H W W 2 l 2 n H ZZ4 l H 2 g A few other SM Higgs production/decay channels A few words on MSSM Higgs Concluding remarks.

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Higgs Boson at CMS: gearing up for discovery

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  1. Higgs Boson at CMS: gearing up for discovery Andrey Korytov

  2. Outline • Introductory remarks: what we already know • LHC, CMS • SM Higgs frontrunners: • H WW2l2n • HZZ4l • H 2g • A few other SM Higgs production/decay channels • A few words on MSSM Higgs • Concluding remarks

  3. SM Higgs Trivia: intelligent design • Start from scalar field • doublet pseudo-scalar in SM • Require local gauge invariance • need massless gauge fields A • lagrangian acquires terms • Mexican hat potential • min V(f) is not at f=0 • non-zero vacuum expectation value v0—ether of 21 century • expand around minimum • effective mass terms for gauge bosons • effective mass for h-field itself • Free lunch: • force f interact with fermions with ad hoc couplings lf • effective fermion masses (within the P-violation framework!) • Two important points: • Higgs boson mass is the only free parameter • (Higgs-particle coupling) ~ (mass of particle) • Production mechanisms: first one needs to produce heavy particles • Decay channels: higgs likes to decay to heaviest particles it can decay to

  4. non-perturbative New Physics Energy Scale L (GeV) 103 106 109 1012 1015 1018 unstable vacuum 0 200 400 600 Higgs mass MH (GeV) What we know: theory • After renormalization • l l(Q) • If mH were small at 1 TeV, l runs down with Q, flips sign at some scale Q, and vacuum breaks loose • If mH were large at 1 TeV, l runs up with Q, explodes at some scale, theory becomes non-perturbative, and theorists can retire • SM Higgs has a very narrow window of opportunity to be self-sufficient due to a fine-tuned accidental cancellation of large correction factors

  5. } jet (b-tagged) MJJ=MH =? e- jet (b-tagged) b b H LEP Energy 209 GeV } q Z0 jet Z0 e+ q MJJ=MZ=91 GeV jet What we know: direct search at LEP

  6. Tight Cuts What we know: direct search at LEP The final word: No discovery... Consistency with background:~1.7s MH > 114.4 GeV @95% CL Phys. Lett. B565 (2003) 61

  7. What we know: direct search at Tevatron Z/W+H, Hbb HWW At MH= 115 Expected: 3.8xSM Observed: 4.2xSM At MH= 160 Expected: 1.9xSM Observed: 1.1xSM

  8. What we know: circumstantial evidence LEP EWK Working Group • Presence of too light or two heavy Higgs in loops would make various SM precision measurements less self-consistent: • mH<144 GeV at 95% CL (unconstrained by the LEP 114-GeV limit) • mH<182 GeV at 95% CL (if combined with the LEP 114-GeV limit) H W

  9. France 6 miles Geneva airport Switzerland Large Hadron Collider • 2008: first collisions • (Lint ~ 10+ pb-1 ?) • 2009: Lint ~ 1+ fb-1 • 2010: Lint ~ 10+ fb-1 • Beyond: Lint ~ 100 fb-1 / yr

  10. Compact Muon Solenoid

  11. CMS: Conceptual Design (cf. ATLAS) • Large Si Tracker in a Large Magnetic Field: • high momentum measurement precision: ~1% up to 100 GeV • Precision PbWO4 EM Calorimeter: • very good energy resolution for photons/electrons: <1% above 30 GeV • Hadron calorimeter has moderate energy resolution: ~10% above 100 GeV • Muon system: muon trigger and identification (also important for measuring TeV muons)

  12. CMS: all major components are underground Tracker Insertion (Dec’07) Endcap Muon Disks go down

  13. CMS commissioning • Underground Muons, Fall 2007

  14. CMS commissioning • Underground Muons, Fall 2007

  15. CMS Physics Technical Design Report • Physics TDR published in 2006 • Comprehensive/up-to-date overview of CMS physics reach 650 pages 308 figures 207 tables 1.50 kg

  16. SM Higgs: discovery signatures at L=30 fb-1 • Colored cells = { detailed studies available } • YES = { sure discovery in the appropriate range of masses at L=30 fb-1 }

  17. CMS: SM Higgs search frontrunners • Benchmark luminosities: • 0.1 fb-1: exclusion limits will start carving into SM Higgs x-section • 1 fb-1: discoveries become possible around MH~165 GeV • 10 fb-1: SM Higgs is discovered (or excluded) in full range NLO cross sections Systematic errors included NLO cross sections Systematic errors included

  18. HZZ4l • So-called golden channel • I will use this channel to illustrate the level of scrutiny all major search channels now undergo • strategy for data driven background control • strategy for measuring all efficiencies directly from data

  19. ZZ  4 with spectacular peak at m4=mZ (the s-channel contribution was overlookedin all previous studies) Zbb  4 + X tt  4 + X Higgs signal H  4 HZZ4m: dominant 4m backgrounds • tt  Wb + Wb •  mnBX + mnBX •  mn+mnX + mn+mnX •  4m + X • Zbb  mm + BB + X  •  mm + 2(mnX) + X •  4m + X • ZZ  4m (dominant after cuts) s-channel t-channel

  20. Higgs signal H  4 HZZ4m: analysis strategy • Peak in m4m distribution • Cut optimization • mH-dependent (read m4m-dependent) • identify most important and not-correlated cuts • Dm4m mass window • isolation cut on the least isolated muon (i.e., the same cut for all muons) • muon pT cut for the 3rd softest muon • after applying these cuts, others do not help anymore • produce smooth cut(m4m) functions This strategy makes the search automatically optimized for any mass at which Higgs boson may chose to show up • Peak search: • Include statistics and systematics into significance evaluation • Final probabilistic interpretation (significance must be properly de-rated due to a search being conducted in a wide range of masses)

  21. HZZ4m: understanding ZZ bkgd 2 2 + + * + … • Knlo(m4m) • Box-diagram • Control samples • QCD scale uncertainties • PDF scale uncertainties • Isolation cut uncertainties • Muon efficiency uncertainty

  22. Zecher, Matsuura, van der Bij hep-ph/9404295 ~20% over LO HZZ4m: understanding ZZ bkgd • Knlo(m4m) • Box-diagram • Control samples • QCD scale uncertainties • PDF scale uncertainties • Isolation cut uncertainties • Muon efficiency uncertainty Formally (by counting vertices), NNLO However, - it is the LO for ggZZ and - contribution is large due to large gg “luminosity”

  23. HZZ4m: ZZ background • Knlo(m4m) • Box-diagram • Control samples: • qq  Z  2m • very similar origin to ZZ bkgd • huge statistics • ZZ  4m sidebands • would be perfect, if not for rather complicated shape • and very limited statistics • QCD scale uncertainties • PDF scale uncertainties • Isolation cut uncertainties • Muon efficiency uncertainty L=10 fb-1 Total 8 events Exp bkgd 0.8 evts ScL = 4.7

  24. HZZ4m: ZZ background • Knlo(m4m) • Box-diagram • Control samples • QCD scale uncertainties • estimate of higher-order contributions • PDF scale uncertainties • Isolation cut uncertainties • Muon efficiency uncertainty Normalization to Z2m

  25. HZZ4m: ZZ background • Knlo(m4m) • Box-diagram • Control samples • QCD scale uncertainties • PDF scale uncertainties • Isolation cut uncertainties • Underlying Event is the main source for energy flow in vicinity of muons in the irreducible ZZ-bkgd; but UE activity is poorly predicted… • Use data to calibrate UE activity: • UE activity in Z (qq Z) must be very similar to that in ZZ (qq ZZ) • MC studies (random cone and tag-and-probe techniques) confirm this statement • Muon efficiency uncertainty: use data three colors: different UE models — ZZ events - - Z events (random cones)

  26. HZZ4m: ZZ background • Knlo(m4m) • Box-diagram • Control samples • QCD scale uncertainties • PDF scale uncertainties • Isolation cut uncertainties: use data • Muon efficiency uncertainty: use data • single muon trigger; well reconstructed muon m0 • take advantage of muon being measured twice: in Tracker and Stand Alone Muon system • find Z-peak three times… • d(efficiency) ~ 1%

  27. HZZ4m: signal and background after cuts • ZZ is a dominant bkgd

  28. HZZ4m: Higgs signal over ZZ bkgd • Peak search results: • Significance: • Counting Experiment • LLR for m4m spectrum • Luminosity needed • Including systematics

  29. HZZ4m: Higgs signal over ZZ bkgd • Peak search results: • Significance • Luminosity needed • Including systematics

  30. L=10 fb-1 Total 8 events Exp bkgd 0.8 evts ScL = 4.7 HZZ4m: Higgs signal over ZZ bkgd • Peak search results: • Significance • Luminosity needed • Including systematics • significance must be de-rated • effect depends on how we define the control sample: Z2m peak vs ZZ4m sidebands Calculations done for luminosities, at which the expected significance would be 5, if there were not systematic errors systematic errors are under control

  31. HZZ4l: combining four channels new systematic errors are under control electrons are difficult

  32. HZZ4l: significance de-rating Search for Higgs peak Background-only pseudo experiment • Search in a broad range of parameter phase space • mH=115-600 GeV • Probability of finding a local excess somewhere is much higher than naïve statistical significance might imply: e.g. S=3 is almost meaningless • A priori assumptions must be clearly defined — actual probability - - probability implied by local statistical significance

  33. Standard Model Higgs: Hgg

  34. Standard Model Higgs: Hgg L=1 fb-1 Signal x 10 • Backgrounds: • prompt gg • prompt g + jet(brem g, p0g) • dijet • Analysis: • Cut-based • PT, h, isolation, Mgg • events sorted by photon quality (EM shower profile quality) • Optimized (NN) • loose cuts and sorting • event-by-event kinematical Likelihood Ratio • bkgd pdf from sidebands, signal pdf from MC • Systematic errors folded in

  35. Standard Model Higgs: Hgg L=7.7 fb-1, Signal not scaled (all categories of events) L=7.7 fb-1, Signal x 10 (best category of events) CMS CMS Di-photon mass alone does not provide the full power Events sorted by their individual LLR’s

  36. Standard Model Higgs: Hgg CMS

  37. Standard Model Higgs: HWW2l2n

  38. Standard Model Higgs: HWW2l2n Signal Region Control Sample • Backgrounds: • WW, tt, Wt(b), WZ, ZZ • ggWW (box) • Analysis: • KNLO(pTWW) • cuts: • e/m kinematics, isolation, jet veto, MET • counting experiment, no peak • background from a control sample: • signal: 12<mll<40 GeV • control sample: mem>60 GeV • reduce syst. errors; pay stat. penalty • systematic errors are folded in

  39. Standard Model Higgs: HWW2l2n Discovery Exclusion NOT YET PUBLIC CMS

  40. VBF channels qqH qqH, Htt -- is it better than inclusive Hgg? qqH, HWW -- is it better than inclusive HWW2l2n? CMS and ATLAS do not seem to agree…

  41. Difficult (impossible) channel: ttH, Hbb If higgs boson is light, can we use Hbb? • CMS: • careful study of systematic errors in the Physics TDR • syst error control at sub-percent level is needed: not feasible... SM Higgs: ttH, Hbb ATLAS 30 fb-1

  42. mtop=174.3 GeV MSSM Higgs bosons: h, H, A, H± • SUSY stabilizes Higgs mass • 2 Higgs field doublets needed • Physical scalar particles: h, H, A, H± • Properties at tree level • fully defined by 2 free parameters: MA, tanb • CP-even h and H are almost SM-like in vicinity of their mass limits vs MA: hmax and Hmin • large tanb • suppresses coupling to Z and W • enhances coupling to “down” fermions: b and t are very important! • CP-odd A never couples to Z and W: • decays: bb, tt (and tt for small tanb) • H± strongly couples to tb and tn • all Higgs bosons are narrow (G<10 GeV) • Loop corrections • gives sensitivity to other SUSY parameters • mhmax scenario = { most conservative LEP limits }

  43. ATLAS L=300 fb-1 MSSM Higgs or SM Higgs? • SM-like h only: • considerable area… • even at L=300 fb-1 • Any handles? • decays to SUSY particles? • SUSY particle decays? • measure branching ratios?

  44. Summary • Standard Model Higgs: • expect to start excluding SM Higgs at L~0.1 fb-1 • discoveries may be expected already at L~1 fb-1 • SM Higgs, if that’s all we have, is expected to be discovered by the time we reach L~10 fb-1 • MSSM Higgs: • nearly full (M, tanb) plane is expected to be covered at L~30 fb-1 • there is a serious chance to see only a SM-like Higgs…

  45. P.S. Word of caution from Tevatron • MH=110 GeV (the best expectations in 2003) • SM Higgs exclusion at 95% CL was expected at L=1.2 fb-1 • Now at L~2 fb-1, the excluded limit is a factor of 6 behind the early expectations • MH=160 GeV (the best current limit) • It took enormous effort (well beyond of what was contemplated in 2003) to bring the current limit where it is now and approx in agreement with the earlier expectations • The reality may be not as rosy as projections---something to remember as we gear up for the Higgs search at LHC…

  46. Backup slides

  47. jet f jet h ATLAS Standard Model Higgs: qqH, HWW2l2n Signal Region Control Sample • Backgrounds: • tt, WWjj, Wt • Analysis: • 2 high pT leptons + MET • 2 forward jets (b-jet veto) • central jet veto • counting experiment, no peak: • background from data: • Signal: all cuts • Control sample: no lepton cuts • Result • better than inclusive WW (!!!) ATLAS MH=160 GeV HWWe

  48. Standard Model Higgs: qqH, Htt Httem • Backgrounds: • Zjj, tt • Analysis: • two forward jets, central jet veto • two leptons (e, m, t-jet)+MET • ttlnn + lnn • tt lnn + t-jet • mass(l; l or t-jet; pTmis) • despite 3 or 4 n’s present, works quite well in collinear approximation ATLAS 30 fb-1 ATLAS t pTmis H t

  49. MSSM Higgs boson: h, H, A production h H A • x-sections are large, often much larger than SM (dotted line) • bb(h/H/A) production is very important tanb=3 h H A tanb=30

  50. MSSM Higgs: SM-like signatures • ATLAS: • no systematics included • CMS: • better detector simulation • systematics included • contours recessed… CMS 2003 CMS 2006 ATLAS

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