1 / 64

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.

barbie
Télécharger la présentation

Higgs Boson at CMS: gearing up for discovery

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related