1 / 35

Physics at Hadron Colliders Lecture III

Physics at Hadron Colliders Lecture III. Beate Heinemann University of California, Berkeley and Lawrence Berkeley National Laboratory. CERN, Summer Student Lectures, 2008. Outline. Lecture I: Introduction Outstanding problems in particle physics and the role of hadron colliders

orea
Download Presentation

Physics at Hadron Colliders Lecture III

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. Physics at Hadron CollidersLecture III Beate Heinemann University of California, Berkeley and Lawrence Berkeley National Laboratory CERN, Summer Student Lectures, 2008

  2. Outline • Lecture I: Introduction • Outstanding problems in particle physics • and the role of hadron colliders • Current and near future colliders: Tevatron and LHC • Hadron-hadron collisions • Lecture II: Standard Model Measurements • Tests of QCD • Precision measurements in electroweak sector • Lecture III: Searches for the Higgs Boson • Standard Model Higgs Boson • Higgs Bosons beyond the Standard Model • Lecture IV: Searches for New Physics • Supersymmetry • High Mass Resonances (Extra Dimensions etc.)

  3. The Higgs Boson Peter Higgs • Symmetry breaking caused by scalar Higgs field • vacuum expectation value of the Higgs field <> =246 GeV/c2 • gives mass to the W and Z gauge bosons, MW gW<> • fermions gain a mass by Yukawa interactions with the Higgs field, mf  gf<> • Higgs boson couplings are proportional to mass • Higgs boson prevents unitarity violation of WW cross section • (ppWW) > (pp  anything) • => illegal! • Something new must happen at LHC

  4. Higgs Production: Tevatron and LHC Tevatron LHC s(pb) dominant: gg H, subdominant: HW, HZ, Hqq

  5. Higgs Boson Decay BR LEP excluded bb   WW ZZ • Depends on Mass • MH<130 GeV/c2: • bb dominant • WW and tt subdominant • g small but useful • MH>130 GeV/c2: • WW dominant • ZZ cleanest _

  6. Describe what significance means…

  7. High Mass: mH>140 GeV

  8. _ H  WW(*)  l+l-nn • Higgs mass reconstruction impossible due to two neutrinos in final state • Make use of spin correlations to suppress WW background: • Higgs has spin=0 • leptons in H  WW(*)  l+l-nn are collinear • Main background: WW production 10x 160 GeV Higgs

  9. HWW(*)l+l- (l=e, New result! • Event selection: • 2 isolated e/ : • pT > 15, 10 GeV • Missing ET >20 GeV • Veto on • Z resonance • Energetic jets • Separate signal from background • Use matrix-element or Neural Network discriminant to • Main backgrounds • SM WW production • Top • Drell-Yan • Fake leptons em

  10. High Mass Higgs Signals CMS HZZ eeee example signals for mH=150 GeV/c2 HZZ* M. Dührssen et al., hep-ph/0504006 ATLAS HWW* Clean signals on rather well understood backgrounds

  11. Low Mass: mH<140 GeV

  12. WHlbb b jet b jet n e/m • WH selection: • 1 or 2 tagged b-jets • electron or muon with pT > 20 GeV • ETmiss > 20 GeV Now looking for 2 jets Expected Numbers of Events: WH signal: 0.85 + 0.65 Background: 62±13 + 69±12

  13. ZHbb Event selection: ≥ 1 tagged b-jets Two jets ETmiss > 70 GeV Lepton veto Veto missing ET along jet directions Big challenge: Background from mismeasurement of missing ET QCD dijet background is HUGE Generate MC and compare to data in control regions Estimate from data Control: Missing ET direction Missing ET in hard jets vs overall missing ET ET ET mismeasured genuine jet jet jet jet

  14. QCD Jet Background to ZHbb DØ uses data Define variable that can be used to normalize background Asymmetry between missing ET inside jets and overall missing ET Sensitive to missing ET outside jets Background has large asymmetry Signal peaks at 0

  15. Background understanding using MC • CDF use MC and check it in detail against data • “QCD” control region: • Jet aligned with missing ET • Completely dominated by QCD jets and mistags “EWK” control region: Identified lepton in event => Dominated by top Look at data only when control regions look satisfactory

  16. Dijet Mass distributions ZHllbb ZHbb WHlbb • Backgrounds still much larger than the signal: • Further experimental improvements and luminosity required • E.g. b-tagging efficiency (40->60%), NN selection, higher lepton acceptance H signal x10 H signal

  17. Higgs Search with Neural Network no b-tags 1 b-tag 2 b-tags • Construct neural network can be powerful to improve discrimination: • Here 10 variables are used in 2D Neural Network • Critical: • understanding of distribution in control samples SM(ZH)x19

  18. LHC: Low Mass Region • LHC for mH=115 GeV/c2 • L≈10 fb-1 for 5s discovery for single experiment • CMS mostly sensitive to gg decay • ATLAS more sensitive to ttH->ttbb and qqH->qqtt

  19. 115 GeV Higgs: LHC with 10fb-1 Total S/√B=4.2 First evidence possible Difficult to know whether it is the Higgs boson Important to see signal in each channel Gives first idea about branching ratios Diphoton channel will have nice peak, others not ATLAS L=30fb-1 Large K-factor~2 not included

  20. 130GeV Higgs: first year (10fb-1) Total S/√B=6 This is good! Now qqWW and ZZ channels contribute a lot ttH channel really difficult now => cannot measure branching into b’s Nice peaks expected in gg and ZZ complete detector 130 GeV Higgs L = 100 fb-1

  21. Low Mass Higgs Signals ATLAS, 100 fb-1 H CMS, 30 fb-1 VBF H • H challenges: • Background from 0 • Mass resolution requires brilliant calibration • At least 1 photon converts in 50% of events • VBF: Hqqqq challenges: • Central jet veto sensitive to underlying event • Need to understand forward jets • Background from jets looking like tau’s

  22. Ratio to Standard Model • Further experimental improvements and luminosity expected • Will help to close the gap • Expect to exclude 160 GeV Higgs boson soon • At low mass still rather far away from probing SM cross section

  23. LHC SM Higgs Discovery Potential 2004 Fast discovery for high mass, e.g. mH>150 GeV/c2 Harder at low mass=> zoom into low mass region

  24. Ultimate sensitivity ∫ L dt= 6 fb-1 • With 6 fb-1 of LHC data will know if Higgs boson exists • in 2-3 years already (hopefully)!

  25. How do we know what we have found? • After discovery we need to measure: • The mass • The spin • The branching ratio into all fermions • Verify coupling to mass • The total width • Are there invisible decays?

  26. Mass

  27. Couplings at LHC Duehrssen et al hep-ph/0407190 • Measure the couplings of the Higgs to as many particles as possible: • H->ZZ • H->gg • H->WW • H->tt • H->bb • And in different production modes: • gg->H, ttH (tH coupling) • WW->H (WH coupling)

  28. Non Standard-Model Higgs Bosons

  29. Higgs in the MSSM • Minimal Supersymmetric Standard Model: • 2 Higgs-Fields: Parameter tanb=<Hu>/<Hd> • 5 Higgs bosons: h, H, A, H± • Neutral Higgs Boson: • Pseudoscalar A • Scalar H, h • Lightest Higgs (h) very similar to SM • C. Balazs, J.L.Diaz-Cruz, H.J.He, T.Tait and C.P. Yuan, PRD 59, 055016 (1999) • M.Carena, S.Mrenna and C.Wagner, PRD 60, 075010 (1999) • M.Carena, S.Mrenna and C.Wagner, PRD 62, 055008 (2000)

  30. MSSM Higgs Selection • pp +X tt +X : • One t decays to e or m • One t decays to hadrons or e/m • Use “visible mass”, m(ET,l1,l2) for discrimination against background • pp b+X  bbb+X : • Three b-tagged jets • ET>35, 20 and 15 GeV • Use invariant mass of leading two jets to discriminate against background =h/H/A

  31. Mass Distributions e/m+t e+m e/m+t • Good agreement between data and background in all analyses • No sign of deviation bbb

  32. MSSM Higgs: Results • pp  A+Xtt+X • Sensitivity at high tanb • Exploting regime beyond LEP • pp  Ab+X bbb+X • Probes high tanb if m<0 • Combined with tt channel by D0 • Future (L=8 fb-1): • Probe values down to 25-30!

  33. MSSM Higgs Bosons at LHC • At least one Higgs boson observable in all models • Often only one Higgs Boson observable • Could also be produced in SUSY cascades: • Depends on model how well this can be exploited 300 fb-1 • at least one Higgs boson observable for all parameters significant area where only lightest Higgs boson h is observable • can SM be discriminated from extended Higgs sector by parameter determination?

  34. Conclusions • The Higgs boson is the last missing piece in the Standard Model • And arguably the most important SM particle • Searches ongoing at the Tevatron • Chance of a 3sigma evidence • LHC will find the Higgs boson if it exists • And measure some of it’s properties • There might be more than one Higgs boson • E.g. in supersymmetry • They can be found too

  35. Higgs Production at the Tevatron b jet b jet n e/m s(pb) dominant: gg H, subdominant: HW, HZ

More Related