Search for Higgs Bosons at LEP. Haijun Yang University of Michigan, Ann Arbor

Transcription

1 Search for Higgs Bosons at LEP Haijun Yang University of Michigan, Ann Arbor L3 On behalf of the L3 Collaboration American Physical Society Meeting(APS03), Philadelphia April 5-8, 2003

2 OUTLINE Introduction Standard Model Higgs MSSM Neutral Higgs Charged Higgs Fermiophobic Higgs Invisible Higgs Flavour-blind Higgs NO Higgs bosons are discovered at LEP LEP Higgs Working Group Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03)

3 Introduction Standard Model(SM) is a theory for massless particles But, gauge bosons(w Z) and fermions do have masses The Higgs Mechanism : Particles acquire masses by interaction with the Higgs field The Higgs field has quanta - the Higgs bosons One Higgs Doublet Model - SM Higgs boson Two Higgs Doublets Model, MSSM,... The Higgs bosons masses are not predicted by theories Discovery of the Higgs boson(s) is one of the main goals at LEP, Tevatron and LHC Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 2

4 Constraint on the Higgs Mass Global fit to all precision measurements from LEP, SLC, Tevatron, theory uncertainty α had = α (5) ± ± Without NuTeV χ Excluded Preliminary m H [GeV] The fit prefers a low mass Higgs m H 2 GeV at 95% C.L. Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 3

5 Search for SM Higgs Boson Total Integrated Luminosity: 887 pb Center-of-Mass energies: s GeV 2 years operation: Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 4

6 Search for SM Higgs Boson Higgs-strahlung Process WW fusion process 4 Channels: Hq q Hν ν H τ τ q q H bb Branching ratio 0 - H τ + τ H WW * H cc _ H gg 0-2 H ZZ * M H [GeV] Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 5

7 Search for SM Higgs Boson Common Estimator used at LEP: 2lnQ 2 s i n i ln s i b i i Where n i s i b i represent observed events, expected Higgs signal and SM background in the i-th bin, i indicates the i-th bin of final discriminant of each channel and at each s. MC experiments based on Poisson statistics. observed Probability density signal+background background -CL b CL s+b ln(q) CL s CL s b / CL b Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 6

8 -2 ln(q) Search for SM Higgs Boson ALEPH -2 ln(q) DELPHI -2 ln(q) Observed Expected background Expected signal + background m H (GeV/c 2 ) Observed Expected background Expected signal + background L m H (GeV/c 2 ) -2 ln(q) Observed Expected background Expected signal + background m H (GeV/c 2 ) Observed Expected background Expected signal + background OPAL m H (GeV/c 2 ) Expected limit (GeV) Observed limit (GeV) LEP ALEPH DELPHI L OPAL Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 7

9 Higgs Boson Candidates Expt E cm Decay channel m rec H (GeV) ln s b at 5 GeV ALEPH jet ALEPH jet ALEPH jet L E-miss ALEPH 205. Lept ALEPH Taus OPAL jet ALEPH jet L jet DELPHI jet ALEPH jet DELPHI jet OPAL 20.5 E-miss L E-miss ALEPH jet DELPHI jet L jet For m H 5 GeV Data = 7, Signal = 8.42 & Background = 5.9 Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 8

10 Search for MSSM Higgs Bosons Large µ scenario: 0 08 GeV : tanβ LEP GeV Preliminary Large Mu Scan 0 XUnexcluded M SUSY =400 GeV M 2 =400 GeV µ= TeV m gluino =200 GeV X t =-300 GeV Excluded by LEP Previous exclusion based only on e e hz ha h A b b τ τ New searches covered h m A (GeV/c 2 ) c c gg All points excluded at 95% C.L. Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 9

11 tanβ 0 a) Search for MSSM Higgs Bosons -max scenario m A < 0 GeV Not allowed by theory Excluded L3 Not allowed by theory tanβ 0 c) No mixing scenario m A < 0 GeV Excluded Not allowed by theory L3 Not allowed by theory [GeV] [GeV] tanβ 0 max scenario m top = 75 GeV/c 2 M SUSY = TeV/c 2 Excluded at 95% C.L. ALEPH tanβ 0 no mixing scenario m top = 75 GeV/c 2 M SUSY = TeV/c 2 Excluded at 95% C.L. ALEPH (GeV/c 2 ) (GeV/c 2 ) 95% C.L. Limits ALEPH DELPHI L3 OPAL obs (exp) (9.3) 89.7 (88.8) 85.5 (88.5) 79.3 (85.) obs (exp) - m A 90. (9.6) 90.7 (89.7) 86.3 (88.6) 80.6 (86.9) Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 0

12 Search for Charged Higgs Boson Production: e e H H Decay: H cs τν Br(H τν) 0.8 LEP GeV 0.6 Excluded 0.4 Expected limit charged Higgs mass (GeV/c 2 ) 95% C.L. Limits ALEPH DELPHI L3 OPAL obs (exp) 79.3 (76.9) 73.8 (75.4) 66.9 (75.) 72.2 (74.5) Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03)

13 Search for Fermiophobic Higgs Boson e e Zh h γγ dominant for 90GeV B( h 0 xx ) Events/2 GeV 0 Photonic Higgs Search Preliminary 305 obs / expected ADLO data SM Background m=00 GeV signal γγ WW ZZ M h 0 (GeV) M γγ (GeV) Upper Limit on B(h 0 γγ)σ(e + e - h 0 Z)/σ(SM) LEP Combined Excluded Region Fermiophobic BR Limit = 09.7 GeV Expected Limit = 09.4 GeV M h 0 (GeV) 09 7 GeV BR h γγ 6% at 95% C.L. Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 2

14 Search for Fermiophobic Higgs Boson e e Zh h WW ZZ BR(h WW*)+BR(h ZZ*) Excluded L % CL Expected exclusion Benchmark scenario (GeV) Six channels investigated( 93% BR): Zh qqqqqq qqqq ν qq ν ν ννqqqq ννqq ν qqqq Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 3

15 Search for Invisible Higgs Boson Assumption: h inv χ 0 χ 0 might dominant Signal: h inv Z q q two acoplanar jets or leptons missing energy Ratio to SM rate Observed Median Background M H, GeV/c exp GeV at 95% C.L. for R inv Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 4

16 Assumption: h Flavour-blind Higgs Searches b b not necessarily dominant XSEC/SM BR(h hadrons) 0 - LEP PRELIMINARY HZ FLAVOUR BLIND HYPOTHESIS s 209 GeV observed expected for background (GeV) Searches not relying on b-tagging less model-dependent exp GeV at 95% C.L. Haijun Yang Search for Higgs Bosons at LEP (APS-DPF03) 5