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(Final) achievements on Higgs boson searches at LEP2

(Final) achievements on Higgs boson searches at LEP2. Rosy Nikolaidou CEA/DAPNIA/SPP Introduction The Standard Model Higgs boson More exotic scenarios Conclusion. Results will be finalised this year. Introduction. Started operating in 1989. Stopped at November 2000

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(Final) achievements on Higgs boson searches at LEP2

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  1. (Final) achievements on Higgs boson searches at LEP2 Rosy Nikolaidou CEA/DAPNIA/SPP Introduction The Standard Model Higgs boson More exotic scenarios Conclusion Results will be finalised this year

  2. Introduction Started operating in 1989. Stopped at November 2000 Energy spread 91<Ecm< 209. Since 1995 Ecm > mZ  1000 pb-1 per experiment,  688 pb-1 /exp above WW threshold • The rise of energy at LEP open the possibility to explore a wide mass range • Combined results from the 4 experiments ALEPH DELPHI L3 OPAL are denoted LEP ADLO Beyond 2002 conference

  3. Z Ze+e-,+- Hbb Hbb Z+-,q q Zq q Hbb (+-) Hbb SM Higgs boson: production, decays Higgsstrahlung WW,ZZ Fusion mHs - mZ We can go beyond the kinematic limit Hbb dominant Channels: Beyond 2002 conference

  4. IP Fundamental tools: b-tagging Silicon vertex detectors(e.x DELPHI) • Hit precisions: 10 m in R 15 m in Rz • B-hadron characteristics : B-hadron life-time 1.6 ps Flight distance  3 mm Impact Parameter 400 m • Decay products of B-hadrons: large IP, PT ,lead to tracks originating from SV… • Combination of (SV mass, IP, decay length,PT of leptons, e.t.c) to a single b-tagging variable DELPHI SV PV DELPHI Z events u,d,s,c b Data/MC agreement  5% Beyond 2002 conference

  5. LEP2 cross sections qq, Z* dominant but can be reduced ZZ irreducible for H, hAsearches WW irreducible for H+H- searches Background cross sections 103 SM Higgs cross section (… < 1pb ) 102 10 1 10-1  (pb) s GeV Beyond 2002 conference

  6. Tools :Multidimensional analyses Multidimensional techniques used to reach the best S/B discrimination (Neural Network, Likelihood ratio) H L3 Hqq Kinematic fits with E,P conservation and Z mass constraint performed to improve on signal mass reconstruction Beyond 2002 conference

  7. ADLO SM Higgs boson results • 3 selections with increasing purity of a 115 GeV/c2 signal D/Bck 61/60.3 D/Bck 187/188.1 Signal 5.8 Signal 11.4 D/Bck 22/20.2 Signal 2.7 • All channels contribute • Hqq,H,Hll,qq •  event select: A discriminantvariable and a mass Beyond 2002 conference

  8. Tools:Statistical analysis of the results • For a powerful statistical analysis of the combined results (per channel and experiment) : Selection of ‘signal-like’ events in each researched channel  event 2d p.d.f’s of a discriminant variable and a mass Test the compatibility of data with B-only or S+B hypotheses from S and B rates and 2d pdf’s using a likelihood ratio test statistics which simplifies into -2lnQ=2stot-2Niln(1+si/bi) (stot total signal rate, si/bi =s/b prob per pdf bin) e.g Expectations on –2lnQ, one experiment in 2000 Kinematic edge @ 116 GeV Powerful separation for MH=110 GeV/c2 More difficult for MH=115 GeV /c2 DELPHI Beyond 2002 conference

  9. ADLO SM Higgs boson results Median, 1, 2 bands expected from background only Data consistent with a Higgs signal of MH=115.6 GeV/c2 Expected curve from Bck+signal of 115 GeV/c2 • LEP result corresponds to 2.1 over background • At 95%C.L mH>114.1 GeV/c2(obs) mH>115.4 GeV/c2(exp) • Compatibility with background fluctuation hypothesis : 3.4% • Compatibility with a signal of 115.6 GeV/c2:44% Beyond 2002 conference

  10. A golden candidate in the SM search • Event compatible with SM Higgs boson decay e+e-HZ bbqq • A 4-jet event with 2 b-jets • Some properties: • MH=114.3 3 GeV/c2 • Strong S/B probability ratio=4.6 Beyond 2002 conference

  11. SM Higgs boson searches : a summary • From indirect limits from • EW precision measurements • (LEP,SLC,Tevatron) • MH196 GeV/c295% C.L • From direct searches at LEP • MH114.1 GeV/c295% C.L • Likelihood test preferred mass • MH=115.6 GeV/c2 Beyond 2002 conference

  12. 2 Higgs Doublet Models • In these models: 2 complex Higgs doublets (1 2 ) • Five Higgs bosons:(h0, H0,A0, H) • Parameters:tan(=2/1 VEV ratio) (mixing angle of h0 H0 ) (Higgs mass parameter)Ao (trilinear Higgs-sfermion coupling) e.g sin2(-) HZ(SM) cos2(-) HZ(SM) Large at small tan Large at large tan mA mh SM topologies and New topologies searches are used hA bbbb, bb Also h0A0A0,invisible e.x MSSM ghff cos e.x fermiophobic ( =/2) Beyond 2002 conference

  13. ADLO MSSM Higgs searches • Combine hZ , hA searches • Scenario mh-max: Allows max theoretical bound on mh w.r.t tan e+e- hA e+e- hZ mh91GeV/c2 ,mA91.9 GeV/c2 0.5 < tan < 2.4 excluded Beyond 2002 conference

  14. ADLO MSSM Higgs searches • Combine hZ , hA searches • Scenario:No mixing e+e- hA e+e- hZ mh91.5 GeV/c2 ,mA92.2 GeV/c2 0.7 < tan < 10.5 excluded Beyond 2002 conference

  15. ADLO Flavor independent Higgs search In some extensions to SM H-b coupling could be suppressed e.g Search for e+e- hZ, hhadrons (qq, gg), Zanything • Assuming Br(h hadrons) =1 • Min eff (hqq,gg ) used • No more btagging applied Results Mh112.9 GeV/c295% C.L assuming hZ= hZSM Strong cross section limits 112.9 GeV Beyond 2002 conference

  16. ADLO Invisible Higgs searches e+e- Z*  hZ, h(stable, undetectable particles) • h decays In MSSM h  00 Other models h  JJ (Majorons) • Signatures large Emiss + dileptons or dijets with mass close to mZ 114.4 GeV Assuming BR(h inv)=1 mh114.4 GeV/c2 or Up to mh 110 GeV/c2 hZ/ SM  40% Beyond 2002 conference

  17. Non fermionic Higgs searches ADLO Searches in which Higgs couples to bosons only e.g Decay: e+e-hZ()(anything) • h0 small (0.1% at SM , mh=90 GeV) can be enhanced in (2HDM type I models, modified MSSM e.t.c) • Look for energetic isolated photons Assuming SM production rates,BR(hff)=0 mh108.2 GeV/c2 Beyond 2002 conference

  18. Model independent search for scalars • e+e-S0Z, (Zee,) (S0 had, inv.,, lept, stable) • Compare recoil mass spectrum with an expected S0 signal • S0 masses down to 1KeV Limits S0Z= HSMZ<1 mS0 <81 GeV/c2 95%C.L (absolute mass limit for SM coupling strength) Beyond 2002 conference

  19. ADLO Search for Charged Higgses In 2HDM models e+e- H+H- 3 channels cscs, cs,  Assume:(cs, ) saturate the decays But BR’s are not constrained Production cross section (mH) Background from e+e-W+W- • Results Assuming Br(Hcs,  )=1 m H >78.6 GeV/c2 95% C.L 78.6 GeV Beyond 2002 conference

  20. Search for H  WA OPAL • In a 2HDM type I model H  WA, (Abb,+-) dominant if tan>1, kinematically allowed • 3 final states are studied: H+H- WAWAqqbbqqbb H+H- WAWAlbbqqbb H+H- WAqqbb • Results on (mH, mA) plane tan=1 tan=10 68 GeV 42 60 mA tan=100 Beyond 2002 conference 50 60 mH

  21. Search for doubly charged H bosons • Appear in left right symmetric models and can be relatively light • Decay to leptons, preferably  • Signatures: 4l, 4l+Emiss • h unknown Yukawa coupling For h >10-7 H decays fast • Results m H >98.5 GeV/c2 for BR(H  ±  ± )=1 Beyond 2002 conference

  22. Light Higgs Yukawa production Processes I,II suppressed if sin2(-) small, mA+mh>s • Look at Zbb h/A bb  I II III Then III dominant enhancement factor of the bb h/A couplings Beyond 2002 conference

  23. Conclusions For the SM Higgs boson MH<196 GeV/c2 , MH114 GeV/c2 , MH=115.6 GeV/c2 LEP has explored and constrained a lot of models beyond the SM Is the Higgs boson responsible for the EW breaking mechanism? (Tevatron, LHC, Linear Colliders) Still pending question wait for

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