(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 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 R Nikolaidou Beyond 2002 conference

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

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 IP SV PV DELPHI Z events u,d,s,c b Data/MC agreement  5% R Nikolaidou Beyond 2002 conference

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

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 R Nikolaidou Beyond 2002 conference

SM Higgs boson results ADLO 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 discriminant variable and a mass R Nikolaidou Beyond 2002 conference

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 R Nikolaidou Beyond 2002 conference

SM Higgs boson results ADLO 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% R Nikolaidou Beyond 2002 conference

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 R Nikolaidou Beyond 2002 conference

SM Higgs boson searches : a summary From indirect limits from EW precision measurements (LEP,SLC,Tevatron) MH196 GeV/c2 95% C.L From direct searches at LEP MH114.1 GeV/c2 95% C.L Likelihood test preferred mass MH=115.6 GeV/c2 R Nikolaidou Beyond 2002 conference

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 Couples to Type I Type II ’d-type’ leptons 2 1 ’u-type’ quarks ’d-type’ quarks 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) R Nikolaidou Beyond 2002 conference

MSSM Higgs searches ADLO 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 R Nikolaidou Beyond 2002 conference

MSSM Higgs searches ADLO 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 R Nikolaidou Beyond 2002 conference

Flavor independent Higgs search 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/c2 95% C.L assuming hZ= hZSM Strong cross section limits 112.9 GeV R Nikolaidou Beyond 2002 conference

Invisible Higgs searches 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% R Nikolaidou Beyond 2002 conference

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 R Nikolaidou Beyond 2002 conference

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) R Nikolaidou Beyond 2002 conference

Search for Charged Higgses 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 R Nikolaidou Beyond 2002 conference

Search for H  WA OPAL mA 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 R Nikolaidou Beyond 2002 conference 50 60 mH

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 R Nikolaidou Beyond 2002 conference

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 R Nikolaidou Beyond 2002 conference

Still pending question 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