Higgs Searches at LEP2 E. Kneringer University of Innsbruck / Austria Collaboration LAKE LOUISE WINTER INSTITUTE Electroweak Physics February 1999

E. KNERINGERLLWI Motivation indirect searches results from direct searches with Outline  The Standard Model Higgs Boson  SUSY Higgses The Neutral Higgs Bosons of the MSSM Charged Higgses in General Two Doublet Models Invisible Higgs LEP EWWG summer ‘98

E. KNERINGERLLWI The Standard Model Higgs Boson Situation at LEP2 Production and decay

E. KNERINGERLLWI Search in Four Characteristic Event Topologies missing energy 4 jets general requirements:  compatibility with m Z  b-jets general requirements:  compatibility with m Z  b-jets Z 2 isolated high energetic leptons l = e,  Z  candidates

E. KNERINGERLLWI Background Processes  tools to distinguish signal from background processes tagging of b-quark jets lepton identification ( e,  ) kinematic mass fits

E. KNERINGERLLWI Analyses  extensively using Monte Carlo simulations for neural net training for setting cuts  optimizing the analyses for the individual channels to get the best combined analysis highest efficiency with lowest background best Higgs mass exclusion limit if there is no hint for a signal mHmH entries total 11.3  ZZ background subtraction at 189 GeV 189 GeV data

E. KNERINGERLLWI Results for the SM Higgs  all data  observed limit: m H > 90.4 GeV/c 2 at 95 % C.L.* ) from gedanken experiments:  expected limit: m H > 93.4 GeV/c 2  bad luck  other LEP experiments have typically m H > 94 GeV/c 2 (expected and observed)  current best limit from L3: m H > 95.5 GeV/c 2 already above Z mass! *) C.L.: frequentist; P (  signal >  data ) > 95 % ALEPH preliminary

E. KNERINGERLLWI Neutral Higgs Bosons of the MSSM  Reminder 2 parameters to parameterize model at tree-level: m h, tan  (m A,  ) Mass relations at tree-level m h < m Z |cos 2  |(+ radiative corrections !!!) m h < m A < m H m H > m Z (m H > max{m Z,m A }) m H± > m W (m H± 2 = m W 2 + m A 2 ) m A large  decoupling of heavy Higgses  2 complementary production processes: m h   s  m Z hZ Higgs strahlung  sin 2 (  ) m h,m A   s/2 hA associated pair-production  cos 2 (  ) 2 final states

E. KNERINGERLLWI Analyses  candidate: 4 b-jet event m A + m h entries 189 GeV data

E. KNERINGERLLWI Results for the MSSM Higgses  all ALEPH data combined  hA selection at large tan  ( eq. cos 2 (  ) ~ 1 ): m A > 82.2 GeV/c 2 m h > 82.2 GeV/c 2 at 95% C.L.  lucky  reinterpretation of hZ + hA searches at smaller values of cos 2 (  )... expected and observed C.L. for equal h and A masses and cos 2 (  ) = GeV data

E. KNERINGERLLWI sin 2 (  ) - m h plane GeV data hA hZ

E. KNERINGERLLWI m h - tan  plane  results can also be expressed in the m h - tan  plane  for the benchmark sets of MSSM parameters (m top, M SUSY,...)  and for two extreme configurations of stop mixing (controlled by A t,  ) no mixing maximal mixing theoretically excluded theoretically excluded GeV data

E. KNERINGERLLWI Charged Higgs  search for pair-produced H + H   in the framework of general two-doublet models m W = m Z cos  W at tree level, i.e.  = 1 absence of flavor changing neutral currents  decays into heaviest energetically accessible fermion pairs  three analyses for the three channels leptonic   hadroniccs sc mixed  cs  cross section of the same size at LEP2 energies (for the mass limits reachable) m H+ = 59 GeV/c 2

E. KNERINGERLLWI       final state  final state topology: two acoplanar taus missing energy carried away by neutrinos (at least four neutrinos!) charged Higgs

E. KNERINGERLLWI cs    final state  final state topology: two hadronic jets thin  -jet + missing energy  two complementary approaches: global analysis acoplanarity missing momentum thrust topological analysis  -jet reconstruction sensitivity well below m W charged Higgs reconstructed jet-jet mass

E. KNERINGERLLWI Performance of analyses 183 GeV data Expected number of background events from SM processes as a function of m H±, compared to the number of events observed (mixed + four jet channel: sliding mass window): 60 events found by the three selections 57.9 events expected from SM processes consistency + no accumulation in the cssc mass distribution  combine analyses  put limits charged Higgs

E. KNERINGERLLWI Results  only up to 183 GeV data  NO evidence for a signal  mass limits as a function of  (  )  Limit at 95% C.L. on the mass of charged Higgs bosons as a function of  (  ); the hatched region is excluded by the combination of all three channels.  m H± > 59 GeV/c 2 indep. of  (  ) [95% C.L.] charged Higgs

E. KNERINGERLLWI  (  ) for type II doublet models prediction

E. KNERINGERLLWI Invisible Higgs decays Acoplanar leptons with mass ~ M Z Acoplanar jets with mass ~ M Z

E. KNERINGERLLWI Summary  SM Higgs with 250 pb -1 collected with ALEPH at  s = 161, 172, 183 and 189 GeV 95% C.L. lower limit is m H > 90.4 GeV/c 2 very preliminary, very conservative  MSSM Higgs also lower energy data at 130 GeV included m h, m A > 82.2 GeV/c 2 for large tan   Charged Higgs data at 189 GeV not yet included m H+ > 59 GeV/c 2 independent of  (  )  Invisible Higgs h  neutralinos & GUT mass relation (M 1 ~ 0.5 M 2 ) relaxed  2 for  2 = 1: m h > 88 GeV/c 95% C.L.

E. KNERINGERLLWI Confidence Level  define estimator  as a measure for the experiment in question to contain signal events: n = # of observed events s = expected # of signal events P i n = prob. that i signal events are less signal like than observed  small  background like  large  signal like  define observed confidence level c where  data is the estimator of the actual experiment  s is the p.d.f. of the estimator for signal events  expected confidence level average of c where the # of observed events is distributed according to the expected background (i.e. in the absence of signal)  no event observed   = e  s,  data =  min, c = e  s

E. KNERINGERLLWI interpretation of C.L.  m H > 90.4 GeV/c 2 at 95% C.L. means: given the hypothesis that the Higgs mass is and < 90.4 GeV/c 2 be  data the estimator as observed in the data and  signal the estimator for a gedanken experiment where the above hypothesis is true (i.e. distributed according to  s ) then P (  signal >  data ) > 95 % the quoted limit is the largest mass hypothesis for which this is true  it does NOT mean: P(m H > 90.4) > 95% or P(m H < 90.4) < 5%