1 / 22

Motivation indirect searches

Higgs Searches at LEP2 E. Kneringer University of Innsbruck / Austria Collaboration LAKE LOUISE WINTER INSTITUTE Electroweak Physics 14 - 20 February 1999. Motivation indirect searches. results from direct searches with Outline The Standard Model Higgs Boson SUSY Higgses

ulfah
Download Presentation

Motivation indirect searches

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. Higgs Searches at LEP2E. KneringerUniversity of Innsbruck / Austria CollaborationLAKE LOUISE WINTER INSTITUTEElectroweak Physics14 - 20 February 1999

  2. 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 LLWI - 1999

  3. Situation at LEP2 Production and decay The Standard Model Higgs Boson LLWI - 1999

  4. Search in Four CharacteristicEvent Topologies 2 isolated high energetic leptons  candidates missing energy 4 jets Z Z l = e,  general requirements:  compatibility with mZ  b-jets LLWI - 1999

  5. Background Processes • tools to distinguish signal from background processes • tagging of b-quark jets • lepton identification (e, ) • kinematic mass fits LLWI - 1999

  6. 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  189 GeV data entries total 42 11.3 38.0 mH ZZ background subtraction at 189 GeV LLWI - 1999

  7. Results for the SM Higgs • all data • observed limit: mH > 90.4 GeV/c2 at 95 % C.L.*) from gedanken experiments: • expected limit: mH > 93.4 GeV/c2 bad luck • other LEP experiments have typically mH > 94 GeV/c2(expected and observed) • current best limit from L3: mH > 95.5 GeV/c2already above Z mass! *) C.L.: frequentist; P (signal > data ) > 95 % ALEPH preliminary LLWI - 1999

  8. Neutral Higgs Bosons of the MSSM • Reminder • 2 parameters to parameterize model at tree-level: mh, tan (mA, ) • Mass relations at tree-level • mh < mZ |cos 2| (+ radiative corrections !!!) • mh < mA< mH • mH > mZ (mH> max{mZ,mA}) • mH± > mW(mH±2= mW2+ mA2 ) • mA large  decoupling of heavy Higgses • 2 complementary production processes: 2 final states hA associated pair-production  cos2() hZ Higgs strahlung  sin2() mh ,mA s/2 mh s  mZ LLWI - 1999

  9. Analyses 189 GeV data • candidate: 4 b-jet event entries mA + mh LLWI - 1999

  10. Results for the MSSM Higgses 130 - 189 GeV data • all ALEPH data combined • hA selection at large tan (eq.cos2() ~ 1):mA > 82.2GeV/c2mh > 82.2GeV/c2 at 95% C.L. lucky • reinterpretation ofhZ + hA searchesat smaller valuesof cos2() ... expected and observed C.L. for equal h and A masses and cos2() =1 LLWI - 1999

  11. sin2() - mh plane hZ 130 - 183 GeV data hA LLWI - 1999

  12. mh - tan plane 130 - 183 GeV data • results can also be expressed in the mh - tan plane • for the benchmark sets of MSSM parameters(mtop, MSUSY, ...) • and for two extreme configurations of stop mixing(controlled by At, ) • no mixing • maximal mixing excluded theoretically excluded theoretically excluded LLWI - 1999

  13. Charged Higgs • search for pair-produced H+ H • in the framework of generaltwo-doublet models • mW = mZ cosW at tree level, i.e.  = 1 • absence of flavor changing neutral currents • decays into heaviest energetically accessible fermion pairs • three analyses for thethree channels • leptonic   • hadronic cs sc • mixed  cs • cross section • of the same size at LEP2 energies(for the mass limits reachable) mH+ = 59 GeV/c2 LLWI - 1999

  14. charged Higgs   final state • final state topology: • two acoplanar taus • missing energy carried away by neutrinos(at least four neutrinos!) LLWI - 1999

  15. charged Higgs cs final state sensitivitywell below mW • final state topology: • two hadronic jets • thin -jet + missing energy • two complementary approaches: • global analysis • acoplanarity • missing momentum • thrust • topological analysis • -jet reconstruction reconstructed jet-jet mass LLWI - 1999

  16. charged Higgs Performance of analyses • 183 GeV data • Expected number of background events from SM processes as a function of mH± , 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 LLWI - 1999

  17. 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. mH± > 59 GeV/c2 indep. of () [95% C.L.] charged Higgs Results LLWI - 1999

  18. () for type II doublet models 0.972 0.042 prediction LLWI - 1999

  19. Invisible Higgs decays Acoplanar leptons with mass ~ MZ Acoplanar jets with mass ~ MZ LLWI - 1999

  20. 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/c2 • very preliminary, very conservative • MSSM Higgs • also lower energy data at 130 GeV included • m h , mA > 82.2 GeV/c2 for large tan • Charged Higgs • data at 189 GeV not yet included • m H+ > 59 GeV/c2 independent of () • Invisible Higgs • h  neutralinos & GUT mass relation (M1 ~ 0.5 M2) relaxed • for 2 = 1: m h > 88 GeV/c2 @ 95% C.L. LLWI - 1999

  21. 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 • Pin = prob. that i signal events areless 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 <c > 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 LLWI - 1999

  22. interpretation of C.L. • m H > 90.4 GeV/c2 at 95% C.L. means: • given the hypothesis that the Higgs mass isand < 90.4 GeV/c2 • 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% LLWI - 1999

More Related