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Highlights of the searches for Higgs bosons at LEP2

Highlights of the searches for Higgs bosons at LEP2. Marumi Kado CERN. First hints of Higgs boson production. Part I. 206.5 GeV. 205 GeV. How it all began. June 14 th : First candidate at high energy and high reconstructed mass (~115 GeV/c2 ~ 206.5-m Z ).

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Highlights of the searches for Higgs bosons at LEP2

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  1. Highlights of the searches for Higgs bosons at LEP2 Marumi KadoCERN Marumi Kado - CERN

  2. First hints of Higgs boson production Part I Marumi Kado - CERN

  3. 206.5 GeV 205 GeV Marumi Kado - CERN

  4. How it all began • June 14th: First candidate at high energy and high reconstructed mass (~115 GeV/c2 ~ 206.5-mZ) • July 20th LEPC: First combination • limit 113.3GeV/c2 /sensitivity 113.4 GeV/c2 • September 5th LEPC: Second combination • A 2.2s excess is observed • limit 112.3GeV/c2 /sensitivity 113.8 GeV/c2 Marumi Kado - CERN

  5. Repeated confirmations • Septembre 14th Research board: LEP is awarded an additional month of running, to further check these results • Octobre 10th LEPC: Update of the results • The September 5th excess grows up to2.6s • November 2d: end of LEP operations • November 3d LEPC: The new data confirm the excess again. The significance grows up to 2.9s • November 13th: ALEPH publication • November 14th:L3 publication • December 22d: OPAL and DELPHI publication LEP running in 2001 is requested Additional 2001 running not granted Marumi Kado - CERN

  6. Production process at LEP s as a fonction of mH s-mZ «Threshold» • Radiated by a Z* (Higgs-strahlung) 208 204 206 202 s ~ 0.04 pb • Dominant production process s en fonction de s e+e-  HZ 3s Sensitivity: mH s-mZ 5s L~200pb-1 3s Marumi Kado - CERN

  7. Decays • Higgs boson branching ratios • LEP has mostly b quark or t lepton • Gain sensitivity near 115 GeV/c2 OPAL and L3 have also started the study of W+W- decays of the Higgs boson For mH ~ 115 GeV/c2 Br(Hbb) ~ 74% Br(Ht+t-) ~ 7% Br(HW+W-) ~ 7% Marumi Kado - CERN

  8. Background processes A priori irreducible process Zbb s~2 pb _ • Doubly resonant production of W and Z bosons Reducible process s~15 pb Marumi Kado - CERN

  9. Search topologies _ q b Dominant channel: • Mainly determined by the Z boson decay modes • The “four jets” topology _ q b Background processes near threshold: b quark pair with 2 hard gluons Quark pair with gluon splitting into a quark pair Marumi Kado - CERN

  10. _ Second most prolific channel: n b • Missing energy channel _ n b Where the Higgs boson is produced also in the fusion process: non-negligible contribution near threshold (Constructive interference)  No kinematic threshold Main background near threshold: 2 ISR escape undetected at low angle b quark pair 2 photons ISR Marumi Kado - CERN

  11. Good resolution in reconstructed mass: e-,m- b _ • Leptonic channels e+,m+ b Also produced by the fusion process: Negligible contribution (destructive interference) The H or Z decays to taus: b,q t- _ _ b,q t+ Marumi Kado - CERN

  12. Combined results • Results as of November 3d • DELPHI TPC sector and OPAL VDET module • problems were worked around • Analysed luminosity up to October 25th • The ~30 pb-1 collected between 25/10 and 02/11 are not included in this combination s s Marumi Kado - CERN

  13. Illustration of the searches results • Reconstructed mass histograms • Rescalings or fits using the recoil to the Z boson • Each channel in each experiment is optimized with different expected background rates • A possible prescription to illustrate the results of the searches is: • So that each channel and each experiment are represented in a fair way: • S/B for a Signal mass of mH=115 GeV/c2 and for a reconstructed mass greater than 109 GeV/c2 Fixed S/B ratio These histograms are shown for illustration purposes only they do not contain all the information used in the complete interpretation of the results Marumi Kado - CERN

  14. Good agreement data and Monte Carlo Excess at high reconstructed mass • Reconstructed mass distribution (S/B>0.3) 26.5 Marumi Kado - CERN

  15. High mass excess confirmed • For S/B>1 6.6 60 Marumi Kado - CERN

  16. High mass excess again confirmed Most significant events (mH>109 GeV/c2) • For S/B>2 ALEPH: 3 (4J) L3: 1 (Hnn) 2.9 25.5 Marumi Kado - CERN

  17. Complete interpretation For each event i: ~ • - si signal local value of the discriminating variable(reconstructed mass, b-tag, NN) • - bilocal value for the expected background • Estimation of an experiment s, b, N • Estimator  Likelihood ratio: ~ ALEPH Likelihood for the signal & background hypothesis Likelihood for the background only hypothesis Marumi Kado - CERN

  18. where • -2lnQ Estimator used • Trivial combination of channels and experiments • (Sum of the estimators) • For gedanken experiments: Signal & bckg. Bckg. only Light Higgs and large cross section Intermediate Higgs mass and cross section Heavy Higgs and low cross section Marumi Kado - CERN

  19. Estimator for each experiment • For different Higgs boson mass hypotheses: A, L & O: Significant excess at high mass O: Also excess ~105 GeV/c2 D: Deficit Marumi Kado - CERN

  20. Back to the transverse vue for mH=115 GeV/c2 • Illustration of the signal & background and background only expectations for mH=115 GeV/c2 -2lnQ -2lnQ -2lnQ -2lnQ The distribution of the four observations is compatible with the signal hypothesis Marumi Kado - CERN

  21. Combined estimator -2lnQ -2lnQ Minimum at 115 GeV/c2 in coincidence with the most likely value in the signal and background hypothesis mH=115 GeV/c2 Q ~ 30 +0.7 -0.3 Marumi Kado - CERN

  22. Confidence level -2lnQ observed CLb: Fraction of background only “gedanken” experiments with an estimator larger than observed • How much do we trust that this observation is background like? • CLb Official « discovery » estimator (deviation from the Standard Model)  Confidence in the background hypothesis 2.9 Standard deviations (mH=115 GeV/c2) Minimum at 115GeV/c2: 1-Clb = 4.2 10-3 Marumi Kado - CERN

  23. Most significant candidates Observed: 14 Expected (s+b): 14 Signal: 7 Candidates with (s/b)115>0.3 ALEPH: 6 L3: 3 OPAL: 3 DELPHI: 2 Hqq: 9 Hnn: 3 Hl+l-: 1 Htt: 1 (70%) (20%) (7%) (3%) Theoretical branchings Marumi Kado - CERN

  24. _ _ Candidate -a- (21/08/2000) e+e-bbbb s = 206.7 Mrec=1103 GeV/c2 Three well b-tagged jets with two reconstructed displaced vertices Marumi Kado - CERN

  25. Candidate -b- (16/10/2000) e+e-bbnn 206.6 GeV Mrec 114.4 GeV/c2 Missing mass 94 GeV/c2 L3 Two b-tagged jets with two reconstructed displaced vertices (5 mm and 1 mm) Marumi Kado - CERN

  26. Candidate -c- (31/07/2000) _ _ e+e-bbbb s = 206.7 Mrec=1123 GeV/c2 Four well b-tagged jets with tree reconstructed displaced vertices Marumi Kado - CERN

  27. Candidate -d- (14/06/2000) _ _ e+e-bbqq Direction impulsion manquante s = 206.7 Mrec=1143 GeV/c2 Two well b-tagged jets with two reconstructed displaced vertices Marumi Kado - CERN

  28. How about the less significant events? A: (s/b)115<0.7 B: (s/b)115>0.7 ~ -2lnQB -2lnQA The effect is not only due to the four most significant events Marumi Kado - CERN

  29. Estimator for each channel • As a function of the Higgs boson mass hypothesis: Excess at high Higgs mass hypotheses in all channels most significant in the 4 jets channel Marumi Kado - CERN

  30. Back to the transverse vue for mH=115 GeV/c2 -2lnQ -2lnQ -2lnQ -2lnQ All channels are compatible with the signal hypothesis Marumi Kado - CERN

  31. Evolution of the effect with time Coherence in terms of amounts of data taken at high energy 2.9s 2.6s 2.2s 1.1s Coherence in terms of independent samples Results coherent with the presence of a Higgs boson with mass 115 GeV/ c2 Marumi Kado - CERN

  32. Could there be a systematic bias near threshold? • Question mainly answered by looking at data taken at lower centre-of-mass energies: Observed estimator as a function of the distance to threshold with 500pb-1 (per experiment) of data taken between 183 and 206 GeV If a bias similar to that observed above 206 GeV was observed at all centre-of-mass energies Marumi Kado - CERN

  33. Summary of the Standard Model Higgs search as of November 3d LEPC • Each experiment has a 3s sensitivity between 111.5 and 113.5 GeV/c2 • The combined sensitivity of the four experiments reaches 3s at mH=115 GeV/c2 • Coherent picture: • Four experiments • All channels • In terms of production (threshold effect) • Evolution in time and amount of accumulated data • Can we exclude? • LEP (rhetorical question) sensitivity 115.3, mH>113.5 GeV/c2 • DELPHI sensitivity 113.4, mH>113.9 GeV/c2 A 3s excess is observed by the combination in agreement with the production of a Higgs boson weighing 115 GeV/c2 +0.7 -0.3 Marumi Kado - CERN

  34. What have we learned since? • ALEPH (PLB 495(2000)1): • - Combination: excess of 3.4s/ published 3.0s. • Due to mainly two effects: • - Additional luminosity • - Taking into account correlation between discriminating variables • OPAL (CERN-EP-2000-156): • In the combination the significance • was 1.3s, but a quite significant • event was detected after • October 25th: • DELPHI (to appear in PLB) : • Observed limit 113.9114.3 GeV/c2 • Expected limit 113.4113.5 GeV/c2 • L3 (PLB 495(2000)18): • Reprocessing and analysis work is being completed. Opal 29 Oct. 2000 206.6 GeV Mrec= 110.5 GeV/c2 • (1-Clb)03/11/2000=0.19 • (1-Clb)published=0.2 e+e-bbqq Marumi Kado - CERN

  35. Beyond the Minimal Standard Model(Review of the topologies studied) Part II Marumi Kado - CERN

  36. Non minimal extensions of the Standard Model • Any number of Higgs doublets is adequate to break the EW symmetry (r=1 at tree level) • Using two complex scalar Higgs doublets • Mass to gauge bosons and Five physical scalar fields are left: h,H (CP even), A (CP odd) and H • The 6 free parameters of the model are: • - Higgs masses • - a mixing between h and H • - tan b: ratio of the vev of the two doublets • To avoid FCNC fermions of a given charge must couple at most to one doublet only. There are therefore only two convenient types of models: • - Type I: One doublet does not couple to fermions • -Type II: One doublet couples only to “up” type fermions and the other to “down” type fermions. Marumi Kado - CERN

  37. In general 2HDM: Search for charged Higgs bosons • The two main decay channels are H tn/cs • Production • Model dependence is embedded in Br(H tn) assuming Topologies searched for: cscs cstn tntn Main background: W pair production Marumi Kado - CERN

  38. PLB496(2000)34 The L3 excess • 2000 L3 data confirm and reinforce this effect, but... Effect consistently seen at all centre-of-mass energies above 189 GeV Marumi Kado - CERN

  39. the effect is not confirmed by other experiments: Each experiment individually excludes a possible signal at • ~70 GeV/c2 • The combined exclusion in the (mH, Br(H tn)) as of September 5th LEPC (with data since 1998): (LEPC 05/09/2000) Marumi Kado - CERN

  40. Topologies searched for: • In the 2HDM type I, if cosa=0 then no mixing occurs between the two CP-even Higgs bosons, thus one will not couple to fermions whatsoever (fermiophoby) • - Produced by Higgs-strahlung process • - Decays to a pair of photons if mh is small compared to 2mW ggtt~3% ggqq~70% ggnn~20% ggll~7% mqq~mZ mnn~mZ mtt,ll~mZ Main background: two fermions production with two photons Mass lower limit on a Higgs boson decaying to a pair of photons: 107.7 GeV/c2 (obs) 105.8 GeV/c2 (exp) (LEPC 05/09/2000) Marumi Kado - CERN

  41. 2HDM II, Flavour independent analyses (OPAL) • - For certain parameter choices (a and tanb0), the decay modes to a pair of b quarks and tau leptons can be suppressed • - Production process: Higgs-strahlung x2  sMSM qq,ggtt~3% qq,ggnn~20% qqqq,qqgg~70% qq,ggll~7% mqq~mZ mnn~mZ mtt,ll~mZ Main backgrounds: Four fermion production Quasi model independent interpretation: CERN-EP-2000-92 x2 OPAL 170pb-1@183 and 189 GeV Mass lower limit on a Higgs boson decaying to any quark flavour or a gluon pair: 75 GeV/c2 (obs) 89.5 GeV/c2 (exp) mh Marumi Kado - CERN

  42. Flavour independent Flavour dependent • A complete 2HDM type II interpretation, the associated production has also been studied: • Interpretation independent of the parameter a: No mass constraints Ingredients: Associated production Flavor independent Standard searches Marumi Kado - CERN

  43. The MSSM • A 115 GeV/c2 Higgs boson invites SUSY(J. Ellis and D. Ross hep-ph/0012067) • The MSSM Higgs sector is a 2HDM type II, with only two free parameters at tree level (mA and tanb). • - mH< 130 GeV/c2 • - At higher orders: Large number of parameters  Most conservative benchmark maximal mh • Topologies searched for to cover the High tanb region •  h/A  bb/t+t- with mh~mA Complementary Production processes: Associated cos2(-) Higgs-strahlung sin2(-) Use standard searches bbt+t- ~13% bbbb~85% Marumi Kado - CERN

  44. tanb exclusion [0.52,2.25]/expected [0.48,2.48] • Exclusion regions (combined as of October 10th LEPC) • The robustness of the limits on the masse of the h and A Higgs bosons has been proven by large scans of all relevant parameters of the MSSM (Eur. Phys. J. C 17(2000)223) • The tanbexclusion range is very sensitive to the top mass It can be drastically reduced by assuming a large top mass mA> 90.5 (obs.) 94.1 (exp.) GeV/c2 mh> 89.9 (obs.) 93.8 (exp.) GeV/c2 Marumi Kado - CERN

  45. Beyond… • Invisible decays of Higgs bosons can occur in different models: • - MSSM with non universal gaugino masses • - Models with majorons • Produced by Higgs-strahlung process • Quasi model independent interpretation: Main backgrounds: Four fermions qq (invisible) ~70% ll (invisible) ~7% s=sMSM For Br(hinv.) = 100% mh> 113.7 (obs.) mh> 112.8 (exp.) LEP Combination LEPC 05/09/2000 Marumi Kado - CERN

  46. Hypotheses: One Higgs doublet and C and P conservation Where On are combinations of Higgs and Gauge fields • Anomalous couplings: Assuming new physics appear at a scale L, low energy effects can be parametrised with an effective Lagrangian: Anomalous couplings of the type hggandhZg, are allowed Topologies: Marumi Kado - CERN

  47. DELPHI search up to 183 GeV (PLB 458(1999)431) interpretation (All fn’s are equal to F) Marumi Kado - CERN

  48. Interpretation of ALEPH’s MSM and Fermiophobic Higgs boson searches up 202 GeV (Moriond QCD 2000) • Also L3 search @189 GeV(PLB 489(2000)102)with additional interpretations Marumi Kado - CERN

  49. Conclusions • Numerous theoretical scenarios were investigated at LEP and many nonstandard topologies, where the Higgs boson could have escaped otherwise, were covered • Hints of a 115 GeV/c2 standard-model-like Higgs boson were observed, as strong as could be expected at such high mass, close to the operational limit of LEP • This observation corresponds to roughly 3 standard deviations above the background expectation • These indications are not sufficient to claim a discovery • An additional year of LEP running in 2001 would have been essential to conclude • (Final results are scheduled for the forthcoming winter conferences) Marumi Kado - CERN

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