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SM Higgs Search at LEP in Channels other than 4 Jets Enrico Piotto EP Division, CERN

SM Higgs Search at LEP in Channels other than 4 Jets Enrico Piotto EP Division, CERN. e + e - --> H Z--> bb  , e + e - --> W - W +  e  e --> H v e v e --> bb  e  e e + e - --> H Z--> bb e + e - e + e - --> H Z--> bb µ + µ - e + e - --> H Z--> qq  +  -.

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SM Higgs Search at LEP in Channels other than 4 Jets Enrico Piotto EP Division, CERN

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  1. SM Higgs Search at LEP in Channels other than 4 Jets Enrico Piotto EP Division, CERN • e+e- -->HZ-->bb, e+e- --> W- W+ ee-->Hveve -->bb ee • e+e- -->HZ-->bb e+e- • e+e- -->HZ-->bb µ+µ- • e+e- -->HZ-->qq +- Enrico Piotto EP division CERN

  2. SM Higgs branching ratios • For mh =115 Gev/c2: • BR(Hbb)  78 % • BR(H)  7.5 % • BR(Hgg)  7% • BR(HWW)  5 % Enrico Piotto EP division CERN

  3. Topologies @ LEP 2 Jets & missing energy : 16% 2 jets & 2 leptons: 5% 4 jets : 56% qq : 8% Search potential: the probability to confirm a signal if it is true. z qq,  h,bb Enrico Piotto EP division CERN

  4. Backgrounds • For mh=115 Gev/c2, assuming 100% • efficiency, 3.3 events are expected • in a LEP experiment from 2000 data taking • and using the final states other than 4 jets. • Several hundred thousands of background • events are expected. • The strategy: • sequential cuts to remove the bulk of the • background; • likelihood ratio or neural network to further • discriminate the signal from the • background. Enrico Piotto EP division CERN

  5. B-tag: the fundamental tool B hadrons produced at LEP will fly for few mm and their decays products will have large pT wrt jet and large impact parameters with respect to light quark hadrons. Combining different variables with Neural Network or Likelihood a probability that a jet comes from a b quark is derived. OPAL Z-->bb Z-->cc Z-->uds Enrico Piotto EP division CERN

  6. Missing energy channel v b b v Main background near threshold: 2 ISR escape undetected at low angle: b quark pair 2 photons ISR the qq events will have 2 nearly coplanar jets with low pT. DELPHI qqn() Higgs m=115 4 fermions Enrico Piotto EP division CERN

  7. H kinamatic • Double symmetric radiative events • are nearly collinear. • Higgs events may be acollinear even • at the kinematic limit. • The Higgs boost may come from: • widht of Z0 • WW fusion Higgs mh=115 Enrico Piotto EP division CERN

  8. Mass reconstruction In the missing energy channel the energy flow and the jet reconstruction are critical aspects of the analysis. A mass resolution of 3 GeV/c2 can be achieved assuming a Z boson recoiling mass. But: for an event at rest, the Z recoil constraint artificially peaks the visible mass at s-mz DELPHI Enrico Piotto EP division CERN

  9. Most significant H candidates Selecting LEP candidates with (s/b)115 >0.3: Observed: 14 Expected (s+b): 14 Signal: 7 3rd most significant LEP candidate 10th most significant LEP candidate 14th most significant LEP candidate Enrico Piotto EP division CERN

  10. L3 H candidate @ cme 206.6 GeV Mh= 114.4 GeV/c2 missing mass = 94 GeV/c2 M=5.5 GeV 7.3 mm from PV 1.4 mm from PV Two b-tagged jets Enrico Piotto EP division CERN

  11. Leptonic channel e-,m , b e-,m, b • Low background channel. • Neural Network are used to identified. • Good mass resolution also without constrained fit. • Danger: ISR photons, or photons from electron radiation, • can artificially increase the di-jet mass if associated with one jet. OPAL Enrico Piotto EP division CERN

  12. Example If the  is associated to the muon -> a perfect ZZ m If the  is included in the jet: a very high di-jet mass -> good high mass Higgs candidate ! m g Enrico Piotto EP division CERN

  13. Leptonic channel e-,m, b e-,m, b Hee candidate: it is the 5th most significant LEP candidates (ALEPH). Mh=118GeV/c2, Mrec=78.8GeV/c2 b-tag=1.4 S/b H candidate: it is the 8th most significant LEP candidates (ALEPH). Mh=115GeV/c2 Enrico Piotto EP division CERN

  14. ALEPH Hee candidate @ cme 205 GeV Enrico Piotto EP division CERN

  15. ALEPH H candidate @ cme 208 GeV Enrico Piotto EP division CERN

  16. S/b of candidates Hqq: 9 Hnn: 3 Hl+l-: 1 Htt: 1 (70%) (20%) (7%) (3%) Selecting candidates with (s/b)115>0.3: observed: 14 expected (s+b):14 Htt He+e- Theoretical branchings Hnn Hnn Enrico Piotto EP division CERN

  17. 1-CLb Enrico Piotto EP division CERN

  18. Conclusions • bbvv, bb e+e- ,bb µ+µ-, qq +- together have the same potential discovery as the four jets channel; • a coherent view compatible with the expected branching ratios emerges from LEP leptonic and hadronic candidates; BUT • letponic candidates are consistent with both the background and • signal+background hypotheses. • leptonic channels are affected by irreducible background; • all the data presented here are based on the papers published soon after the end of data-taking; • all LEP collaborations are reviewing their analysis taking into account final calibrations and data processing and more MC; • the final LEP word on higgs will be hopefully for summer 2001. Enrico Piotto EP division CERN

  19. Backup Enrico Piotto EP division CERN

  20. Backup: qq cross section Enrico Piotto EP division CERN

  21. Backup Enrico Piotto EP division CERN

  22. Backup Higgsstrahlung WW-fusion Total Mh=115 Gev Ecm=206Gev Enrico Piotto EP division CERN

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