Siena 7 th Topical Seminar: The Higgs in two jet modes

1. Siena 7th Topical Seminar:The Higgs in two jet modes W. J. Murray RAL Standard Model results

2. Thanks to Steve Myers and friends LEP Accelerator • Very clean environment • Excellent performance! 2.5 fb-1 @ Ecm>189 GeV

3. Higgs Searches at LEP 1 • These were typically two-jet modes • Many modes: • Stable,gg,ee,mm,pp,tt,bb • Clean Z decays (ll, nn) used • Prior to LEP only some patchy constraints • The mass range to 0 now excluded, no holes. Events expected at LEP1 0.0  mH 65 GeV/c2 Excluded at 95% C.L.

4. Closing in on the Higgs! EW fits assume a Higgs Search looks for one Bayesian Frequentist After A. Wagner, ICHEP 2000

5. Total LEP Higgs Predicted LEP events, 4 experiments 17 @ 115GeV

6. LEP Higgs production: HZ ‘Higgstrahlung’ process Make a Higgs and a Z together So need Energy greater than Higgs mass plus Z mass

7. Higgs and Z decay channels Assuming: Both Z & H are made! Z decay modes Higgs decay modes WW nn B quarks Any quark Marumi described green x green – I get the odds and ends!

8. Fusion: WW and ZZ • These diagrams allow non-resonant Higgs production • WW always much larger than ZZ • Has been suggested as way to beat kinematic threshold • (Ebeam-Mz) WW e+ n Any quark H W n e-

9. Higgs  cross-section fb s=192GeV WW Any quark MH • Cross-section ~ few fb at 115: • Not accessible with ½ fb-1per experiment

10. LEP Higgs channels Higgs decays, 115GeV This talk Search channels

11. How is the Higgs search done? • Classify events into one category, pre-select • Assign measured particles to four initial fermions: quark, e/m, t, n • tag b quark jets, mvtx+leptons • Decide which fermions are from Z, which from H • Not obvious in t cases… • Require compatibility with Z mass for Z pair • Fit for MH, assuming E, p conserved, Mz at least for Hnn channel

12. Higgs backgrounds

13. Channel comparison at 115GeV

14. Channel sensitivity (3s evidence) 45% Probability of 3s assuming MH 2 jets and 4 jets comparable Need to combine all 20%

15. B-tagging – crucial step LOG SCALE • Tag of b’s reduces many backgrounds • Semi-leptonic W’s dangerous • Mis-tag is well controlled b c uds FACTOR >500 IN WW REJECTION REACHED

16. ZZ cross-checks • B-tag important for ZZ selection • Proof that this very similar channel can be measured • A major background is under control

17. EXP Energy Z decay M s/b w 1 ALEPH 206.7 quarks 114.3 4.6 1.73 2 ALEPH 206.7 quarks 112.9 2.4 1.21 3 ALEPH 206.5 quarks 110.0 0.9 0.64 4 L3 206.4 nn 115.0 0.7 0.53 5 OPAL 206.6 quarks 110.7 0.7 0.53 6 DELPH 206.7 quarks 114.3 0.6 0.49 7 ALEPH 205.0 ee 118.1 0.6 0.47 8 ALEPH 208.1 tt 115.4 0.5 0.41 9 ALEPH 206.5 quarks 114.0 0.5 0.40 10 OPAL 205.4 quarks 112.6 0.5 0.40 Significant high mass events

18. Top three candidates: • L3 Hnn candidate, s/b=0.7 • 114GeV Higgs mass • But very collinear – too much? • ALEPH Hee candidate, s/b=0.6 • 118GeV mass • Evidence for brehmstrahlung…Mass 99? • ALEPH Htt candidate, s/b=0.5 • 115GeV mass • Kinematic fit poor All well b-tagged All have problems not reflected in their s/b estimates

19. L3 Hnn candidate: Two clear b-quarks Lots of energy missing Could be HZ, with Z decaying to invisible neutrinos Mass is 114.5GeV Or maybe it is just a pair of quarks. Importance dropped in publication

20. L3 Hnn characteristics Mass and neural network output both at signal peak Used for discriminator

21. Collinearity of Hnn We cannot add criteria in the light of the data. Otherwise classical statistical analysis is impossible. Four fermion backgrounds acollinear Two fermion collinear Total well modelled L3 candidate

22. How is significance assessed? • Maximum likelihood fit to observed distribution • Done in 1D or 2D (e.g. Mass,Neural-Net) • Each bin needs signal and background estimates, from simulation, dependent upon Ecms, channel etc. `Qi’ • This is a weighted sum of events. • L is compared with distributions expected for background and signal to quantify probabilities

23. Log-likelihood for two jets • Positive – • background like • Negative – • signal like • Height gives power • Green – 1s • Yellow – 2s Most powerful

24. Log-likelihood for two jet channels Good agreement Tiny excess at 117

25. PDF’s for channels No single leptonic channel has much discrimination Results in each compatible with either hypothesis.

26. Combined leptonic PDF • Disfavours signal • Excludes nothing

27. Comparison with Hqq PDF Rather similar distributions (sensitivity) – results somewhat different

28. Combined two-jet limit MH>114.2GeV/c2 Exactly what would be expected

29. CLb for two jets combined Good agreement with background expectation

30. Hqq alone 1-CLb LEP 1-CLb Marumi showed you this allready Significance reduced to 2.1s

31. Conclusions from Two Jet search: • MH > 114.2GeV/c2, nothing unusual seen • Statistical analysis has its limitations • Just as powerful as four jet search • Results should only be used combined with four jets: MH>114.1GeV/c2

32. My own conclusions: Lancon will explain this • Precision electroweak measurements DEMAND a Higgs: • MW agrees with Higgs predictions to 1 per mille, • Mtop agreement to 10% mH = 88 GeV/c2 + 53 - 35 Just a statistical fluke? • Direct Searches (~2.1s effect) mH = 115.6 GeV/c2 + 0.8 In about 4 years, Tevatron+LHC should decide - 0.8 More Precision Measurements with Lepton Colliders should follow after 2012 We need to build TESLA/NLC

33. Acollinearity of Hnn Higgstrahlung Higgstrahlung rather collinear Total much less so WW fusion total L3 candidate