Higgs Search at LEP

1. Higgs Search at LEP A lecture on: Physics, Statistics, History & Sociology Nikos Konstantinidis (UCL)

2. Overview • The physics • Production and decay of the SM Higgs at LEP • Event topologies and characteristics • Statistics • Results • Lessons for the future • Outlook Higgs Search at LEP

3. SM Higgs production – decay @ LEP e+ H Z* e_ Z Production Decays • Cross section ~0.1pb • Looking for near-threshold • production • ( threshold = Ecm - MZ ) Higgs Search at LEP

4. HZ - Topologies (1) BR50% (2) BR15% (3) BR5% (4) BR8% Higgs Search at LEP

5. bbll: the “golden” channel • Low rate, but excellent Higgs mass resolution (~3.5GeV) and almost no tails • MH calculated as the recoil to the dilepton system • Dominant bkg: ZZ • Small but nasty tails due to Final State Radiation (FSR) from the leptons • Dilepton mass lower, recoil mass higher… • Also: ZZ with Initial State Radiation (ISR) • Dilepton mass OK, but recoil mass high! Higgs Search at LEP

6. bbnn • Second most important channel, but half of the event is missing, so Higgs mass resolution not good (~5GeV) and with tails • Main bkg • ZZ -> bbnn • ee -> ggbb : Difermion production with double Initial State Radiation Higgs Search at LEP

7. 4jets • More sensitive than all the other channels put together • Good MH resolution (~3.5GeV), but tails from • mismeasurements • overlapping jets • wrong jet pairings • Main bkg: • ZZ → bbqq (tails also from the Z width) • ee → bb → bbgg (double gluon radiation) • WW → cscs Higgs Search at LEP

8. Reconstructed Higgs mass in 4jets LEP : excellent energy resolution (~20MeV) LEP detectors: good jet angular resolution (~2 degrees) fix jet directions and rescale their energies/momenta to satisfy energy-momentum conservation (Ei,pi) → ai(Ei,pi) SaiEi = Ecm andSaipi = 0 (4c-fit slightly more sophisticated) In the end, because m12 and m34 are anti-correlated: MH = m12 + m34 -mZ Higgs Search at LEP

9. Putting it all together: confidence levels • Many channels and a lot of info to combine: • mass resolution of individual channels • BR’s and purity of individual channels • different centre-of-mass energies • other discriminating variables (e.g. b-tagging) Combine all info to produce: the confidence level for the signal+bkg hypothesis (CLs+b) the confidence level for the bkg only hypothesis (CLb) Higgs Search at LEP

10. How to construct CLs+b and CLb • Assume a signal (e.g. SM Higgs: m=115GeV) • Put all the properties of signal/bkg in one estimator e (e.g. LR: Q=L(s+b)/L(b)) • Use a lot of bkg-only/sig.+bkg toy expts, and find the distribution of e • Find edata and Separation between the two dist’ns shows the sensitivity of the expt. Higgs Search at LEP

11. LEP result [ LEPHWG ’03 ] Higgs Search at LEP

12. 1-CLb: bkg compatibility ALEPH excess near 116 GeV due to 2-3 “golden” 4jet events “Nothing” observed by DLO • At mH=116GeV: • ALEPH: 3.0s (<5s) • LEP: 1.7s (<5s) [ LEPHWG ’03 ] Higgs Search at LEP

13. Other distributions [ LEPHWG ’03 ] Weights of candidates Overall, numbers consistent with Higgs production at ~115-116GeV Higgs Search at LEP

14. “The results of the experiment were inconclusive so we had to use statistics…” Higgs Search at LEP

15. Consistency checks • Why only in ALEPH? • When we are talking about ~2 events anything is possible • ALEPH had the best single-experiment sensitivity • Why only in 4-jets? • Based on BRs, efficiencies etc you should expect • 3 four-jet events before you see 1 bbnn event and • 6 four-jet events before you see 1 bbll event Better shown with stat. estimators  Higgs Search at LEP

16. Estimators per experiment Higgs Search at LEP

17. Estimators per channel Higgs Search at LEP

18. Other systematic checks on… • behaviour as a function of CM energy • b-tagging • kinematical fits • etc… All showed no inconsistencies/discrepancies Higgs Search at LEP

19. Higgs Search at LEP

20. Candidate 54698/4881 (recorded on 14/06/2000) • Properties • Two clear b-jets (dec. length, inv. mass of tracks in vertex) • Event well-measured: Pmis in direction of jet with m from vertex • Planar event, as in threshold production of two heavy particles • b-jets: 55GeV & 59GeV, like in a decay almost at rest • Non-b jets: 43GeV & 49GeV, like in a Z decay almost at rest • Raw invariant mass of non-b jets 92.3GeV • non-b jets: leading parton effect, low multiplicity (q vs. gluon) • Impossible to be a WW, very unlikely to be bbgg, very unlikely a ZZ*→qqbb (and if it is we were very unlucky!) • One candidate is not a discovery, but if mH~116GeV, this event was the first Higgs ever observed! Higgs Search at LEP

21. Lessons for the future • Event selections should be not only sensitive but also robust • Some “golden” candidates disappeared after “data reprocessing”  bad for LEP’s credibility. • Define analyses a priori; define systematic checks and consistency checks a priori • Hard to have a statistically meaningful result otherwise. • Improve analyses by inspection of candidates • But improvements to be used in new data only! Higgs Search at LEP

22. Search window vs. mass resolution • What is the probability to see such a peak anywhere in the mass range that I am looking at? MH=200GeV Signal Bkg Events / 0.5 GeV CMS 10 fb-1 H  4l (l=e,) m (4l) Higgs Search at LEP

23. LHC: Channels for mH~115GeV  H  total S/ B gg H   ttH  tt bb  bl bjj bb VBF: qqH  qq b b Different production/decay modes Different backgrounds  complementary! *(K-factors  (NLO)/(LO)  2 not included) (10fb-1) Higgs Search at LEP

24. Outlook • The journey goes on • It has produced great excitement & there is more to come! • We should know the final answer by the end of this decade! The future is bright… Higgs Search at LEP