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

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

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

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

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! Nikos Konstantinidis Higgs Search at LEP

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 Nikos Konstantinidis Higgs Search at LEP

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 Nikos Konstantinidis Higgs Search at LEP

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 and Saipi = 0 (4c-fit slightly more sophisticated) In the end, because m12 and m34 are anti-correlated: MH = m12 + m34 -mZ Nikos Konstantinidis Higgs Search at LEP

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) Nikos Konstantinidis Higgs Search at LEP

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. Nikos Konstantinidis Higgs Search at LEP

LEP result [ LEPHWG ’03 ] Nikos Konstantinidis Higgs Search at LEP

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 ] Nikos Konstantinidis Higgs Search at LEP

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

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

Consistency checks Why only in ALEPH? Why only in 4-jets? 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  Nikos Konstantinidis Higgs Search at LEP

Estimators per experiment Nikos Konstantinidis Higgs Search at LEP

Estimators per channel Nikos Konstantinidis Higgs Search at LEP

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

Nikos Konstantinidis Higgs Search at LEP

Candidate 54698/4881 Properties (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! Nikos Konstantinidis Higgs Search at LEP

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! Nikos Konstantinidis Higgs Search at LEP

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) Nikos Konstantinidis Higgs Search at LEP

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

The future is bright… 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… Nikos Konstantinidis Higgs Search at LEP