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Lesson #3 Higgs boson searches at LEP1, LEP2

Standard Model. Lesson #3 Higgs boson searches at LEP1, LEP2. Z*. H. Z. Z. Z*. H. E CM =206 GeV. Higgs searches at LEP. The coupling of the Higgs field to the vectorial bosons and fermions it’s fully defined in the Standard Model

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Lesson #3 Higgs boson searches at LEP1, LEP2

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  1. Standard Model Lesson #3 Higgs boson searches at LEP1, LEP2

  2. Z* H Z Z Z* H ECM=206 GeV Higgs searches at LEP The coupling of the Higgs field to the vectorial bosons and fermions it’s fully defined in the Standard Model The cross section of the Higgs production and the decay modes as a function of it’s mass are predicted by the theory

  3. MH(GeV/c2) ECM=206 GeV The dominating Higgs production mechanism at LEP1 and LEP2 is the “Higgs-strahlung” Higgs-strahlung WW fusion + interference Dominant mode m(H)  s-m(Z)

  4. Higgs decay channels For mH 120 GeV, the most important decay chanel is H bb “b-tagging” is relevant ! Reaserch topology: Hbb 85% 4 jets 2 jets & missing energy 19% 60% Htt 8% 2 jet & 2 lepton 6% Or a tinstead of the b

  5. Ezio Torassa Higgs searches at LEP1 Neutrino decay channel • The signature is one unbalanced hadronic event. • The background is due to Z decay into b quarks • Background reduction: • invariant mass of the two jets  MZ • jets not in collinear directions • b-tagging 2 jets & missing energy b c uds c uds b Leptons transverse momentum Tracks impact parameters

  6. Data analysis example (1991-1992) Zqq Z H (55GeV)X (1) Preselection: Acollinearity > 8 0 20 GeV < Minvariant < 70 GeV Eff. ( Z HX) = 81.2% Eff. (Zqq) = 1.5 % Z HX (2) Neural network: Zqq Neural network with 15 input variables. The output is a single quality variables: Q takes values between 0 and 1 Q > 0.95 Eff. ( Z HX) = 65.8% Eff. (Zqq) = 0.23 % ( to be multiplied with the previous Eff. ) Q ( )

  7. Results Sum of the tree decay channels:Z Zee Z # observed events: 0 # expected background events : 0 # expected signal events For MH = 55.7 GeV we have 3 expected signal events events. The expected number of event is a mean number (l=3) with a Poisson distribution: The probability to observe 0 events is 5%. l=3

  8. For MH larger than 55.7 GeV the probability to observe zero events il smaller than 5%. Your confidence level is 95%. DELPHI 1991-1992: 1 M hadronic events ~380 k events nn ee mm Higgs mass limit: MH > 55.7 GeV al 95 % di C.L. LEP1 : 1989-1995 4 detectors , all channels m(Higgs) > 65 GeV /c2 at 95%CL LEP1 1989-1995 17 M hadronic events

  9. Exclusion and discovery • Large number of events  Gauss distribution approximation • Small number of events  Poisson distribution • n = number of observed events • m = mean number of events • Contributions to the mean value l: background (b) and signal (s) : • n is the measurement; • Exclusion (at least at 95% CL): the probability to observe n events  5% • Discovery (5  significance): signal 5 times larger than the error

  10. EXCLUSION The observed small number of events could be due to a statistical fluctuation with prob.  5×10-2 DISCOVERY The observed large number of events could be due to a statistical fluctuation with prob.  5.7×10-5 • Lexclusion • Increasing the Integrated luminosity the background uncertainty decreases. When the difference between background and background+signal is 2 the Luminosity for the exclusion is reached. • Ldiscovery • Similar definition for the discovery • Really observe n events and expect to observe n events at a given luminosity is not the same. • At the exclusion (or discovery) Luminosity • the probability to reach the goal is 50%

  11. Significance When the background b can be precisely estimated With high statistics, for few units of significance, the denominator is only √b The inclusion of the background error Db with a Gaussian distribution needs a specific calculation, with the Gaussian approximation for the number of events n the significance can be expressed with the following relation:

  12. The “blind analysis” • With a large number of observed events (n>>n), the statistical fluctuations do not have a big impact in the final result; for small numbers is the opposite: • small changes in the selection can produce big differences (i.e. 0 evts  2 evts) • None is “neutral” , good arguments can be found to modify a little bit the cuts to obtain a sensible change of the final result; • The selection criteria must be defined a priori with the MC to optimize the signal significance, only at the end we can open the box and look the impact on the real data. This method is called “blind analysis”.

  13. ECM=206 GeV MH Higgs searches at LEP II The “Higgs-strahlung” is dominant production also at LEP II. At higher s - the diboson fusion increas the relative relevance; - higher Higgs masses can be produced.

  14. Higgs decay channels at LEP II The most relevant decay channel is H bb like at LEP I Over 115 GeV (LHC region) other decay channels (WW e ZZ) becames relevant or dominant Research topology: Hbb 85% 4 jets 2 jets & missing energy LEP I LEP II 19% 60% Htt 8% 2 jet & 2 lepton 6% Or a tinstead of the b

  15.  e+ H Z e- Z e+ - e+ W+,Z,  W+ H ,e W- e-  W-, Z,  e-  f’ e+ Z e+ e+ q e-   f  q e- e- In addition to Zff we have also the WW , ZZ and g-g production and decays. e+e- →e+e-qq

  16. mH=100 GeV mH=115 GeV Invariant mass distribution for MC and real data. Final LEP selections for 115 GeV search (Loose and Tight)

  17. Statistic approach for the global combination • We need to combine the results from different channels (Hqq, Hnn, Hll) and different energies Ecm. They are grouped in the same two-dimensional space (mHrec , G) • mHrec reconstruced invariant mass • G discrimanant variable (QNN, b-tag) • For every k channel we obtain: • bk estimanted background • sk estimated signal (related to mH) • nk number of Higgs candidate from the real data We build the Likelihood for two hypothesis: • - candidates coming from signal + background Ls+b • - candidates coming from background Lb G mHrec

  18. We want to discriminate the number of observed events (n) w.r.t. the mean number of expected signal plus background (b+s) or only background (b) The following is the probability for b+s , s is a function related to mH : The Likelihood is the product of the probability density (k channel density)

  19. The comparison between the two hypothesis is provided by the Likelihood ratio. We choose to describe the results with the log of the ratio because it provides the c2 difference : • We look to the function -2ln(Q(mH)) • For the real data • For the MC with n=b • For the MC with n=b+s

  20. green: 1 s from the background yellow: 2 s from the background background (higher c2 for b+s) signal+background (higher c2 for b)

  21. Over 114 GeV/c2 the real data line (red) is closer the the s+b line (brown) anyway the real data line is always (every mH ) within 2s from the background line Finally we can estimate the exclusion at 95% of confidence level (CLs = CLs+b / CLb) mH > 114.4 GeV/c2 at 95% CLs LEP I mH > 65 GeV/c2 LEP II mH > 114.4 GeV/c2

  22. The “window” for MHiggs 171 GeV 114.4 GeV This exclusion window is at 95% of C.L. , masses outside this window are not forbidden, they have a smaller probability

  23. Higgs searches at LEP I : Z Physics at LEP I CERN 89-08 Vol 2 – Higgs search (pag. 58) Search for the standard model Higgs boson in Z decays – Nucl Physics B 421 (1994) 3-37 Higgs searches at LEP II : Search for the Standard Model Higgs Boson at LEP – CERN-EP/2003- 011

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