1. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Lesson #3 Higgs boson searches at LEP1, LEP2 Standard Model

2. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Higgs searches at LEP Z Z* H H Z E CM =206 GeV 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. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Higgs-strahlungWW fusion Dominant mode m(H)   s-m(Z) + interference M H (GeV/c 2 ) E CM =206 GeV The dominating Higgs production mechanism at LEP1 and LEP2 is the “Higgs-strahlung”

4. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Higgs decay channels For m H  120 GeV, the most important decay chanel is H  bb “b-tagging” is relevant ! 4 jets 2 jets & missing energy 19% 60% Or a   instead of the b 2 jet & 2 lepton 6% H  bb 85% H  8% Reaserch topology:

5. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Padova 12 Aprile 2011 Ezio Torassa Neutrino decay channel 2 jets & missing energy 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  M Z jets not in collinear directions b-tagging Leptons transverse momentum b c uds Tracks impact parameters uds c b Higgs searches at LEP1

6. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 (1) Preselection: Acollinearity > 8 0 20 GeV < M invariant < 70 GeV Z  qq Z H (55GeV)  X Eff. ( Z H  X) = 81.2% Eff. (Z  qq) = 1.5 % (2) Neural network: Neural network with 15 input variables. The output is a single quality variables: Q takes values between 0 and 1 Data analysis example (1991-1992) Q ( ) Z H  X Z  qq Eff. ( Z H  X) = 65.8% Eff. (Z  qq) = 0.23 % Q > 0.95 ( to be multiplied with the previous Eff. )

7. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Results M H (GeV)50556065 Eventi (simulati HZ) 7.9  0.43.6  0.21.4  0.10.41  0.05 # expected signal events # observed events: 0 # expected background events : 0 Sum of the tree decay channels: Z  Z  ee Z  For M H = 55.7 GeV we have 3 expected signal events events. The expected number of event is a mean number ( =3) with a Poisson distribution: The probability to observe 0 events is 5%. =3

8. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 For M H larger than 55.7 GeV the probability to observe zero events il smaller than 5%. Your confidence level is 95%. Higgs mass limit: M H > 55.7 GeV al 95 % di C.L. LEP1 : 1989-1995 4 detectors, all channels m(Higgs) > 65 GeV /c 2 at 95%CL DELPHI 1991-1992: 1 M hadronic events ~380 k events ee  LEP1 1989-1995 17 M hadronic events

9. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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 : 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 Exclusion and discovery

10. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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 L exclusion 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. L discovery 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. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Significance When the background b can be precisely estimated The inclusion of the background error  b 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: With high statistics, for few units of significance, the denominator is only √b

12. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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”. The “blind analysis”

13. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Higgs searches at LEP II MHMH E CM =206 GeV 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. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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 4 jets 2 jets & missing energy 19% 60% Or a   instead of the b 2 jet & 2 lepton 6% H  bb 85% H  8% Research topology: LEP I LEP II

15. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 e+e+ f’ e-e- f ZZ   W +, Z,  e+e+,e e-e- W -, Z,  e+e+ H e-e- Z ZZ e+e+ - e-e- W+W+ W-W- H  In addition to Z  ff we have also the WW, ZZ and  production and decays. e+e+ e-e- e+e+  e-e-  q q e + e - → e + e - qq

16. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 m H =100 GeV Invariant mass distribution for MC and real data. m H =115 GeV Final LEP selections for 115 GeV search (Loose and Tight)

17. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 Statistic approach for the global combination We need to combine the results from different channels (Hqq, H, Hll) and different energies E cm. They are grouped in the same two-dimensional space (m H rec, G) m H rec reconstruced invariant mass G discrimanant variable (Q NN, b-tag) For every k channel we obtain: - b k estimanted background - s k estimated signal (related to m H ) - n k number of Higgs candidate from the real data We build the Likelihood for two hypothesis: - candidates coming from signal + background L s+b - candidates coming from background L b m H rec G

18. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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 m H : The Likelihood is the product of the probability density (k channel density)

19. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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  2 difference : We look to the function -2ln(Q(m H )) (i)For the real data (ii)For the MC with n=b (iii) For the MC with n=b+s

20. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 green: 1  from the backgroundyellow: 2  from the background background (higher  2 for b+s) signal+background (higher  2 for b)

21. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 m H > 114.4 GeV/c 2 at 95% CL s Finally we can estimate the exclusion at 95% of confidence level (CL s = CL s+b / CL b ) 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 m H ) within 2  from the background line LEP I m H > 65 GeV/c 2 LEP II m H > 114.4 GeV/c 2

22. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 The “window” for M Higgs 114.4 GeV 171 GeV This exclusion window is at 95% of C.L., masses outside this window are not forbidden, they have a smaller probability

23. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014

24. XXIX Ph.D in Physics Ezio TorassaPadova, May 9th 2014 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