1. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 LHC Physics Lesson #2 Higgs boson searches at LEP1, LEP2 and LHC IDPASC school

2. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 From the experimental observables: line shape  (s) FB asymmetries A FB (s)  polarization P  (cos  ) pseudo-osservables can be extrapolated: M Z  Z   h A l FB etc.. Using a fit program (ZFITTER) with 2 loop QE W D and 3 loop QED the best fit can be obtained for the parameters of the model and for the masses having some uncertainty (m t,,m H ). The current version of ZFITTER (in C++) is Gfitter. Global fits are performed in two versions: the standard fit uses all the available informations except results from direct Higgs searches, the complete fit includes everything Global Electroweak Fit

3. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 20 pseudo-osservables 5 fitted parameters With the fitted parameters we can obtain also the fitted pseudo-osservables

4. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 usage of latest experimental input: Z-pole observables: LEP/SLD results [ADLO+SLD, Phys. Rept. 427, 257 (2006)] M W and  W : latest LEP+Tevatron averages (03/2010) [arXiv:0908.1374][arXiv:1003.2826] m top : latest Tevatron average (07/2010) [arXiv:1007.3178] m c and m b : world averages [PDG, J. Phys. G33,1 (2006)]  had (5) (M Z 2 ): latest value (10/2010) [Davier et al., arXiv:1010.4180] direct Higgs searches at LEP and Tevatron (07/2010) [ADLO: Phys. Lett. B565, 61 (2003)], [CDF+D0: arXiv:1007.4587] Updated Status of the Global Electroweak Fit and Constraints on New Physics July 2011 arXiv:1107.0975v1arXiv:1107.0975v1  2 min /DOF = 16.6 / 14

5. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 m H =81 +52 -33 GeV (2002) m Higgs < 193 GeV 95% C.L. m H =91 +58 -37 GeV (2003) m Higgs < 211 GeV 95% C.L. m H =96 +60 -38 GeV (2004) m Higgs < 219 GeV 95% C.L. MWMW A b FB, A c FB, R b, R c m H =96 +31 -24 GeV (2011) m Higgs < 171 GeV 95% C.L.

6. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

7. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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”

8. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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:

9. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

10. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 (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. )

11. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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 probability to observe 0 events from a Poisson distribution with mean value 3 is 5%. 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

12. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Large number of events  Gauss distribution approximation Small number of events  Poisson distribution n = number of observed events m = mean number of events n=0  m  3 @ 95% CL n=2  m  6.3 @ 95% CL For the Higgs search m is related to the Higgs mass m  xx  M H ≥ yy Contributions to the mean value m: 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

13. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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%

14. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Signaficance 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

15. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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”

16. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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.

17. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

18. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

19. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 ALEPH HZ  4jet,  s=192 GeV m H =90 GeV, L = 500 pb -1 OPAL HZ  2jet 2,  s=192 GeV, m H =80 GeV, L = 1000 pb -1. Invariant mass distribution for the signal and the backgrounds (MC) After the selection dibosons are the main source of background m H =80 GeV m H =90 GeV

20. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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)

21. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

22. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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)

23. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

24. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 green: 1  from the backgroundyellow: 2  from the background background (higher  2 for b+s) signal+background (higher  2 for b)

25. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

26. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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

27. Second IDPASC school Ezio TorassaUdine, February 1 st 2012

28. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Higgs serches at LHC E CM = 7 TeV L max = 3.54 10 33 cm -2 sec -1 CMS

29. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Cosmic Rays LHC ~ 100 mb (AKENO, FLY’S EYE) SPS (SppS) (UA1, UA4 UA5) TEVATRON (CDF, E710, E811) ( ISR ) LHC 7 TeV Total cross section at LHC EPL Volume 96, Number 2, October 2011 First measurement of the total proton-proton cross-section at the LHC energy of √s =7TeV

30. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Padova 19 Aprile 2011 Ezio Torassa protone Main interaction ISR e FSR Jets from high pt particles Fragmentation and hadronization Multi partonic interacions Beam Remnant

31. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Underlying Event and Minimum Bias The Underlying Event is the residual part of the event excluding the high pt process: ISR, FSR, Multi partonic interactions, Beam remanent Together with the p-p interaction producing the high pt process, we can find additional p-p interactions in the same beam-crossing  PileUp protone

32. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Δ E i = 0 Elastic scattering (25%) Double diffractive inelastic (8%) Not diffractive inelastic (55%) Single diffractive inelastic (8%) Minimum Bias: soft inelastic scattering - Observable fro the detector (Pt min ~100 MeV) - None (or few) tracks produced at significant Pt (~ 2 GeV)

33. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 E.W. background LEP  10 3  10 7 QCD background H H  1/year LHC LHC: Higgs factory inside a little bit hostile environment  1/hour From LEP to LHC

34. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 SM Higgs production cross section including NNLO/NLO QCD corrections Higgs boson production at LHC m H (GeV)

35. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Higgs branching ratios Higgs boson decays For Higgs masses over 135 GeV the main decay channels are WW (*) and ZZ (*) under 135 GeV they are bb,  +  - and  The coupling constant of the Higgs to the fermions and bosons are proportional to the mass of the particles: When m H is high enough to open a new decay channel this one becomes the dominant m H (GeV)

36. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 BR(h  WW) / BR(h  ZZ) = g 2 hWW / g 2 hZZ = 4m W 2 / m Z 2 ~ 3 This rule can be broken when the two mass are very close: BR(WW) > BR (ZZ) but m W < m Z In the Lagrangian the ZZ has a factor two of penalty in comparison to WW because they are indistinguishable. This factor 2 it becomes a factor 4 in the BR, reduced to a factor 3 considering the different masses

37. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 The Higgs boson width The width changes from few MeV for low masses to hundreds of GeV for high masses due to his dependece on m 3 H (from H→VV coupling) m H (GeV)

38. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 m H (GeV)

39. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Higgs search at LHC Higss search status report CERN seminar December 13 th, 2011 In high mass region the discovery can be obtained using the WW and ZZ channels In the low mass region the contribution from several channels can be useful ATLAS CMS

40. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Direct production of WW Wt The signal signature is: - 2 high Pt leptons - missing Et - veto for high energy Jet - angular correlation between W-W DY H  WW (*)  2 l 2 Signal Background

41. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 Data describes the predicted background well Exclusion window: Expected: 129 < M H < 236 GeV Observed: 132 < M H < 238 GeV

42. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 In the region m H < 140 GeV 3 events are observed: two 2e2μ events (m=123.6 GeV, m=124.3 GeV) and one 4μ event (m=124.6 GeV) H  ZZ (*)  4 l

43. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 In the region m H < 160 GeV 13 events are observed: The excess is distributed in a wider mass range w.r.t. ATLAS

44. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 135 < m H < 156 GeV 181 < m H < 234 GeV 255 < m H < 415 GeV 134 < m H < 158 GeV 180 < m H < 305 GeV 340 < m H < 460 GeV

45. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 H  CMS PAPER HIG-11-033

46. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 46 November 2011 CMS PAS HIG-11-023, ATLAS-CONF-201-157 LEP (95%CL) m H > 114.4 GeV Tevatron exclusion (95%CL): 100 < m H < 109 GeV 156 < m H < 177 GeV ATLAS+CMS combination: based on data recorded until end August 2011 (~2.3 fb -1 / exp.) Excluded 95% CL : 141-476 GeV Excluded 99% CL : 146-443 GeV (except ~222, 238-248, ~295 GeV) Higgs exclusion window 114 - 141

47. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 47 H  ZZ  4 μ candidate with m 4 μ = 124.6 GeV p T (μ -, μ +, μ +, μ - )= 61.2, 33.1, 17.8, 11.6 GeV m 12 = 89.7 GeV, m 34 = 24.6 GeV

48. Second IDPASC school Ezio TorassaUdine, February 1 st 2012 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 Higgs searches at LHC: CMS PAS HIG-011-32 SM Higgs Combination