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LHC Physics Lesson #2 Higgs boson searches at LEP1 , LEP2 and LHC

IDPASC school. LHC Physics Lesson #2 Higgs boson searches at LEP1 , LEP2 and LHC. Global Electroweak Fit. From the experimental observables : line shape s (s) FB asymmetries A FB (s) t polarization P t (cos ) pseudo-osservables can be extrapolated:

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LHC Physics Lesson #2 Higgs boson searches at LEP1 , LEP2 and LHC

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  1. IDPASC school LHC Physics Lesson #2 Higgs boson searches at LEP1 , LEP2 and LHC

  2. Global Electroweak Fit From the experimental observables: line shape s(s) FB asymmetries AFB(s) t polarization Pt(cos) pseudo-osservables can be extrapolated: MZ GZs0h AlFB etc.. Using a fit program (ZFITTER) with 2 loop QEWD and 3 loop QED the best fit can be obtained for the parameters of the model and for the masses having some uncertainty (mt, ,mH ). 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

  3. 20 pseudo-osservables 5 fitted parameters With the fitted parameters we can obtain also the fitted pseudo-osservables

  4. Updated Status of the Global Electroweak Fit and Constraints on New Physics July 2011 arXiv:1107.0975v1 c2min /DOF = 16.6 / 14 • usage of latest experimental input: • Z-pole observables: LEP/SLD results • [ADLO+SLD, Phys. Rept. 427, 257 (2006)] • MW and GW: latest LEP+Tevatron averages (03/2010) • [arXiv:0908.1374][arXiv:1003.2826] • mtop: latest Tevatron average (07/2010) [arXiv:1007.3178] • mc and mb: world averages [PDG, J. Phys. G33,1 (2006)] • Dahad(5)(MZ2):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]

  5. MW mH=81+52-33 GeV (2002) mHiggs< 193 GeV 95% C.L. mH=91+58-37 GeV (2003) mHiggs< 211 GeV 95% C.L. AbFB , AcFB , Rb , Rc mH=96+31-24 GeV (2011) mHiggs< 171 GeV 95% C.L. mH=96+60-38 GeV (2004) mHiggs< 219 GeV 95% C.L.

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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 ( )

  11. 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 probability to observe 0 events from a Poisson distribution with mean value 3 is 5%. 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

  12. 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 • 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  MH ≥ 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

  13. 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%

  14. Signaficance 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:

  15. 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”.

  16. 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.

  17. 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

  18.  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

  19. Invariant mass distribution for the signal and the backgrounds (MC) After the selection dibosons are the main source of background mH=90 GeV mH=80 GeV OPAL HZ2jet 2, s=192 GeV, mH=80 GeV, L = 1000 pb-1. ALEPH HZ4jet, s=192 GeV mH=90 GeV, L = 500 pb-1

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

  21. 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

  22. 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)

  23. 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

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

  25. 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

  26. 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

  27. Higgs serches at LHC ECM = 7 TeV CMS L max = 3.54 1033 cm-2 sec-1

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

  29. Ezio Torassa Main interaction ISR e FSR Jets from high pt particles Fragmentation and hadronization Multi partonic interacions Beam Remnant protone protone

  30. Underlying Event and Minimum Bias protone protone 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

  31. Δ Ei = 0 • Minimum Bias:soft inelastic scattering • Observable fro the detector (Pt min ~100 MeV) • None (or few) tracks produced at significant Pt (~ 2 GeV) Elastic scattering (25%) Not diffractive inelastic (55%) Double diffractive inelastic (8%) Single diffractive inelastic (8%)

  32. From LEP to LHC LHC LHC: Higgs factory inside a little bit hostile environment LEP E.W. background QCD background 107 103 H H  1/hour  1/year

  33. Higgs boson production at LHC SM Higgs production cross section including NNLO/NLO QCD corrections mH (GeV)

  34. Higgs boson decays Higgs branching ratios For Higgs masses over 135 GeV the main decay channels areWW(*)and ZZ(*) under 135 GeV they are bb , t+t-and gg The coupling constant of the Higgs to the fermions and bosons are proportional to the mass of the particles: When mH is high enough to open a new decay channel this one becomes the dominant mH (GeV)

  35. This rule can be broken when the two mass are very close: BR(WW) > BR (ZZ) but mW < mZ 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 BR(hWW) / BR(hZZ) = g2hWW / g2hZZ = 4mW2 / mZ2 ~ 3

  36. The Higgs boson width The width changes from few MeVfor low masses to hundreds of GeVfor high masses due to his dependece on m3H (from H→VV coupling) mH (GeV)

  37. mH (GeV)

  38. Higgs search at LHC ATLAS 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 CMS Higss search status report CERN seminar December 13th, 2011

  39. HWW (*) 2l 2n Signal • The signal signature is: • - 2 high Pt leptons • missing Et • veto for high energy Jet • angular correlation between W-W Background Direct production of WW Wt DY

  40. Exclusion window: Expected: 129 < MH < 236 GeV Observed: 132 < MH < 238 GeV Data describes the predicted background well

  41. HZZ (*) 4l In the region mH < 140 GeV3 events are observed: two 2e2μ events (m=123.6 GeV, m=124.3 GeV) andone 4μ event(m=124.6 GeV)

  42. In the region mH < 160 GeV13 events are observed: The excess is distributed in a wider mass range w.r.t. ATLAS

  43. 181 < mH < 234 GeV 135 < mH < 156 GeV 255 < mH < 415 GeV 180 < mH < 305 GeV 134 < mH < 158 GeV 340 < mH < 460 GeV

  44. Hgg CMS PAPER HIG-11-033

  45. Higgs exclusion window November 2011 CMS PAS HIG-11-023, ATLAS-CONF-201-157 LEP (95%CL) mH> 114.4 GeV 114 - 141 Tevatron exclusion (95%CL): 100 < mH< 109 GeV 156 < mH< 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)

  46. HZZ  4μ candidate with m4μ= 124.6 GeV pT (μ-, μ+, μ+, μ-)= 61.2, 33.1, 17.8, 11.6 GeV m12= 89.7 GeV, m34= 24.6 GeV

  47. 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

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