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XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Lesson #3 Higgs boson searches at LEP1, LEP2 and LHC Standard Model.

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Presentation on theme: "XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Lesson #3 Higgs boson searches at LEP1, LEP2 and LHC Standard Model."— Presentation transcript:

1 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Lesson #3 Higgs boson searches at LEP1, LEP2 and LHC Standard Model

2 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

3 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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”

4 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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:

5 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

6 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 (1) Preselection: Acollinearity > 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 ( ) 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 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Results M H (GeV) Eventi (simulati HZ) 7.9     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 : detectors, all channels m(Higgs) > 65 GeV /c 2 at 95%CL DELPHI : 1 M hadronic events ~380 k events ee  LEP M hadronic events

8 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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  95% CL n=2  m  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

9 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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%

10 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

11 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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”

12 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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.

13 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

14 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

15 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

16 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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)

17 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

18 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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)

19 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

20 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 green: 1  from the backgroundyellow: 2  from the background background (higher  2 for b+s) signal+background (higher  2 for b)

21 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 m H > 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 > GeV/c 2

22 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 The “window” for M Higgs 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 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012

24 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Higgs serches at LHC E CM = 7 TeV L max = cm -2 sec -1 CMS

25 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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 ( 98.3 ± 0.2 stat ± 2.8 sys ) mb

26 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Padova 29 Giugno 2009 Ezio Torassa protone Interazione principale ISR e FSR Creazione dei Jet Frammentazione e Adronizzazione Interazioni Multi Partoniche Beam Remnant

27 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Underlying Event, Minimum Bias, Pile-Up 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 (~ protons/buch)  Pile-Up protone

28 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012  * 1.5m  1m Number of interactions / bunch crossing

29 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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)

30 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

31 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 SM Higgs production cross section including NNLO/NLO QCD corrections Higgs boson production at LHC m H (GeV)

32 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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)

33 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

34 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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)

35 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 m H (GeV)

36 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Higgs search at LHC 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 arXiv: v1 Feb 7, 2012 ATLAS-CONF March 7, 2012

37 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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

38 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Data describes the predicted background well

39 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 < 90° (1.55 rad) < 1.8 rad Prima del taglio m ll <50 GeV Dopo il taglio m ll < 45 GeV

40 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 ATLAS Exclusion window: Expected: 127 < M H < 234 GeV Observed: 130 < M H < 260 GeV ATLAS-CONF Mar 2012 CMS Exclusion window: Expected: 129 < M H < 236 GeV Observed: 132 < M H < 238 GeV CMS arXiv: v1 7 Feb 2012

41 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Anziché mostrare il CLs in funzione della massa, si è scelto di moltiplicare la sezione d’urto del segnale per un fattore opportuno (maggiore o minore di 1) in modo da ottenere sempre l’esclusione al 95% per tutte le masse. Ovviamente solo dove non serve una sezione d’urto superiore a  SM si ha una vera esclusione per l’Higgs SM.

42 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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 ATLAS-CONF arXiv: v3arXiv: v3 1 Mar 2012

43 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 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 CMS arXiv: v1 9 Feb 2012

44 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th < m H < 156 GeV 182 < m H < 233 GeV 256 < m H < 265 GeV 268 < m H < 415 GeV 134 < m H < 158 GeV 180 < m H < 305 GeV 340 < m H < 465 GeV

45 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 H  CMS arXiv: v1 7 Feb 2012ATLAS arXiv: v1ATLAS arXiv: v1 7 Feb 2012

46 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012

47 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 ATLAS-CONF Mar 2012 Excluded from 129 GeV to 539 GeV SM Higgs combination CMS arXiv: v1 7 Feb 2012 Excluded from 127 GeV to 600 GeV GeV to GeV GeV to GeV

48 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 arXiv: v1

49 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th November 2011 CMS PAS HIG , ATLAS-CONF LEP (95%CL) m H > 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 : GeV Excluded 99% CL : GeV (except ~222, , ~295 GeV) Higgs exclusion window ~ 130

50 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th H  ZZ  4 μ candidate with m 4 μ = 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

51 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Look elsewhere effect arXiv: v3 The statistical significance that is associated to the observation of new phenomena is usually expressed using a p-value, that is, the probability that a similar or more extreme effect would be seen when the signal does not exist. p-value = p0CLs = (1 - p1) / (1 - p0) Looking everywhere (elsewhere) i.e. the invariant mass in a wide mass range, the probability to observe somewhere a background fluctuation is boosted. The effect can be quantified in terms of a trial factor, which is the ratio between the probability of observing the excess at some fixed mass point, to the probability of observing it anywhere in the range. p-value (ATLAS Hgg) = 2.8  (1.5  L.E.E.)

52 XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Higgs searches at LEP I : Z Physics at LEP I CERN 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/ Higgs searches at LHC: CMS PAS HIG SM Higgs Combination


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