XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Lesson #4 Higgs boson searches at LHC Standard Model

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Higgs serches at LHC E CM = 7,8 TeV L max = cm -2 sec -1 (Hz / nb ) CMS

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Integrated Luminosity TeV Delivered 23.3 fb -1 Recorded 21.8 fb -1 Integrated Luminosity TeV Delivered 6.13 fb -1 Recorded 5.55 fb -1

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Cosmic Rays LHC 8 ~ 100 mb (AKENO, FLY’S EYE) SPS (SppS) (UA1, UA4 UA5) TEVATRON (CDF, E710, E811) ( ISR ) LHC 8 TeV Total cross section at LHC CERN-PH-EP December 11, 2012 ( ± 2.9 ) mb in agreement with the extrapolation from lower energies

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Padova 29 Giugno 2009 Ezio Torassa protone Interazione principale ISR e FSR Creazione dei Jet Frammentazione e Adronizzazione Interazioni Multi Partoniche Beam Remnant

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The pileup is the multiple interaction in the same bunch crossing. Increasing the luminosity (i.e. with more protons / beam) will produce also higher pileup environment for the triggered events Average PU 2011 RUNA 5 Average PU 2011 RUNB 8 Average PU 2012 all 21

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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)

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC Received 31 July 2012 Accepted 11 August fb -1 7 TeV 5.3 fb -1 8 TeV M = ± 0.4 (stat) ± 0.5 (sys) GeV

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013  H (m H = 126 GeV) ~ pb gg fusion 1.57 pb VBF 1.06 pb ZH,WH 0.13 pb ttH (10 5 pb for W) Higgs boson production at LHC SM Higgs production cross section NNLO/NLO QCD corrections

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 H →  H→WW, H→ZZ In the low mass region all the channels can help to increase the significance in addition we are interested to check all the theoretical branching ratios The Higgs does not couple with gluons and photons because they are massless particles anyway the gg fusion is the dominant contribution in the production and the H→  is a fundamental decay channel for the discovery and mass measurement H →  b b _ H →bb Higgs boson decays

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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 The coupling constant of the Higgs to the fermions and bosons are proportional to the mass of the particles:

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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) tt turn-on WW/ZZ turn-on bb

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 LEP results were described with the CLs plots. The mass limit is the mass corresponding to CLs=5%. (exclusion at 95%). At LHC we decided to multiply the signal cross section with a factor (>1 or <1) needed to exclude the signal at 95%. The real exclusion is the mass range were This factor is equal or lower than 1 (where you do need a cross section larger than  SM to obtain the exclusion.

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Higgs search at LHC

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 H  ZZ (*)  4 l 4 JulyHCP (Nov. 2012)Moriond (March 2013) N 1 = 9 B 1 =4 N 2 =21 B 2 =20 Significance 3.2  N 2 = 47 B 2 =37 Significance 4.5  R 1 : ÷ GeVR 2 : 110 ÷ 160 GeV N 2 = 71 B 2 =46 Significance 6.7 

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Dominant ZZ background FSR Z   background Signal

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Considering m H < 2 m Z one of the two Z is off shell All the 4l combinations (4e, 4 , 2e2  ) show the excess at M=126 GeV

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 N TOT = = 71

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The SM Higgs can be excluded everywhere with CL > 95% except in the region where we see the mass peak and for m H > 800 GeV Exclusion plot Discovery plot 6.7  (right) corresponds to a probability to be a statistical fluctuation of (left)

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Higgs mass best fit

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 signal strength  = best fit  /  SM

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 H  Large background contribution: -irriducible  from QCD - mis identification  -jet and di-jet Small branching ration: BR H  WW ~ BR H  ZZ ~ BR H→  ~ but no additional BR (ZZ  4 ,4e,2e2  ) Signal Background

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Several photon quality selection (class ) and different tags dominated by different production diagrams.

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The diphoton invariant mass distribution with each event category weighted by its S/(S+B). The lines represent the fitted background and signal 4 JulyMoriond (March 2013) Significance 4.1  (expected 2.8  Significance 3.2  (expected 4.2 

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Exclusion plot Discovery plot

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 m H = ± 0.6 ± 0.5 GeV m H = ± 0.5 ± 0.2 GeV ZZ  m H = ± 0.2 ± 0.7 GeVm H = ± 0.5 ± 0.6 GeV

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013  (at m H =125.4 GeV) = 0.78 ± 0.28  (at m H =126.8 GeV) = 1.6 ± 0.4  ZZ  (at m H =125.8 GeV) = 0.91 ± 0.30  (at m H =124.3 GeV) = 1.7 ± 0.5

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Three categories are considered: WW + 0 jets, WW + 1 jet, WW + 2 jet Distributions of the azimuthal angle difference between two selected leptons in the 0-jet category for data, for the main backgrounds, and for a SM Higgs boson signal with m H = 125 GeV.

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Distributions of the transverse mass in the 0- jet category compared with the backgrounds and the signal m H = 125 GeV expected The cut-based H → WW selection, except for the requirement on the transverse mass itself, is applied. The transverse mass is the invariant mass between W + W - with the missing energy used to estimate the neutrino momentum and all the z components set to zero.

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Exclusion plot Discovery plot 125 GeV 4.0  (expected 5.1 

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 ATLAS largest m H =140 GeV CMS largest m H =135 GeV Is interesting to compare the mass having largest significance with the mass fitted for the H→ZZ and H→  channels  (at m H =125 GeV) = 0.76 ± 0.21  (at m H =125 GeV) = 1.0 ± 0.3

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The fact that the reconstruction of the  pair decay kinematics is underconstrained by the measured observables is addressed by a maximum-likelihood fit method. The mass m  is reconstructed by combining the measured observables E x miss and E y miss with a likelihood model H 

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Exclusion plotDiscovery plot 120 GeV 3.0  (expected 2.6 

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 best-fit value of the signal strength  =1.1±0.4

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The signal strength is a measurement of the coupling but the relation is not trivial due to the different production channels. In the following table k i are the coupling scale factors. Only for VBF/VH production and H→VV decay the signal strength factor is simply k 2 VV Combined results

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Remeber coupling with fermions (and bosons) are proportional to the messes Summary of the fits for deviations in the coupling for the generic five- parameter model not effective loop couplings, expressed as function of the particle mass. (the minimum of the Higgs potential) is a common constant

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013  (at m H =125.7 GeV) = 0.80 ± 0.14 CMS m H =125.7 ± 0.3 ± 0.3 GeV ATLAS m H =125.5 ± 0.2 ± 0.5 GeV  (at m H =125.5 GeV) = 1.3 ± 0.2

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 It is crucial to determine the spin and quantum numbers of the new boson We can use the H  ZZ decay channel to distinguish between the scalar (SM Higgs) J P =0 + and the pseudoscalar J P =0 - hypothesis. We start to remember the M.E. likelhood analysis used in the mass estimation Considering the following angles:   : angle between Z direction (z’) and z axis:     : angles between the leptons and the Z  : angle between the two leptons pair planes   angle between z’z plane and two lept. plane We can build a kinematic discriminant between signal and background (MELA) Matrix Element Likelihood Analysis Higgs J P measurement

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The mass of the boson is measured with a maximum-likelihood fit to 3D distributions combining for each event: - m4l, -  m4l (from the individual lepton momentum errors) - K D BKGSGN Expected density of points (max set to 1) for background and signal as a function of m 4l and K D

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The fit is performed using the D JP (parity) and the the K D (sgn-bkg) discriminants. The discriminating power of D 0 - becomes more clear looking to the right plot where the condition D BKG > 0.5 has been applied A similar kinematic discriminant (pseudo-MELA) ca be used to distinguish scalar J P =0 + from pseudoscaral J P =0 -

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 The fit is performed using the D JP (parity) and the the K D (sgn-bkg) discriminants. The relevant distribution is the log-likelihood ratio -2 ln ( L 0- / L 0+ ) from pseudoexperiments under the assumptions of either a pure pseudoscalar or a pure scalar model. The arrow indicates the observed value. The data disfavor the pseudo scalar hypothesis with 3.3  or 0.16% Excluded 0.16%

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Excluded < 0.1% Excluded 1.5%

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th H  ZZ  4 μ candidate

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 H  WW  2 μ + MET candidate

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Higgs searches at LHC: CERN-PH-EP TOTEM Luminosity measurement CERN-PH-EP/ arXiv: v1 (19 March 2013) Observation of a new boson with mass near 125 GeV in pp collisions at √s = 7 and 8 TeV CMS-PAS-HIG Combination of standard model Higgs boson searches and measurements of the properties of the new boson with a mass near 125 GeV CMS-PAS-HIG Properties of the Higgs-like boson in the decay H to ZZ to 4l in pp collisions at √s =7 and 8 TeV

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Backup

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013

XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 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.)