Observation of the Higgs particle in γγ events and search for the Higgs particle in Z γ events at ATLAS LIU Kun 1 Laboratoire de Physique Nucl´ eaire et de Hautes Energies (LPNHE) 2 University of Science and Technology of China (USTC) June 24 th , 2014 June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 1 / 72
Outline Outline The Standard Model Higgs Boson Thesis motivation Experimental Setup Photon Performance Search for a Higgs Boson in H → Z γ → ℓℓγ ( ℓ = µ, e ) Observation of the Higgs Boson in γγ Events Summary June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 2 / 72
The Standard Model Higgs Boson Success of the Standard Model (SM) Success of the Standard Model (SM), but... Spin- 1 2 fermions, grains of matter. Spin-1 bosons for interactions. Most of the SM predictions have been confirmed in experiments. Spontaneous Symmetry Breaking through Higgs mechanism: origin of particle masses. The spin-0 Higgs boson is pre- dicted by the Higgs mechanism. The Higgs boson had not been detected before July 2011. The Higgs boson mass is a free parameter. June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 3 / 72
The Standard Model Higgs Boson Constraints on the SM Higgs Boson Mass Constraints on the SM Higgs boson mass in July 2011 Theoretical bounds: the existence of the Higgs boson leads to the cancellation of the ultraviolet divergence in the scattering amplitude of W + L → W + L W − L W − L , provided the Higgs boson mass is not too heavy: √ � 4 π 2 m H < ∼ 700 GeV 3 G F Experimental direct limits (95 % CL) , in July 2011, from LEP, Tevatron. Exclusion region: m H < 114.4 GeV m H within [156, 177] GeV. June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 4 / 72
The Standard Model Higgs Boson Constraints on the SM Higgs Boson Mass Constraints on the SM Higgs boson mass in July 2011 A global fit to electroweak data (from Hfitter group): Input parameters: W , Z , top masses and widths, cross sections Best fitted m H = 91 +30 − 23 GeV (without direct limits on m H as inputs) This constraint favours a low mass value for the SM Higgs boson June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 5 / 72
The Standard Model Higgs Boson The Higgs Boson Production and Decay The Higgs boson production and decay in the SM Five main production processes ( ggH , VBF , WH , ZH , t ¯ tH ): 2 H+X) [pb] 10 LHC HIGGS XS WG 2012 q q s = 8 TeV g pp → H (NNLO+NNLL QCD + NLO EW) V t , b H H 10 V g → q q (pp p p → q q H (a) (b) ( N N L O σ Q C 1 pp D + pp N L → O E → ZH (NNLO QCD +NLO EW) WH (NNLO QCD + NLO EW) W ) pp → g t ttH (NLO QCD) d / u q -1 10 H H H W Z W ± Z u / d q 10 -2 g t 80 100 200 300 400 1000 (c) (d) (e) m [GeV] H Higgs BR + Total Uncert 1 LHC HIGGS XS WG 2013 WW For a light Higgs boson in the SM b b gg -1 ZZ 10 τ τ H → γγ and H → ZZ → 4 ℓ ( ℓ = µ, e ) c c clean final states and large sensitivity -2 10 H → Z γ → ℓℓγ ( ℓ = µ, e ) and H → µµ : Z γ γ γ clean final states but small BR -3 10 µ µ -4 10 80 100 120 140 160 180 200 m [GeV] H June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 6 / 72
Motivation The H → γγ and the H → Z γ → ℓℓγ Decays The H → γγ and the H → Z γ → ℓℓγ decays in the SM They are loop-induced decays, dominated by W loop. Two kinds of backgrounds: irreducible and reducible background γγ (75%), γ +jet(jet+ γ ), jet+jet (for H → γγ ) ℓℓ + γ (82%), ℓℓ +jet (for H → Z γ → ℓℓγ ) June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 7 / 72
Motivation The H → γγ and the H → Z γ → ℓℓγ Decays The H → γγ and the H → Z γ → ℓℓγ decays in the SM At m H = 125 GeV, σ H × BR H → γγ = 40 (50) fb at √ s = 7 (8) TeV, while σ H × BR H → Z γ → ℓℓγ ( ℓ = µ, e ) = 1.8 (2.3) fb . H → γγ and H → Z γ → ℓℓγ decays clean final states excellent photon and lepton energy response resolution loop-induced decays: sensitive probe for new physics Reminders on the photon performance high photon selection efficiency and jet rejection needed to improve √ S / B accurate measurements of the photon trigger and identification efficiencies are needed for precise measurements of cross sections and Higgs boson properties. June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 8 / 72
Experimental Setup The Large Hadron Collider (LHC) The Large Hadron Collider (LHC) June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 9 / 72
Experimental Setup The ATLAS Detector Running parameters in 2011 (2012) ATLAS recorded 5.1 (21.3) fb − 1 of pp collisions at √ s = 7 (8) TeV (left plot) Mean number of interactions per crossing was 9.1 (20.7) in 2011 (2012) (right plot) 30 /0.1] -1 180 fb ATLAS Online Luminosity ATLAS -1 Total Integrated Luminosity ∫ 25 Preliminary 2012, s = 8 TeV Recorded Luminosity [pb 160 -1 µ s = 8 TeV, Ldt = 21.7 fb , < > = 20.7 ∫ -1 -1 s = 7 TeV, Ldt = 5.2 fb , < µ > = 9.1 LHC Delivered Delivered: 22.8 fb 140 -1 Recorded: 21.3 fb 20 ATLAS Recorded 120 100 15 80 2011, s = 7 TeV 10 -1 60 Delivered: 5.46 fb -1 Recorded: 5.08 fb 40 5 20 0 0 0 5 10 15 20 25 30 35 40 45 n p r u l t n p r u l t a A J c a A J c J O J O Mean Number of Interactions per Crossing Month in Year June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 10 / 72
Experimental Setup The ATLAS Detector The ATLAS detector components June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 11 / 72
Experimental Setup The ATLAS Detector Excellent performance of the ATLAS tracker system The precision tracking detectors the Inner detector (ID) : vertex and track reconstruction, precision track momentum measurement the Muon Spectrometer (MS) : muon track reconstruction and precision momentum measurement Very high vertex and track reconstruction efficiencies 1 Vertex Reconstruction Efficiency 0.995 0.99 Simulation 0.985 t t Z → µ µ Z ee → ATLAS Preliminary 0.98 0 5 10 15 20 25 30 35 40 45 µ June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 12 / 72
Experimental Setup The ATLAS Detector The Electromagnetic Calorimeter (EMC) A Pb-LAr sampling dete- ctor with accordion-shaped kapton electrodes. 3 layers to reconstruct γ direction. 1 st layer segmentation to separate prompt γ from photon pairs from π 0 , η ... A thin LAr presampler ( | η | < 1.8) to estimate the energy lost before the accordion calorimeter. June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 13 / 72
Photon performance Photon Reconstruction and Identification Photon reconstruction Unconverted photons: clusters not matched to tracks. Converted photons: clusters matched to at least one track (and ambiguity resolution w.r.t. electrons). About half of the photons convert before the EMC. Photon reconstruction efficiencies are very high 1 Reconstruction efficiency 0.98 0.96 0.94 0.92 0.9 0.88 ATLAS Preliminary Simulation 0.86 All photons 0.84 Unconverted photons Converted photons 0.82 0.8 20 40 60 80 100 120 p [GeV] T June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 14 / 72
Photon performance Photon Reconstruction and Identification Photon/electron isolation Discriminating γ/ e from fake and non-direct γ/ e coming from jets. Calorimeter isolation: collecting energy deposited in a cone ∆ R = 0 . 4 around the γ/ e object E Tcone 40 : from all cells E Topo Tcone 40 : from cells belonging to topological clusters Track isolation: in a cone of ∆ R = 0 . 2 around the γ/ e object p Tcone 20 : scalar sum of p T of tracks nu cone 20 : number of tracks June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 15 / 72
Photon performance Photon Reconstruction and Identification Photon performance [ATLAS-CONF-2012-123] Photon identification efficiency measurements, Photon trigger June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 16 / 72
Photon performance Photon Reconstruction and Identification Photon identification 9 discriminating variables As example E ratio , ∆ E Entries/0.02 Entries/160 MeV ATLAS Preliminary ATLAS Preliminary 5 10 4 ∫ ∫ 10 -1 -1 s =8 TeV, Ldt=20.3 fb s =8 TeV, Ldt=20.3 fb γ γ Converted Unconverted 4 10 Z → ll γ data Z → ll γ data 3 10 Z → ll γ corrected MC Z → ll γ corrected MC 3 10 Z( → ll)+jet corrected MC Z( → ll)+jet corrected MC 2 10 2 10 10 10 1 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 1000 2000 3000 4000 5000 6000 7000 8000 ∆ E E [MeV] ratio June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 17 / 72
Photon performance Photon Identification Efficiency Measurements Photon identification efficiency measurements Two identification algorithms cut-based: loose, tight ID neural-network based ID (only used in H → γγ at √ s =7 TeV) Three data-driven methods: using Final State Radiative (FSR) photons from Z → ℓℓγ decays matrix method electron extrapolation 1 Z radiative decays 0.8 Extrapolation data-driven from Z ee → 0.6 Matrix method 0.4 0.2 Data-MC correction on signal MC MC-based 0 10 20 30 100 200 1000 2000 E [GeV] T June 24 th , 2014 LIU Kun (LPNHE&USTC) Thesis defense 18 / 72
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