Measurement of the Higgs Couplings by Means of an Exclusive Analysis of its Diphoton decay

Transcription

1 Measurement of the Higgs Couplings by Means of an Exclusive Analysis of its Diphoton decay i.e. what do we know about the Higgs Marco Grassi

2 The Discovery of a New Boson Evidence of a new boson with 5 GeV/c mass Reported by both Atlas and CMS with 5σ statistical significance So far properties very similar to the Standard Model Higgs Boson Is that really the Standard Model Higgs? ( )ev( 5( ( G ( G 5( ( ) ) m( ± σ ± σ m γγ ( S( )ev( m γγ ( Events / GeV CMS Data Z+X Zγ*, ZZ m H =5 GeV s = 7 TeV, L = 5. fb Events / GeV 5 s = 8 TeV, L = 5. fb 6 >.5 K D 6 m (GeV) m (GeV) Higgs BR + Total Uncert τ τ cc bb γ γ gg Zγ WW 6 8 [GeV] ZZ M H LHC HIGGS XS WG

3 The Discovery is not enough Higgs Potential Higgs mechanism: Electroweak (EW) symmetry breaking New minimum in EW Lagrangian It defines the vacuum state Higgs Potential Tunneling Higgs Field High values of the Higgs Field: Higgs potential driven by its self-coupling New minimum could emerge Our vacuum could not be The Vacuum What if Tunneling prob. < Age of the Universe? Study of the Higgs Couplings

4 The Exclusive H γγ Analysis Measure the couplings Disentangle Higgs production mechanism g g t H Gluon-gluon Fusion Cross section: 9.5 pb Final state: Higgs alone q q W/Z H Vector Boson Fusion Cross section:.56 pb Final state: two forward jets where H γγ q q W/Z H Associated Production Cross section:.9 pb Final state: W/Z boson

5 The Compact Muon Solenoid Large Hadron Collider (LHC) Proton-proton collisions at s=8tev Peak Luminosity > 7 cm - s More than fb delivered Compact Muon Solenoid (CMS) T T Silicon Tracker Electromagnetic Calorimeter Hadron Calorimeter Superconducting Solenoid Iron return yoke interspersed with Muon chambers m m m m m 5 m 6 m 7 m 5

6 Higgs via Gluon-Gluon Fusion m H = p E E ( cos ) Energy Correction Issues: Precise Energy Reconstruction Vertex Identification Regression targeting photon energy Solutions: Multivariate analysis exploiting pt(γ γ) Fraction of events Vertex Identification..9.8 z - z true < mm reco.7.6 CMS Preliminary Simulation <PU> = p (γ γ) (GeV) T 6

7 Higgs via Vector Boson Fusion Two forward jets (in addition to two photons) Forward: Δη(jet,jet)> Large dijet invariant mass M(jet,jet) > 5 GeV 7

8 Higgs via Associated Production H W/Z Leptons Electron / muon Easy to tag Low branching ratio Low statistics Neutrinos / Mis-reconstructed leptons Higher statistics Challenge of Missing Energy reconstruction N ev /6pb 6 5 Longer Tail in Data ZZ+WZ+WW H M() W+jets QCD tt+jets γ + jets DY+jets di-γ jets di-γ box N ev /6pb 8 6 Large Asymmetry ZZ+WZ+WW W+jets tt+jets di-γ box H M() QCD γ + jets DY+jets di-γ jets PF Met [GeV] - - Φ(PF Met) 8

9 Missing Transverse Energy Recovery Missing Transverse Energy (Met) needs to be corrected Met composed mainly by hadronic jets: Simulated energy resolution not describing resolution in data Energy scale in data underestimates real jet energy Asymmetric noise + subdetector mis-alignment After all corrections have been deployed: N ev /6pb 6 5 ZZ+WZ+WW W+jets tt+jets H M() QCD γ + jets DY+jets di-γ jets N ev /6pb 8 6 ZZ+WZ+WW W+jets tt+jets di-γ box H M() QCD γ + jets DY+jets di-γ jets di-γ box Smeared & Shifted Met [GeV] - - Φ(Smeared & Shifted Met) 9

10 Diphoton Invariant Mass Spectra Inclusive: max η <.5, min(r9)>.9 Simulation Both γ in barrel, min(r9)>.9 s=8tev L=5./fb. Simulation Parametric model.5 σ eff =. GeV. FWHM/.5 =.8 GeV.5. Events / ( GeV) 5 Data Bkg Model 5 ± σ ± σ xsm m H = 5 GeV Gluon-Gluon Fusion Expected signal: ev (in 5. fb ) Signal/Background~ Invariant Mass γγ (GeV) 6 8 Invariant Mass γγ (GeV) MET Tagged Category Simulation MET Tagged Category s=8tev L=5./fb Simulation Parametric model σ eff =.99 GeV FWHM/.5 =.78 GeV Events / ( GeV) 6 Data Bkg Model 5 ± σ ± σ xsm m = 5 GeV H Associated Production (Events with Met) Expected signal:.5 Signal/Background~.. Invariant Mass γγ (GeV) 6 8 Invariant Mass γγ (GeV)

11 Statistical Combination Simulation Upper Limit on Higgs cross section Evaluate exclusive analysis significance Disentangling Higgs production mode improves sensitivity /σ(h γγ) σ(h γγ) SM 95%CL Simulation Inclusive & VBF Categories Adding Lepton Tagged Category Adding Met Tagged Category Higgs Mass (GeV) Data Fit to the Higgs boson cross section Y axis: likelihood shape w.r.t. to minimum Best fit:. x SM prediction (large uncertainty ~8%) Δ(-LogLikelihood) Higgs Mass 5 GeV 6 Inclusive Inclusive+VBF Inclusive+VBF+VH Systematic Uncertainty 8 6 s = 8TeV L = 5. fb 5 Strength Modifier σ/σ SM

12 Measuring the Higgs Couplings Higgs Production Higgs Decay Number of Events t γ γ N~ (.5 - ) W γ N~ (.5 - ) γ : coupling Higgs - bosons (cv) : coupling Higgs - fermions (cf)

13 Couplings: Results Coupling To Fermions c - Injected SM Signal: fb û 7 m h = GeV D SM 68% 9% 95% categories H95%L Coupling To a Bosons Coupling To Fermions: g(hψψ)/g(hψψ) SM - s = 8TeV L = 5./fb Data σ band σ band Coupling To Bosons: g(hvv)/g(hvv) SM Δ(Negative LogLikelihood) Coupling To Fermions: g(hψψ)/g(hψψ) SM - s = 8TeV L = 5./fb Data σ band - No syst error σ band - No syst error σ band - Full syst σ band - Full syst Coupling To Bosons: g(hvv)/g(hvv) SM Simulation Data Systematics Plot axes: measured coupling values over Standard Model prediction Blue region: best fit values Data prefers larger coupling w.r.t. Standard Model prediction All the models where Higgs couples only to bosons are excluded at 99% CL

14 Conclusions First step in the field of Higgs properties measurement Motivated by strong interest (theoretical and experimental) on Higgs couplings Goal: probe the Electroweak Symmetry Breaking mechanism Analysis can be improved adding other production modes (tth) Uncertainty still dominated by statistical error: add remaining data. Exciting times foreseen in Higgs Physics