Search for the Standard Model Higgs Boson in the two photon decay - PowerPoint PPT Presentation

Search for the Standard Model Higgs Boson in the two photon decay mode Nicholas Wardle QM IOP - April 4, 2012 1 / 21

◮ Introduction ◮ Analysis Strategy ◮ Results ◮ Conclusions 2 / 21

Introduction Search performed at CMS on 4 . 8 fb − 1 pp dataset at √ s = 7 TeV Update from recent paper. 3 / 21

Event Selection ◮ Two high p T photons ◮ MVA ID quality cut to reject fakes. p T m γγ > 1 3 , 1 ◮ isolation variables to reject jets 4 ◮ shower shape variables for π 0 ◮ Vertex from track information. ◮ Correct vertex found 85% of the time. 4 / 21

Search Strategy Sensitivity dominated by H → γγ peak Width of the peak is driven by the experimental resolution Search is performed in a ± 2% window around the peak 5 / 21

Event Classification ◮ Photon Kinematics ◮ p γ 1 T / m γγ , p γ 2 T / m γγ ◮ η γ 1 , η γ 2 , cos(∆ φ ) ◮ Photon Discriminator ◮ IDMVA γ 1 , IDMVA γ 2 ◮ Event Resolution ◮ σ ( correct ) / m γγ , σ ( wrong ) / m γγ ◮ vtxprob Combine with (∆ M / M H ) using a second BDT Inputs invariant with m H 6 / 21

Di-jet Category Jets back to back with di-photon system. ) 2 CMS preliminary 18 Data Events / ( 1 GeV/c Bkg Model -1 s = 7 TeV L = 4.76 fb ± σ 1 16 ± σ 2 BDT >= 0.05 VBF Tag 1xSM m =120 GeV 14 H 12 10 8 6 Look for two high E T forward Jets. 4 2 0 100 120 140 160 180 2 m (GeV/c ) γ γ 7 / 21

Signal Model 7 ) CMS preliminary 2 Events / ( 0.35 GeV/c Simulation Simulation 6 Signal model derived from MC Parametric Model simulation. All Categories Combined 5 σ 2 = 1.82 GeV/c eff 4 MC corrected for detector 2 FWHM = 3.29 GeV/c effects (data-driven 3 measurements in Z → ee and Z → µµγ ) 2 1 Dominant systematics are energy scale and resolution 0 100 110 120 130 2 m (GeV/c ) γ γ 8 / 21

Background Model - Normalization f bkg ( m γγ ) = N ( m − a γγ + bm − c γγ ) Total background normalization taken from fit to sidebands. Systematic included for remaining potential bias from wrong function choice. Single parameter in the Likelihood 9 / 21

Background Model - Shape Define sidebands outside the signal region Shape in BDT extracted from linear fits in m H ◮ Only assumption is variation as a function of mass is linear ◮ Systematic included to account for potential higher order terms 10 / 21

Results CMS preliminary, s = 7 TeV ∫ Data Data -1 L = 4.8 fb 4 10 Higgs, m Higgs, m =124.0 GeV (x5) =124.0 GeV (x5) H H Background Background ± ± σ σ 1 1 3 10 ± ± σ σ 2 2 2 10 10 1 BDT Bin 1 BDT Bin 2 BDT Bin 3 BDT Bin 4 BDT Bin 5 BDT Bin 6 BDT Bin 7 Di-jet Categories optimized so as to achieve Likelihood constructed as a the best sensitivity to signal. combination of counting experiments. 11 / 21

Exclusion Limits 4 SM Observed CLs Limit CMS preliminary ) γ Median Expected CLs Limit -1 γ ± σ s = 7 TeV L = 4.8 fb 3.5 1 Expected CLs → ± σ 2 Expected CLs (H σ 3 / 95%CL 2.5 ) γ γ → 2 (H σ 1.5 1 0.5 0 110 115 120 125 130 135 140 145 150 2 m (GeV/c ) H ◮ Exclusion limits extracted in the range 110 < m H < 150 ◮ Expected exclusion limit less than 1.5xSM in 110 < m H < 140 ◮ Several ranges excluded at 95% confidence level (CL). 12 / 21

Significance Scan 3 µ 2 1 0 ◮ Significance and best-fit 110 115 120 125 130 135 140 145 150 signal strength are p-value extracted in the range 1 110 < m H < 150 σ 1 ◮ Largest local significance -1 10 around 3 σ σ 2 ◮ Look elsewhere effect -2 10 reduces this to 1 . 9 σ 1xSM Higgs Median Expected σ Local P-value 3 (combined) -3 10 Local P-value CMS preliminary (no di-jet) Local P-value -1 (di-jet only) s = 7 TeV L = 4.8 fb Observed p-value (Ensemble) -4 10 110 115 120 125 130 135 140 145 150 m (GeV) H 13 / 21

Conclusions ◮ A search for the SM Higgs has been performed on 4 . 8 fb − 1 of pp collisions at CMS ◮ Exclusions placed on the SM Higgs at several ranges of mass hypotheses. ◮ No significant excess is observed in the search range. ◮ Can expect to become sensitive to SM Higgs by summer with twice the current dataset. ◮ The analysis is described in CMS public document HIG-12-001. 14 / 21

Backup 15 / 21

Analysis Workflow 16 / 21

VBF Selection Jet Tag E T > 30 / 20 GeV m jj > 350 GeV | η j 1 − η j 2 > 3 . 5 | | φ jj − φ γγ > 2 . 6 | | 0 . 5( η j 1 − η j 2 ) − η γγ | < 2 . 5 17 / 21

Signal MC Corrections Fraction of events 1 0.8 z -z < 10 mm reco true 0.6 0.4 CMS preliminary 0.2 Simulation 〈 〉 n =9.5 PU 0 0 50 100 150 200 250 p (H) (GeV/c) T MC additional smearing measured Vtx Probabilities measured in using Z → ee MC-data likelihood fit in categories. Z → µµγ as a function of boson p T , MC reweighted. 18 / 21

Signal Systematics 19 / 21

MVA Validation using Z → ee MC simulation is validated using Z → ee MC-data comparisons. 20 / 21

CLs Limit setting with the CLs procedure: ◮ Fit toy data, once conditionally with a fixed µ and once with µ floating. Number of toys -1 6 CMS Preliminary s =7 TeV L =0.2-0.9 fb 10 ◮ Systematics are incorporated into int Higgs Combination at m = 250 GeV H Likelihood, constraints randomized in the 5 f(q ) for signal+bkgd pseudo-data 10 µ f(q ) for bkgd-only pseudo-data fits (nuisance parameters). µ q observed µ 4 10 ◮ Ratio of Likelihoods is the Profile Likelihood, we use the test statistic - 3 10 q µ = − 2 ln L ( µ, ˆ θ ) µ, ˆ 2 ˆ 10 L (ˆ θ ) ◮ Throw toys from model fit to data to 10 generate the distribution of under 1 signal+background and background only 0 5 10 15 20 hypotheses µ Test Statistic q ( =2) µ ◮ Exclude at 95% confidence when ratio of p-values CL s = CL s + b / CL b is 0.05. 21 / 21