ttH can be feasible using ttH can be feasible using shape ….?shape ….?

Samir FERRAGSamir FERRAG

University of GlasgowUniversity of Glasgow Les Houches, 23/06/2007Les Houches, 23/06/2007

Not Official ATLAS results

HHbb, via ttH with mH< 130 GeVbb, via ttH with mH< 130 GeV• Complex and challenging channel (HComplex and challenging channel (Hbb):bb):

– One lepton (trigger)One lepton (trigger)– 4 b-jets + 2 jets4 b-jets + 2 jets

Need excellent b-tagging: Need excellent b-tagging: signal efficiency ~O(signal efficiency ~O(44bb))

– Fully reconstructed tFully reconstructed t– High jet multiplicity High jet multiplicity

• CombinatoricsCombinatorics• Difficult to simulateDifficult to simulate

• Background:Background:– Reducible: tt+jj, W+jets, WWbbjjReducible: tt+jj, W+jets, WWbbjj

• Extracted from dataExtracted from data• Reduced by imposing reconstructed tt pair Reduced by imposing reconstructed tt pair

– Irreducible: ttbb, combinatoricsIrreducible: ttbb, combinatorics• Slight difference in kinematics with the signal (Likelihood)Slight difference in kinematics with the signal (Likelihood)

Evolution of resultsEvolution of results• After an optimistic evaluation on

the TDR:– Updating pdf: 20% smaller cross sections– Matrix element based Monte Carlo for ttbb

background higher jet multiplicity

• Realistic environment– Significance down to < 2

Counting experiment:Counting experiment:

Constraints on the ttH analysisConstraints on the ttH analysis • Higgs mass=120GeV• Higgs mass=130GeV

• 10 fb-1

• 100 fb-1

ATLFASTanalysis

ATLFASTanalysis

The constraints on ttH are challenging

CE: Constraint on ttH for 10fbCE: Constraint on ttH for 10fb-1-1

5

4

3

2

6% of PDF uncertainties (on ttbb+ttjj)

S/B

systematic uncertainty on B (%)

Requested S/BFor 3 significaneFor 6% PDF uncertainty

Need 1 to 2 % of accuracyOn background to reach 2 significance

• Higgs mass=120GeV

S/B=0.14 From Markus analysis(ATLFAST)

Signal ReconstructionSignal Reconstruction

The signal is plagued by wrong combinatorial pairs (>70%) which makes it looking-like the background --> - lower the S/B power

Signal Background

Wright combinations

Likelihood2 for the Signal/BackgroundLikelihood2 for the Signal/Background

Robustness of distributions validated by a comparison between MC@NLO and ACERMC

Likelihood1 to reduce combinatoricsLikelihood1 to reduce combinatorics

CE: Sensitivity results CE: Sensitivity results (without systematics)(without systematics)

Current Analyses:

A1- Cuts Based Analysis: C. Collins-Tooth s/b=1.9

A2- Selected Likelihood: G. Aad s/b=3.1

90 -150 GeV

s/b is computed in thisInterval (Signal Interval)

– For 100fb-1 (in 2011) and Background accuracy of 5%5% :• S/B=30% for a 5 discovery (mHiggs=120 GeV)

• S/B=16% for a 3 evidence (mHiggs=120 GeV)

Shape Analysis: Getting and using Shape Analysis: Getting and using 22

H0

H0 ---> t0 ( where t0 is a template for the null hypothesis, H0. )

• H1 ---> t1 ( where t1 is a template for the signal plus background hypothesis, H1. )

• ‘Data’ ---> gives us one 02 (H0) and 1

2 (H1).

• Generate ‘Data’ 10000 times (in pseudo experiments).

2lnQ =-2

H1

2 = 9.876e.g.

• 1 - CLb = Probability that the background will fluctuate to look like the signal

Equivalent in counting experiment: …….5 1-CLb = 2x10-7.

•CLs+b = Confidence Level for excluding H0.

Use:-

CLs+b 1 - CLb

Improvement from using shapeImprovement from using shape

Shape Analysis 2

(CE) One Bin 2

H1H0

Effect of shapesEffect of shapes

Probability of measuring with 30fb-1

3 5

Counting Experiment 19% <1%

Shape 38% 3%

Shape 15% on Signal X-sec

30% 1%

Probability of measuring with 30fb-1 2 3

Counting Experiment 30% 6%

Shape 34% 8%

Cut based analysis

Likelihood analysis

The hape of distributions helps for both likelihood and shape analysis but is it robust?

Backup slides

CE: Constraint on ttH for 100fbCE: Constraint on ttH for 100fb-1-1

6% of PDF uncertainties

S/B

systematic uncertainty on B (%)

Requested S/BFor 3 significaneFor 6% PDF uncertainty

5

4

3

2

Need 4% of accuracyOn background to reach 3 significance

S/B=0.14 From Markus analysis(ATLFAST)

• Higgs mass=120GeV