Sung-Won Lee 1

Study of Hadronic W Decays in the Jets + MET Final State

@ LHCKittikul Kovitanggoon* & Sung-

Won LeeTexas Tech University

Michael Weinberger & Teruki Kamon

Texas A&M University

Introduction1) W’s may present in the SUSY events and could be an

indicator to distinguish various SUSY scenarios.

2) Since we have LSP’s, reconstructing W’s using lepton plus MET is difficult. Thus Wjj will be important, even if we face a huge combinatorial background.

3) We are testing a data-driven method to estimate the dijet mass distribution from non-W sources.

Hadronic W decays in the Jets+MET Final State at the LHC 2

EventEvent JetsJets M(jj)M(jj) M(jj)M(jj)

1 1a, 1b, 1c

2 2a, 2b M(2a, 1a), M(2a, 1b), M(2a, 1c)M(2b, 1a), M(2b, 1b), M(2b, 1c)

M(2a, 2b)

3 3a, 3b, 3c, 3d M(3a, 2a), M(3a, 2b), M(3b, 2a), M(3b, 2b), M(3c, 2a), M(3c, 2b), M(3d, 2a), M(3d, 2b)

M(3a, 3b), M(3a, 3c), M(3a, 3d),M(3b, 3c), M(3b, 3d), M(3c, 3d)

4 For each jjii in Event XX, MM((jji i jjkk) ) is calculated with jjkk in Event XX11

Data Driven MethodForming M(jj) Distributions

Same Event: any pairs of jets of the current event

Mixed Event: any pairs of jets of the current + previous events

[Step.1] [Step.2]

Hadronic W decays in the Jets+MET Final State at the LHC 2

tt production @ LHC

Hadronic W decays in the Jets+MET Final State at the LHC

)( )( bWbWttpp jj l

Data: TTbar_0jet_Et30alpgen/Summer08_IDEAL_V12_AODSIM_v1/AODSIM

M(jj) (GeV/c2)

Same event

Mixed event after normalization

Expected shoulder due to b-jets contamination

M(jj) (GeV/c2)MW

Peak position

3

Generator Level Study for

Hadronic W decays in the Jets+MET Final State at the LHC

tt

• In order to verify the data-driven method and investigate the shoulder from b jets the generator level jets were used

After subtraction

Before subtraction

M(jj) (GeV/c2)

After subtraction

Before subtraction

M(jj) (GeV/c2)

M(jj) for jets from W and b M(jj) for jets only from W

4

Comparing with and without B jets

Hadronic W decays in the Jets+MET Final State at the LHC

All Jets

Exclude B Jets

M(jj) (GeV/c2)

5

b-tag analysis

Hadronic W decays in the Jets+MET Final State at the LHC

• According to the gen level analysis, the shoulder is due to the b jets contamination• In order to remove this shoulder, b-tag algorithm is needed • There are 2 types of b-tag algorithms recommended to use in the PAT version 1 on CMSSW_2_2_9

1. "Track Counting" algorithm : This is a very simple tag, exploiting the long lifetime of B hadrons. It calculates the signed impact parameter significance of all good tracks, and orders them by decreasing significance. Its b tag discriminator is defined as the significance of the N'th track , where good tracks are N = 2 for high efficiency and N = 3 for high purity

2. "Simple Secondary Vertex" algorithm: This reconstructs the B decay vertex using an adaptive vertex finder, and then uses variables related to it, such as decay length significance to calculate its b tag discrimatinator

•The cut number is recommended by b-tagging analysis group for Summer08 sample• HighEff less than 2.03• HighPur less than 1.47• Simple Secondary Vertex less than 1.25

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b-tag analysis

Hadronic W decays in the Jets+MET Final State at the LHC

For CaloJet ET > 30 GeV and abs(eta) < 2with MC b matching

• b-tag removing efficiency can be calculated from this results

HighEff 70.7%HighEff 70.7%

HighPur 54.8%

simSecondVer 56.6%simSecondVer 56.6%

No b tagHighEff b tagHighPur b tagsimSecondVer b tag

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b-tag analysis

Hadronic W decays in the Jets+MET Final State at the LHC

• The result shows that the HighEff b-tagging can remove b jets better than other b discriminators• Apply the HighEff b-tagging into the data-driven method to eliminate the b jets contamination

Same event

Mixed event after normalization

M(jj) (GeV/c2)

Before subtraction

After subtraction

M(jj) (GeV/c2)

8

b-tag analysis

Hadronic W decays in the Jets+MET Final State at the LHC

Zoom-in M(jj) after

Subtraction

Zoom-in M(jj) after

Subtraction

MW

Peak position

9

b-tag analysis

Hadronic W decays in the Jets+MET Final State at the LHC

• Using maximum entry to normalize with and without HighEff b-tag algorithm and imposing both on the same histogram • It is clearly see that b-tagging algorithm can eliminate the shoulder from b jet contamintaion

No b tag algorithm

HighEff b tag algorithm

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Conclusions and Plans

Hadronic W decays in the Jets+MET Final State at the LHC

• At this point, data-driven method gives good result for analyzing hadronic W decay, however we need to test this method by mixing the signal with some backgrounds• The generator level study confirms the result and identifies the reason of the shoulder due to b jet contamination• The High Efficiency b –tagging algorithm is used to eliminate the shoulder from the calo level jets• Without b jet contamination, dijet mass distribution shows good peak as expected • However, peak position is still higher than 80 GeV i.e. W mass • The JES correction is the next step to do in order to fix the higher than expect peak position • Also, moving the analysis to the new version of PAT on CMSSW3xx

11

Back up

Case 1: tt production @ LHC

Event Selection - Electron pT > 20 GeV/c - Electron Isolation < 0.1 - Standard Electron Identification  - Electron < 2.5 - Missing Transverse Energy > 20 GeV

Event Selection - Electron pT > 20 GeV/c - Electron Isolation < 0.1 - Standard Electron Identification  - Electron < 2.5 - Missing Transverse Energy > 20 GeV

M(jj) Reconstruction - Jet pT > 30 GeV/c - ΔR(jj) > 0.5

M(jj) Reconstruction - Jet pT > 30 GeV/c - ΔR(jj) > 0.5

Hadronic W decays in the Jets+MET Final State at the LHC

)( )( bWbWttpp jj l

Data: TTbar_0jet_Et30alpgen/Summer08_IDEAL_V12_AODSIM_v1/AODSIM

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Case 2: SUSY Production @ LHC))()(( 0

102 ~bWbW~ttg~

Event Pre-Selection• ET

miss > 180 GeV;

• N(J) > 2 with ET J1,J2 > 200 GeV;

• ETmiss + ET

J1 + ETJ2 > 600 GeV

N(jjii) > 2 with pT > 30 GeVΔR(jj) > 0.5

W

W -b0

2~

01~

tt

• J : represented the 1st and 2nd leading jets• j : represented the other jets that are not the 1st and 2nd leading jets

Hadronic W decays in the Jets+MET Final State at the LHC

Data: SUSY_LM7-sftsht/Summer08_IDEAL_V9_AODSIM_v1/AODSIM

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Hadronic W decays in the Jets+MET Final State at the LHC

Data Driven M(jj) Extraction for SUSY Step 1: Normalization

300-500 GeV/c2

Log Scale

Normalization Region

Normalize the mixed event to the shape of the tail of the same event with the ratio

M(jj) (GeV/c2) M(jj) (GeV/c2)

Same event

Mixed event after normalization

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Hadronic W decays in the Jets+MET Final State at the LHC

Data Driven M(jj) Extraction for SUSY Step 2: Subtraction

Peak position

After subtraction Before subtraction

Zoom-in M(jj) after

Subtraction

Zoom-in M(jj) after

Subtraction

M(jj) (GeV/c2)M(jj) (GeV/c2)MW

MW

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