sung-won lee 1 study of hadronic w decays in the jets + met final state @ lhc kittikul kovitanggoon...

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

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

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]

2

tt production @ 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


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


All Jets

Exclude B Jets

M(jj) (GeV/c2)

5

b-tag analysis


• 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

6

b-tag analysis


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

7

b-tag analysis


• 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


M(jj) (GeV/c2)

Before subtraction

After subtraction

M(jj) (GeV/c2)

8

b-tag analysis


Zoom-in M(jj) after

Subtraction

Zoom-in M(jj) after

Subtraction

MW

Peak position

9

b-tag analysis


• 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

10

Conclusions and Plans


• 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


)( )( bWbWttpp jj l

Data: TTbar_0jet_Et30alpgen/Summer08_IDEAL_V12_AODSIM_v1/AODSIM

13

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


Data: SUSY_LM7-sftsht/Summer08_IDEAL_V9_AODSIM_v1/AODSIM

14


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


15


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

16

sung-won lee 1 study of hadronic w decays in the jets + met final state @ lhc kittikul kovitanggoon...

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