CDFにおけるWH→lνbb過程を用いた
ヒッグス粒子探索
永井 義一
Outline• Introduction• Analysis（筑波大学）
Feb. 23th 2010

 Event Selection
 Background Estimation
 Analysis Optimization• Result and Conclusion
フレーバー物理研究会 ＠ 大洗1

Why WH → lν bb channel?

Current constraint on the SM Higgs– LEP II direct search exclusion M H < 114.4 GeV c– Tevatron direct search exclusion 163 < M H < 166 GeV c 2– Precision measurement of W boson and top quark mass
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M H = 87 +−35GeVc26

M H < 157 GeV c 2

Current results imply fairly
light Higgs boson!!

Low mass Higgs Search M H < 135 GeV c 2Dominant decay for this region is:

qq→WH: 2nd highest
W leptonic decay[pb]gg→H: Highest cross section
Huge QCD background→ one of most promisingchannel in low mass

H → bb

Tevatron and CDF

Tevatron• Proton-antiproton collisionat
S = 1.96TeVCDF• One of the general
purpose detector• Currently > 6.0 fb-1 data on

tape. For this analysis, weuse 4.3 fb-1 data.
Hadron calorimeter

Fermilabp
CDF
Main Injector& Recyclerp
Tevatron

EM calorimeterSolenoidp
Trackingchamber
Silicon detectorpMuon chamber

3Event selection
Event trigger
Central electron/muon (CENTRAL)
Forward electron (FORWARD)
Missing ET + 2jets or Missing ET (MET)

Baseline selection• High pT electron, muon or isolated track (PT > 20GeV)• Large missing ET (MET > 20 GeV , > 25 GeV for FORWARD)• Exactly two high ET jets (ET > 20 GeV )• At least one b-tagged jet

3 exclusive
triggercategories

W bosonselection

W + 2jetsselectionb flavor tagging
Use 3 b flavor tagging algorithms: SECVTX, JETPROB, and Neural Network
b-tagging is very important to improve S/B
Main background: W+jets

S/B ~1/5000 (no b-tag requirement)
S/B ~1/100 (2-SECVTX tagged jet)4

SECVTX b-tagging

b-tagging algorithm

Efficiency ~40%
Fake rate ~1%

Jet Probability b-tagging

Efficiency ~50%
Fake rate ~5%

NN b-tagging

Efficiency~ 40% (non-muon jet)
~ 75% (muon jet)
Fake rate 5-10%• Require at least one-SECVTX tagged jet S/B
Double SECVTX Tight tagged (ST+ST)
One SECVTX Tight + One JetProb (5%) (ST+JP)One SECVTX Tight + One NN (ST+NN)
One SECVTX Tight tagged (1-ST)

highlow

4 exclusive
b-taggingcategories

5Background Estimation• QCD multi-jets fake events (non-W QCD)
• W + light flavor events (falsely tagged jet, Mistag)
Estimated from Data
• W + heavy flavor events (W+bb, W+cc, W+c)
Estimated from Data and MC
WW

12.4 +/- 0.25 pb

WZ3.96 +/- 0.06 pb

ZZ1.58 +/- 0.05 pb

stop-s

0.88 +/- 0.11 pb

stop-t1.98 +/- 0.25 pb

Z + jets

787.4 +/- 50.0 pb

tt

6.7 +/- 0.9 pb• Top events (tt, single-top)• Diboson events (WW, WZ, ZZ)• Z + jets events
Estimated from MC using theoreticalcross section

WH (H->bb, 115GeV) 0.136 pb
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Background Estimation
ST+ST

ST+JP

Top
W+HF
W+LF

Top
W+HF

signal region

signal region

ST+NN

Top
W+HF
W+LF
signal region

Expected signal (MH = 115GeV)
3.21 (ST+ST)
2.62 (ST+JP)
1.22 (ST+NN)
6.62 (1-ST)
W+HF
W+LF

1-ST

signal region7Analysis Optimization– Neural network b-jet energy correction
Improve di-jet invariant mass resolution– Bayesian neural network discriminant
Improve signal-background separation8

b-jet energy correction• Di-jet invariant mass is the most sensitive variable in WH->lνbb• We develop Neural Network b-jet energy correction method• We use 8 input variables (Jet ET , LXY , Track pT , etc … )Apply NN correction

9• Di-jet invariant mass resolution is improved ~4%

Bayesian Neural Network (BNN)• Separately train “ST+ST”, “ST+JP & ST+NN”, “1-ST”

Input(ST+ST): Mjj, PtImbal, max(Mlνj), Qxηlep, Σ(LooseJetEt), Pt(W), Ht
Input(ST+JP): Mjj, Σ(LooseJetEt), Qxηlep, min(Mlνj), Ht, Pt(W), Met

7 input
7 input

Input(1-ST): Mjj, Σ(LooseJetEt), Qxηlep, Pt(W), Ht, Met, PtImbal

7 input

PtImbal: pT(j1) + pT(j2)+ pT(l) – MET

Ht: pT(j1) + pT(j2)+ pT(l) + MET

BNN inputs

BNN output
・・+・

Bayesian Neural Network

Other 6 input variables

10All lepton combined BNN output
ST+STST+NN

ST+JP

1-STNo significant excess in BNN output distribution→ Set upper limit on Higgs boson production cross section

Final result• We set σ(pp->WH) x Br(H->bb) upper limit using BNN output(binned likelihood)absolute upper limit in [pb](normalized to SM expectation)

Observed (Expected) upper limit @ mH = 115 GeV/c2
5.26 (3.98) x σ(Standard Model)

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CDF Combination

Combined channels
WH->lνbb, WH+ZH->MET+bb, ZH->llbb, H->WW->lνlν
WH+ZH->jjbb, H->ττ + 2jets
13Tevatron Combination and Future• MH = 163 – 166 GeV/c2 is excluded at the 95% C.L.
• Observed (expected) upper limit @ 115 GeV/c2: 2.7 (1.8) x SM
• Observed (expected) upper limit @ 165 GeV/c2: 0.9 (0.9) x SM
• Tevatron plan to run through 2010 -> expected > 8/fb of data
• We can reach to the SM sensitivity even at the MH = 115 GeV/c214

Summary• We search for the Higgs boson in the WH->lνbb channel using 4.3/fbof data Employ 3 b-tagging algorithm Develop NN b-jet energy correction method
 Develop Neural Network discriminant (BNN)• No evidence of the Higgs boson signal
 We set the 95% C.L. limit 3.98 – 37.6 x σ(SM) for 100 – 150 GeV• Tevatron can reach the Standard Model sensitivity with > 8/fb of data,
even at the low mass region.

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Backup16

b-jet energy correction• Di-jet invariant mass resolution is improved ~4%17

Dijet Mass Distribution with NN b-jet Energy Correction

ST+ST

ST+JP

ST+NN

1-ST

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BNN input variables (Pretag)

Mjj (L5)

Mjj (L5 + NN)(for reference)

ALLQ x ηlep
ALL

Pt Imbalance(LepPt + ΣjetEt Met)
STST, 1-ST

ΣloosejetEt
ALLPt(W)ALL

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BNN input variables (Pretag)

Htmax(Mlνj)

ALL

ST+STmin(Mlνj)
ST+JP

MET
ST+JP & ST+NN, 1-ST

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Pretag BNN outputs

CentralPHX

IsoTrkCentralPHX

IsoTrk21

Background Estimation

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Event Selection (Baseline)

CEM,CMUP,CMX (4.3/fb)•High pT lepton triggered
electron/muon (Pt > 20GeV)•Large missing Et (MET > 20GeV)•Di-lepton veto, Z veto•

QCD veto (0-tag/1-tag)•Jets Et > 20 GeV, |η| < 2.0

PHX (4.3/fb)• MET15_PEM20 triggered electron(Pt > 20GeV)• Large missing Et (MET > 25GeV)• Di-lepton veto, Z veto
• QCD veto (all events)• Jets Et > 20 GeV, |η| < 2.0

IsoTrk (MET2J (3.9/fb),
MET45 (4.2/fb))• MET trigger fired (details shown later)• One isolated track (Pt > 20GeV)• Large missing Et (MET > 20GeV)• Di-lepton veto, Z veto
• Tight lepton veto, Two track veto
• Tight jet veto (tracks not in ∆R< 0.4)
• QCD veto (0-tag/1-tag)

MET2J only
• at least two jets with:
Et > 25 GeV, ∆R(j1 - j2) > 1.0
one of two leading jets with |η| < 0.9

Lepton categories: Central, PHX, IsoTrk
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