SM Low Mass search secondary channels γγ, HX->ττjj, VH->qqbb) … · 2010. 7. 29. · H→γγ...

Subhendu ChakrabartiSUNY @ Stony Brook

for D0 and CDF collaboration

Higgshunting Workshop 07-29-2010

SM Low Mass search "secondary channels" (H->γγ, HX->ττjj, VH->qqbb)

Outline

• Motivation

• H->γγ @ D0/CDF

• XH->ττjj @ D0/CDF

• VH->qqbb @CDF

• Results

• Summary

2

Analysis presented here 2.3-5.4 fb-1

Motivation

• Tevatron collecting large dataset where we maximize sensitivity for Higgs searches

• For secondary channels, γγ/ττ decays have small BR and bb decay has large QCD background.

• Experimental challenges include difficult detection of γ/τ final states

• However they can add up to Higgs sensitivity

• Moreover, relative model independence of these channels can be used to search for new physics

3

H→γγ @D0/CDF

• SM Higgs decay to two photons

• BR only 0.2% but better mass resolution

• Diphoton mass bump observable in falling spectrum

• Background model by data driven techniques (sideband technique)

• Search can probe for any narrow resonance decaying into di-photons in quasi-model independent way

• Event Selection

• DiEM cal-only trigger suites, Single photon with high pT (50-70 GeV) threshold CDF only

• Primary vertex should be inside acceptance of tracking

• EM object in central calorimeter

• pT>25/15 GeV (D0/CDF)

• Mγγ > 60/30 GeV

L=4.2/5.4 fb-1

H→γγ Preselection

• High EM fraction/cluster in shower

• Calorimeter and tracker isolation

• Transverse shower profile and no associated track

• D0 also uses a 5 variable NN cut >0.1

• Reducible backgrounds : Electrons misidentified as photons Z/γ*->ee, Jets misidentified as photons,dijet and γ+jet, using NNLO MC

• Irreducible backgrounds : direct QCD di-photon production using a sideband fitting method from data

• CDF analysis use the sum of all backgrounds is taken from a inclusive sideband fitting method

Photon Identification

Main Backgrounds

H→γγ Results @D0/CDF

GeVhM110 115 120 125 130

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GeVhM110 115 120 125 130

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CDF Run II Preliminary

=120.0h

signal at M! ! "h

SM) !Signal scaled to expected limit (19.7 SM) !Signal scaled to observed limit (22.8

DataBackground Model

hM100 110 120 130 140 150

) /

SM

! !

" h(

Br

!

#

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)-1 (5.4 fb! ! "Limits for h

CDF Run II Preliminary

Central PhotonsObserved limitExpected limit

1 sigma region

2 sigma region

)-1 (5.4 fb! ! "Limits for h

• Mγγ spectrum in the search region for derivation of Obs(Exp) limits 15.8(18.5) D0/24.6(20.8) CDF times SM higgs cross section @Mh=115 GeV

VH/VBF→ττjj @D0• W(→ qq') H(→ τ+τ-)

• Z(→ qq) H(→ τ+τ-)

• H(→ bb) Z(→ τ+τ-)

• VBF qHq'→ q' τ+τ-q

• gg → H → τ+τ-+≥ 2jets

7

• Event Selection

• Only one isolated muon

• One hadronic tau candidate pT>15 GeV

• At least two jets pT>20 GeV and |η|<3.4

• Opposite sign mu-tau pair requirement

• Electron veto and no b tagging

L=4.9 fb-1

Tau reconstruction in D011%

9%

17%

8%

55%

Signal contribution after preselection

WH ZH HZVBF GGF

D0: simple cone algorithm, cone size R = 0.3. Isolation cone, R = 0.5EM cells in other layers Associate up to 3 tracks with pT > 1.5 GeV to the tau

• Simultaneous search by grouping all possible tau signals

• Difficult with only 7% higgs ->tautau BR

ττjj Preselection @D0• After preselection Multivariate technique

used for optimum sensitivity

• Variables used for BDT training showing good data montecarlo agreement

8

ST

T S

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Diboson

Signal x 100

Event Yield

• QCD estimated from data used in BDT training

(GeV)T

Leading jet p

0 20 40 60 80 100 120 140 E

ntr

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data

MJ

ttw+jets

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Diboson

Signal x 100

leading jet pT

Total Signal 1.33

data 433

ttbar 66.7

w+jets 81.5

z+jets 222.7

diboson 10.2

QCD 80.7

total 439.9

signal 1.33

BDTs for ττjj @D0

• 32 Boosted Decision Trees for four signals to four bkgd BDTs output in two mass regions

• 17 variables used for each BDT

9

• BDT training WH signal vs different backgrounds

(HW,ttbar) BDT

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(HW,Wjets) BDT

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(HW,MJ) BDT

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BDTs for ττjj@D0

• Final distribution weighted average Zjets BDT for limit setting

• Major systematics Tau ES 4.5% Lumi 6.1% QCD 15% Cross sec 10% JES 7.5%

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L=4.9 fb-1

Wjets BDT

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MJ BDT

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dataMJ

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ttbar BDT

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Zjets BDT

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DibosonSignal x 50


low mass

high mass

wjets ttbarmj• Final selection cut on average over maxBDT of ttbar,wjets and MJ BDTs

VH/VBF→ττjj @ CDF

11

• W(→ qq') H(→ τ+τ-)

• Z(→ qq) H(→ τ+τ-)

• VBF qHq'→ q' τ+τ-q

• gg → H → τ+τ-± 2jets

L=2.3 fb-1

27%

21% 21%

31% WH ZHVBF GGF

CDF: Start with a calorimeter tower, ET > 6 GeV.Add up to 6 contiguous towers with ET > 1 GeV.Associate tracks with the calorimeter cluster, At least one track with pT > 6 GeV.Tau cone defined by seed track,Isolation annulus 1 or 3 tracks, Reconstruct π0's.Require M(tracks,π0's) < 1.8 GeV

τ identification @CDF

Simultaneous search for tau signals

Orthogonal to b tagged low mass searches

Event Yield @ CDF

12

DATA 965, 166 (1,2 jet)

Z-tautau+jets 357.9, 59.3

zmumu+jets 26.4, 4.8

ttbar 3.9, 16.3

diboson 4.6, 0.9

QCD 483, 64

w+jets 45.8, 14.1

total background 921.7, 159.4

Total signal (mh=120) 0.746, 0.477

• Event Selection

• Exactly 1 Lepton: Central (|η| < ~1.0) isolated electron or muon with Pt > 10 GeV

• Exactly 1 Hadronic Tau: Central hadronic τ (|η| < 1.0, 1 or 3 track in signal cone) with visible Pt > 15 GeV

• OS requirement: Lepton and Hadronic τ candidates have to be opposite sign

• At least 1 Jet and 2 or more jet: Et > 20 GeV and |η| < 2.5

• Z boson veto (for Z→ee/µµ)

] 2[GeV/c!lep,M0 50 100

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] 2[GeV/c!lep,M0 50 100

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! ! "* #Z/

ee"* #Z/

! ! "* #Z/

fakes from SS data

add-on W+jets

top

diboson

-1 L dt = 2.3 fb$CDF Run II Preliminary

2 jet signal channel%

BDTs selection @ CDF

13

• Three BDTs trained with mixed Signal vs zee,top, qcd backgrounds

• Final discriminator minimum of three BDTs

• Major Systematics JES 15% Lumi 6.0% tau ID 3% Signal 10-22%

BDT output (QCD vs Higgs)-1.0 -0.5 0.0 0.5 1.0

eve

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! ! "Z

ee"Z

! ! "Z

fakes from SS data

add-on W+jets

top

diboson

signal X 100

-1 L dt = 2.3 fb#CDF Run II Preliminary

2=120 GeV/cHM

2 jets channel$

vs Higgs)!!"BDT output (Z-1.0 -0.5 0.0 0.5 1.0

events

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! ! "Z

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2=120 GeV/cHM

2 jets channel$

BDT output (top vs Higgs)-1.0 -0.5 0.0 0.5 1.0

eve

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! ! "Z

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fakes from SS data

add-on W+jets

top

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signal X 100


2=120 GeV/cHM

2 jets channel$

Limits

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• CDF uses a Bayesian approach

• CDF sets limits for ττjj analysis with 2.3 fb-1 data

• The expected limit ranges from 23.9 to 82.6

D0 ττ

)2

(GeV/cHm105 110 115 120 125 130 135 140 145

LL

R

-1.5

-1

-0.5

0

0.5

1! 2-

BLLR

! 1-B

LLR

BLLR

S+BLLR

OBSLLR

-1DØ Preliminary, L=4.9fb

)2

(GeV/cHm100 105 110 115 120 125 130 135 140 145 150

)!!

(SM

Hig

gs

wit

h

"L

imit

/

1

10

210

-1DØ Preliminary, L=4.9 fb

Observed Limit

Expected Limit

Standard Model

Observed Limit

Expected Limit

Standard Model

• Good agreement with background prediction. No significant excess in signal region

• D0 uses modified frequentist approach to set 95% CL LLR plots shown below

• D0 sets limits for combined ττjj analysis expected limit range 13.4 to 61.4

Mh=115 Exp Obs

D0 15.9 27.9

CDF 24.5 27.9

)2 (GeV/cHM

100 120 140

)! ! "

(SM

) *

BR

(H

#(9

5%

C.L

. L

imit)/

# 0

50

100

150

Expected

# 1 !

# 2 !

Observed

-1 L dt = 2.3 fb$CDF Run II Preliminary

Standard Model

VH/VBF→qqbb@CDF• Large signal yield as it profits from

the largest cross-section x branching ratio

• Complete event information. No missing energy to infer

• large QCD background

L=4.0 fb-1

• Event Selection

• Pass CDF multi-jet trigger

• Veto lepton events

• 4 or 5 jets with ET > 15 GeV

• Small MET significance to reduce contribution from ttbar background

• Exactly two jets must be tagged as b-jets Two b tagging methods in CDF referred as SS/SJ

• Sum of the transverse energy of the selected jets > 220 GeV to suppress QCD

Preselection qqbb @CDF• VH Signal Region: 75 < Mbb 175

& 50 < Mqq < 120

• VBF Signal Region: 75 < Mbb 175 & Mqq > 120

• Backgrounds:

• QCD (98%) ,ttbar , Z + jets (where Z decays to b/c quarks), Single-Top, W + bb/cc jets,WW/WZ/ZZ

Four orthogonal channels were studied: 1. •VH where both b-jets are tagged by tight SecVtx (SS)

2. •VH where one b-jet is tagged by tight SecVtx and the other by JetProb (SJ 1%)

3. •VBF where both b-jets are tagged by tight SecVtx

4. •VBF where one b-jet is tagged by tight SecVtx and the other by JetProb(1%)

The results from the 4 channels are combined which gives a better result than the individual channels.

• QCD Modeling : Data-based approach using Tag-Rate-Function (TRF), the probability that a jet is b-tagged given an event with a tagged b-jet as a function of three variables: ET, η of the probe jet and Δ R between the b and the probe jet.

• The TRF is parameterized in the TAG region and applied into the signal region to predict the double b-tagged QCD multi-jet events in the signal region.

Limits qqbb@CDF

• Good DATA-MC agreement

• Put a limit on SM Higgs production cross section*BR

Major Systematics Sources

• QCD Modeling (shape) • Luminosity (6%) • b-tag scale factor (7.6-9.7%) • Jet Energy Scale (7%)

Expected cross section x BR limit/SM < 18 @115GeV

Summary• Presented results from Higgs search at Tevatron with secondary search

channels

• These channels were included in the combined CDF D0 limits shown below.

• Sensitive at low and intermediate mass region.

• These channels can serve as search for New physics.

• With more data and improved analysis techniques results will be updated. Stay tuned.

18

SM Low Mass search secondary channels γγ, HX->ττjj, VH->qqbb) … · 2010. 7. 29. · H→γγ...

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