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
!
#
0
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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
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ST
T S
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-1 DØ Preliminary, L=3.9 fb
data
MJ
ttw+jets
z+jets
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
ies
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45
-1 DØ Preliminary, L=3.9 fb
data
MJ
ttw+jets
z+jets
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
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
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-1 DØ Preliminary, L=3.9 fb
data
MJ
ttw+jets
z+jets
Diboson
Signal x 300
(HW,Wjets) BDT
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
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-1 DØ Preliminary, L=3.9 fb
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MJ
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Signal x 300
(HW,MJ) BDT
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-1 DØ Preliminary, L=3.9 fb
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Signal x 300
ttbar mjwjets
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%
10
L=4.9 fb-1
Wjets BDT
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dataMJ
ttw+jetsz+jets
DibosonSignal x 300
MJ BDT
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trie
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dataMJ
ttw+jetsz+jets
DibosonSignal x 300
ttbar BDT
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-1 DØ Preliminary, L=3.9 fb
data
MJ
ttw+jets
z+jets
Diboson
Signal x 300
Zjets BDT
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12dataMJ
ttw+jetsz+jets
DibosonSignal x 50
-1 DØ Preliminary, L=3.9 fb
Zjets BDT
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4.5dataMJ
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DibosonSignal x 50
-1 DØ Preliminary, L=3.9 fb
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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] 2[GeV/c!lep,M0 50 100
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60 DATA
! ! "* #Z/
ee"* #Z/
! ! "* #Z/
fakes from SS data
add-on W+jets
top
diboson
-1 L dt = 2.3 fb$CDF Run II Preliminary
2 jet signal channel%
BDTs selection @ CDF
13
• Three BDTs trained with mixed Signal vs zee,top, qcd backgrounds
• Final discriminator minimum of three BDTs
• Major Systematics JES 15% Lumi 6.0% tau ID 3% Signal 10-22%
BDT output (QCD vs Higgs)-1.0 -0.5 0.0 0.5 1.0
eve
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BDT output (QCD vs Higgs)-1.0 -0.5 0.0 0.5 1.0
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BDT output (QCD vs Higgs)-1.0 -0.5 0.0 0.5 1.0
eve
nts
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60DATA
! ! "Z
ee"Z
! ! "Z
fakes from SS data
add-on W+jets
top
diboson
signal X 100
-1 L dt = 2.3 fb#CDF Run II Preliminary
2=120 GeV/cHM
2 jets channel$
vs Higgs)!!"BDT output (Z-1.0 -0.5 0.0 0.5 1.0
events
0
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vs Higgs)!!"BDT output (Z-1.0 -0.5 0.0 0.5 1.0
events
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vs Higgs)!!"BDT output (Z-1.0 -0.5 0.0 0.5 1.0
events
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DATA
! ! "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$
BDT output (top vs Higgs)-1.0 -0.5 0.0 0.5 1.0
eve
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BDT output (top vs Higgs)-1.0 -0.5 0.0 0.5 1.0
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BDT output (top vs Higgs)-1.0 -0.5 0.0 0.5 1.0
eve
nts
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40 DATA
! ! "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$
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.
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