search for standard model higgs in zh l + l bb channel at d Ø
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Search for Standard Model Higgsin ZH l+lbb channel at DØ
Shaohua FuFermilab
For the DØ Collaboration
DPF 2006, Oct. 29 – Nov. 3
Honolulu, HawaiiMain Injector& Recycler
(new)
Tevatron
DØ
p source
CDF
Chicago
Booster
pp
1.96 TeV
DPF 2006, Honolulu Shaohua Fu, Fermilab 2
Tevatron and DØ Detector
Run II goal: 4 ~ 8 fb1 in 2009 Analyses presented here used
840-920 pb1
Used almost the wholeRun IIa data ~ 1fb1
DØ detector in Run IIa Run IIb: Layer0 for SMT,
trigger upgrade
DPF 2006, Honolulu Shaohua Fu, Fermilab 3
Constraints on SM Higgs
Standard Model Higgs is the key to Electro-Weak symmetry breaking and gives masses to elementary particles, with its own mass unpredicted
Limit from direct searches at LEP2: mH > 114.4 GeV at 95% C.L. Indirect limit from fits to precision EW measurements from LEP, SLC, and
Tevatron: mH < 166 GeV at 95% C.L. (< 199 GeV if LEP2 limit included) Indirect best fit value: 85 +39
28 GeV at 68% C.L. A light Higgs is favored
LE
PE
WW
G J
uly
2006
DPF 2006, Honolulu Shaohua Fu, Fermilab 4
SM Higgs Production and Decay
SM Higgs production at Tevatron Gluon fusion ~ 0.8-0.2 pb (mH 115-180 GeV) Associated production with a W or Z boson ~ 0.1-0.03 pb
Dominant decays Low mass (mH<135 GeV): Hbb, high mass (mH>135 GeV): HW+W
This analysis: ZH l+lbb, in e+e and + channels
TeV 96.1s
Exc
lud
ed a
t E
xclu
ded
at
LE
PL
EP
DPF 2006, Honolulu Shaohua Fu, Fermilab 5
Data Selection
Integrated luminosity = 920 (840) pb–1 for e+e (+) sample Using all EM (Muon) triggers with efficiency ~100% (75%) for e+e (+) sample
e+e sample 2 electrons pT > 15 GeV, || < 1.1 or 1.5 < || < 2.5, central track match Z candidate: 65 GeV < Mee < 115 GeV
+ sample 2 muons pT > 15 GeV, central track matched (|| < 2.5), and isolated
Z candidate: 70 GeV < M < 110 GeV, Z pT > 20 GeV
At least 2 jets pT > 15 GeV, || < 2.5
b-tagging Secondary vertex Large impact parameter of the tracks Neural Net tagging algorithm ~60% efficiency and ~3% light-jet fake rate
(b-tagging and taggability)
BImpact Parameter
Decay Length Hard Scatter
(Signed) Track
DPF 2006, Honolulu Shaohua Fu, Fermilab 6
Backgrounds
Z( l+l) + jets Including Z( l+l) + light-parton jets and Z( l+l) + c jets
Z( l+l) + b jets Hard to reduce Z+bb background, which will pass b-tagging just like signal
Top pair, WZ, ZZ, etc. Small contribution, e.g. tt l+lbb is reduced by l+l invariant mass cut
MC Normalization Z( l+l) + jets normalized to #data under Z peak, other MC normalized to
Z+jets using NLO cross sections Multijet background – QCD process
e+e channel: selecting QCD enhanced sample from data by inverting the electron shower shape and track-matching requirement, then fitting Mee distribution to normalize QCD sample.
+ channel: fitting M distribution of data with a Gaussian (Z) and an exponential (Drell-Yan + QCD) to get the fraction of QCD, depending on jet multiplicity and b-tagging.
DPF 2006, Honolulu Shaohua Fu, Fermilab 7
Data and Backgrounds
Z peak for Z( e+e) + 2 jets (before b-tagging) 2900 events within 65 GeV < Mee < 115 GeV 102 events estimated for QCD background Total background estimated to be 2860 470 events Signal ZH (mH=115 GeV): 0.78 0.03 events
Mee Z pT
DPF 2006, Honolulu Shaohua Fu, Fermilab 8
Data and Backgrounds
Z( l+l) + 2 jets (before b-tagging): e+e and + combined 5386 events observed in data 5610 930 events expected as total background Signal ZH (mH=115 GeV): 1.50 0.06 events
Leading and second lepton pT distributions:
DPF 2006, Honolulu Shaohua Fu, Fermilab 9
Data and Backgrounds
Z( l+l) + 2 jets (before b-tagging): e+e and + combined Leading and second jet pT distributions:
Then apply b-tagging
DPF 2006, Honolulu Shaohua Fu, Fermilab 10
With 0 b-tagged jet
0 b-tagged jet (Mjj 60~110 GeV)
e+e +
Data 832 647
Bckg. 808 134 671 111
QCD
Z + jets
Z + bb
tt
ZZ
WZ
28.5
740
19.3
1.0
10.9
8.7
6.71
662
21.2
0.9
6.9
6.6
ZH 115 0.111 0.004
0.11 0.004
#events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel
DPF 2006, Honolulu Shaohua Fu, Fermilab 11
Exclusive Single b-tagging
1 b-tagged jet (Mjj 60~110 GeV)
e+e +
Data 126 99
Bckg. 111 22 88 18
QCD
Z + jets
Z + bb
tt
ZZ
WZ
4.4
82.4
17.8
2.0
2.2
2.3
0.4
75.5
16.0
1.8
1.4
2.0
ZH 115 0.243 0.011
0.21 0.009
#events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel
DPF 2006, Honolulu Shaohua Fu, Fermilab 12
Inclusive Double b-tagging
2 b-tagged jets (Mjj 60~110 GeV)
e+e +
Data 8 10
Bckg. 9.8 3.4 8.7 3.1
QCD
Z + jets
Z + bb
tt
ZZ
WZ
0.25
3.7
3.8
1.3
0.11
0.75
1.8
3.7
3.4
1.2
0.07
0.65
ZH 115 0.169 0.014
0.14 0.012
#events in di-jet range 60 (70) ~ 110 GeV for e+e (+) channel
DPF 2006, Honolulu Shaohua Fu, Fermilab 13
Systematic Uncertainties
Sources of uncertainties Lepton identification efficiency uncertainty: 4% Jet energy scale correction uncertainty: 1-7% for backgrounds, and 1-2% for
ZH signals b-tagging uncertainty (double b-tagging): 7-8% for backgrounds and signals NLO cross section uncertainty: 15% for Z+jets, 50% for Z+bb, 6-8% for
other backgrounds QCD normalization uncertainty: 30%
Total uncertainties Correlations among each background uncertainty taken into account For double b-tagging: 35% on total background, 9% on ZH signals
DPF 2006, Honolulu Shaohua Fu, Fermilab 14
Cross Section Limits
e+e and + combined No excess observed, so set cross section limits Modified frequentist approach (CLS), using di-jet mass distribution
95% C.L. upper limits on ZH cross section: 3.3-1.6 pb for mH=105-155 GeV
*CDF results with NN selection
*
DPF 2006, Honolulu Shaohua Fu, Fermilab 15
Summary
ZH l+lbb searches performed at DØ in e+e and + final states, using about 1 fb–1 integrated luminosity
No excess of events is observed, thus upper limits on ZH cross section are set as 3.3-1.6 pb for mH=105-155 GeV
ZH mH=115 GeV cross section limit is about 30 times of the SM production
To improve sensitivity Optimization techniques
(e.g. Neural Net selection) Better detector understanding Layer0 silicon detector
better b-tagging Combine all channels More data!
Int.
Lum
inos
ity p
er E
xp.
(fb-1
)
LEP
Tevatron
8
4
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