search for standard model higgs in zh l + l bb channel at d Ø

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


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


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


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


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.


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



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:



Z( l+l) + 2 jets (before b-tagging): e+e and + combined Leading and second jet pT distributions:

Then apply b-tagging


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


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



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



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


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

*


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

search for standard model higgs in zh l + l bb channel at d Ø

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