Search for Standard Model Higgs in ZH  l + l  bb channel at DØ Shaohua Fu Fermilab For the DØ Collaboration DPF 2006, Oct. 29 – Nov. 3 Honolulu, Hawaii Main Injector & Recycler (new) Tevatron DØ p source CDF Chicago Booster pp 1.96 TeV

DPF 2006, HonoluluShaohua Fu, Fermilab2 Tevatron and DØ Detector Run II goal: 4 ~ 8 fb  1 in 2009 Analyses presented here used pb  1 Used almost the whole Run IIa data ~ 1fb  1 DØ detector in Run IIa Run IIb: Layer0 for SMT, trigger upgrade

DPF 2006, HonoluluShaohua Fu, Fermilab3 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: m H > GeV at 95% C.L. Indirect limit from fits to precision EW measurements from LEP, SLC, and Tevatron: m H < 166 GeV at 95% C.L. (< 199 GeV if LEP2 limit included) Indirect best fit value:  28 GeV at 68% C.L. A light Higgs is favored LEPEWWG July 2006

DPF 2006, HonoluluShaohua Fu, Fermilab4 SM Higgs Production and Decay SM Higgs production at Tevatron Gluon fusion  ~ pb (m H GeV) Associated production with a W or Z boson  ~ pb Dominant decays Low mass (m H 135 GeV): H  W + W  This analysis: ZH  l + l  bb, in e + e  and  +   channels Excluded at LEP

DPF 2006, HonoluluShaohua Fu, Fermilab5 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 p T > 15 GeV, |  | < 1.1 or 1.5 < |  | < 2.5, central track match Z candidate: 65 GeV < Mee < 115 GeV  +   sample 2 muons p T > 15 GeV, central track matched (|  | < 2.5), and isolated Z candidate: 70 GeV 20 GeV At least 2 jets p T > 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) B Impact Parameter Decay Length Hard Scatter (Signed) Track

DPF 2006, HonoluluShaohua Fu, Fermilab6 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 + l  bb 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, HonoluluShaohua Fu, Fermilab7 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 (m H =115 GeV): 0.78  0.03 events MeeZ p T

DPF 2006, HonoluluShaohua Fu, Fermilab8 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 (m H =115 GeV): 1.50  0.06 events Leading and second lepton p T distributions:

DPF 2006, HonoluluShaohua Fu, Fermilab9 Data and Backgrounds Z(  l + l  ) +  2 jets (before b-tagging): e + e  and  +   combined Leading and second jet p T distributions: Then apply b-tagging

DPF 2006, HonoluluShaohua Fu, Fermilab10 With 0 b-tagged jet 0 b-tagged jet (Mjj 60~110 GeV) e+ee+e ++ Data Bckg. 808   111 QCD Z + jets Z + bb tt ZZ WZ ZH   #events in di-jet range 60 (70) ~ 110 GeV for e + e  (  +   ) channel

DPF 2006, HonoluluShaohua Fu, Fermilab11 Exclusive Single b-tagging 1 b-tagged jet (Mjj 60~110 GeV) e+ee+e ++ Data12699 Bckg. 111  2288  18 QCD Z + jets Z + bb tt ZZ WZ ZH   #events in di-jet range 60 (70) ~ 110 GeV for e + e  (  +   ) channel

DPF 2006, HonoluluShaohua Fu, Fermilab12 Inclusive Double b-tagging  2 b-tagged jets (Mjj 60~110 GeV) e+ee+e ++ Data810 Bckg. 9.8   3.1 QCD Z + jets Z + bb tt ZZ WZ ZH   #events in di-jet range 60 (70) ~ 110 GeV for e + e  (  +   ) channel

DPF 2006, HonoluluShaohua Fu, Fermilab13 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, HonoluluShaohua Fu, Fermilab14 Cross Section Limits e + e  and  +   combined No excess observed, so set cross section limits Modified frequentist approach (CL S ), using di-jet mass distribution 95% C.L. upper limits on ZH cross section: pb for m H = GeV *CDF results with NN selection *

DPF 2006, HonoluluShaohua Fu, Fermilab15 Summary ZH  l + l  bb 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 pb for m H = GeV ZH m H =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. Luminosity per Exp. (fb -1 ) LEP Tevatron 8 4