Search for the standard model Higgs boson in Z H ---> nu anti-nu b anti-b channel at D0 Page: 3 of 4
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have #T > 40 GeV. To suppress multijet background we apply a cut on "#T significance" S, which
takes into account the resolution of jet energies to separate real #T from expected fluctuations in
measured jet energies. S > 5 ensures the observed #T is not due to such fluctuations. In signal
events, the vectorial sum p9k of the charged particle transverse momenta is expected to point in a
direction opposite to that of the #T, while this is not expected in multijet events where a jet energy
has been mismeasured. Advantage is taken of this feature by requiring D = AO(#T Pk) < ir/2. To
reject backgrounds from W+jets, top, and diboson production, events containing an isolated elec-
tron or muon are rejected. To define a sample dominated by the multijet background we require
D>w/2 and there we substract contributions from all simulated SM processes.
Monte Carlo simulation of the SM backgrounds is verified in a control sample mainly com-
posed of W+jets events, by requiring the presence of a muon instead of vetoing it. Multijet mod-
elling is verified in a seperate control sample by lowering the cut on #T without a cut on S.
Large branching fraction for H -> bb is utilised by b-tagging of one or both of the two leading
jets. The double tag sample is selected with asymmetric requirements on the output of a neural
network b tagging algorithm [9], such that one jet is tagged with an efficiency of 70% ("loose tag")
and other with an efficiency of 50% ("tight tag"). This asymmetric tagging procedure provides the
best sensivity to a Higgs boson signal. To enhance search sensitivity an independent single tag
sample is defined by requiring one of the jets to be tight tagged and other failes to be loose tag.
A BDT technique [10] is used for the final discrimination which takes advantage of the differ-
ent kinematics of signal and background processes. For each Higgs mass value (mH) a decision tree
is trained to discriminate signal against multijet background, using 23 kinematic variables, and is
optimized to remove most of the multijet background while retaining most of the signal. Additional
training is done to discriminate the signal from other SM backgrounds.
Systematic uncertainities are assigned and their impact is assessed in overall normalization
and shape of final discriminants. Agreement between data and expectation from SM and multijet
background is observed both for number of selected events and for distributions of final discrimi-
nants. A modified frequentist approach [11] is used to set limits on cross section for the SM Higgs
production. The test statistic is a joint log-likelihood ratio (LLR) of the background only and of sig-
nal+background hypothesis, obtained by summing LLR values over the bins of final discriminant.
The impact of systematic uncertainities on the sensitivity of analysis is reduced by maximizing a
profile likelihood function [12] in which these uncertainities are given Gaussian constraints asso-
ciated with their priors.
4. RESULTS
Fig. 1 and Table 1 shows the observed and expected limit [13], in terms of the ratio of the ex-
cluded cross section to the SM expected production cross section multiplied by branching fraction
of H -> bb. The LLRs are also shown in Fig. 1. For a Higgs boson of mass 115 GeV, the observed
and expected limits on the combined cross section of ZH and WH production multiplied by the
branching fraction of H -> bb are factors of 3.7 and 4.6 times the SM value, respectively.3
Standard Model Higgs Search
Abhinav Dubey
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Dubey, Abhinav & U., /Delhi. Search for the standard model Higgs boson in Z H ---> nu anti-nu b anti-b channel at D0, article, January 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1013503/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.