Search for Standard Model Higgs boson using the H → ZZ → 2l2ν channel in pp collisions at CMS

A search for the Standard Model (SM) Higgs boson in pp collisions at the LHC at a center-of-mass energy of 7 TeV is presented. The results are based on a data sample corresponding to an integrated luminosity of 1.6 f b−1 recorded by the CMS experiment. The search is conducted in the decay channel H → ZZ → 2l2ν. No excess is observed in the transverse mass distributions. Limits are set on the production of the Higgs boson in the context of the Standard Model and in the presence of a sequential fourth family of fermions with high masses.


Introduction
The search for the SM Higgs boson and its discovery are central to the goals of the experiments at the LHC. The primary production mechanism of the Higgs at the LHC is gluon fusion with the small contribution from vector boson fusion. This note presents search for SM Higgs boson in the H → ZZ → 2l2ν channel which is sensitive to Higgs searches in the high mass range (250 − 600GeV/c 2 ). The braching fraction of this decay channel is about six times higher than that of golden channel H → ZZ → l − l + l − l + . This may lead to better sensitivity to SM Higgs boson production at higher masses, where background can be effectively suppressed kinematically.

Event Selection
Since, this analysis is carried out for Higgs mass above 250GeV/c 2 so H → ZZ → 2l2ν event is characterized by the presence of a boosted Z boson decaying to e + e − or µ + µ − and large missing transverse energy (MET) arising from the decay of the other Z boson into neutrinos. Muons are measured with the silicon tracker and the muon system [3]. Further identification criteria based on the number of hits in the tracker and muon system, the fit quality of the muon track and its consistency with the primary vertex, are imposed on the muon candidates to reduce fakes. Electrons are detected in the ECAL as energy clusters and as tracks in the tracker [4]. These reconstructed electrons are further required to pass certain identification criteria based on the ECAL shower shape, track-ECAL cluster matching and consistency with the primary vertex. They are measured in pseudorapidity range |η| < 2.4 for muons and |η| < 2.5 for electrons, though for electrons the transition range between the barrel and endcap, 1.44 < |η| < 1.57, is excluded. Events are selected such that there are two well-identified, isolated, opposite charge leptons of the same flavor with p T > 20GeV/c that form an invariant mass consistent with a e-mail: Arun.Kumar@cern.ch b e-mail: Kirti.Ranjan@cern.ch Z mass. With this selection the principal backgrounds in this analysis are: -Z+jets: with fake missing transverse energy due to jet mismeasurement and detector effects.
see Table 2 MET see Table 2 M T see Table 2

Background Estimation
ZZ/WZ backgrounds are modeled using simulation, while the remaining backgrounds (Z+jets and all non-resonant ones) are estimated using the data-driven methods descibed below.

Z+jets Estimation
Photon+jets (γ+jets) events are used to estimate the Z+jets events in data because both processes have same MET response from detector and γ+jets have higher cross section than that of Z+jets. Re-weighting of γ+jets data is done event-by-event, so that the p T spectrum of photon agrees with the observed dilepton p T spectrum. An additional reweighting is done to match the jet multiplicity between γ+jets and dilepton events in the 0 and 1 jet bins (jets with p T > 30 GeV/c are considered). Then, a mass sampled from a fit to the observed Z mass spectrum is assigned to each photon. The yield of γ+jets events is normalized to the observed yield of dilepton events. The transverse mass M T is computed and the full analysis selection is applied to the weighted γ+jets events. This procedure produces an accurate model of the MET distribution in Z+jets as can be seen in Fig1 and Fig2.

Top/WW/W+jets Estimation
To estimate the non-resonant background using data, events with final state comprising of e + µ − /e − µ + pairs and passing the full analysis selection are used. The non-resonant background in the e + e − /µ − µ + final states is estimated by  applying a scale factor (α) to these events: This scale factor α is computed from the sidebands (SB) to the Z peak (40GeV/c 2 <m H < 70GeV/c 2 and 110GeV/c 2 < m H < 200GeV/c 2 ) using the following relations: where N S B ee , N S B µµ and N S B eµ are events in the sidebands in the e + e − , µ + µ − and e + µ − /e − µ + final states respectively, passing all the analysis requirements that are independent of the Higgs mass (with the exception of anti-btag) and MET>70 GeV. This method cannot distinguish between the non-resonant background and H → WW → 2l2ν events, which are very small. Table3 and Table4 list the predicted yields for the non-resonant backgrounds with integrated luminosity of 1.6 f b −1 . Statistical uncertainties on these estimates are also quoted.

Systematics and Results
The systematic uncertainies are summarized in Table5. No evidence of SM Higgs boson production is found in H → ZZ → 2l2ν channel with integrated luminosity of 1.6 f b −1 . The 95% mean expected and observed C.L. upper limits on the cross section, σ × BR(H → ZZ → 2l2ν), for masses in the range 250-600GeV/c 2 has been measured.
Results are obtained using a CLs approach with a flat prior for the cross-section. The ratio R of the σ upperlimit to the σ S M at 95% CL is shown in Fig.3. σ upperlimit as a function of the Higgs mass m H at 95% CL has also been measured and is shown in Fig.4. With 1.6 f b −1 , the SM Higgs with masses in the range 340-375 GeV/c 2 can be excluded at 95% CL.