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Summary of the analysis strategy:

Higgs production processes

Gluon Fusion    VH→VZZ    Vector Boson Fusion    ZH→ZWW  

This analysis focuses on the search of a Standard Model Higgs Boson that decays into a Z0 boson pair, reconstructed in the 4 lepton final state. Even if the analysis is optimized to detect a Higgs produced through gluon fusion, the search in this final state is sensitive also to some minor contributions coming from associated production and Vector Boson Fusion (VBF). Therefore we will include in our search also signal coming from WH, ZH, VBF where the Higgs boson decays to ZZ and produce 4 leptons in the final state, as well as ZH→ZWW, where two leptons come from the associated Z decay and the other two from the W(→lν)W(→lν) decay.

Event selection

We select events that contains exactly 4 leptons reconstructed in the detector, that can be either 4 electrons, 4 muons or 2 electrons and 2 muons. We identify our sample requiring also that:
  • Leading lepton: pT > 20 GeV/c² (triggered the event)
  • All other leptons: pT > 10 GeV/c²
  • min(ΔR(leptons)) > 0.1
  • Mll-1, Mll-2 in the range 20 - 140 GeV/c².
In the case of 4e or 4μ, the leptons can be arranged in several ways to reconstruct the 2 Z's, we choose the combination for which the two dilepton invariant mass are the closest to the nominal mZ.

Background modeling

The signature considered is shared also by other processes, which are background contribution to our search.
  • Non resonant SM ZZ production: simulated using a PYTHIA based Monte Carlo simulation.
  • Mis-identified leptons background from Z(γ)+jets events: estimated from data and Monte Carlo simulation.
The rate of the background coming from fake leptons is evaluated from data, measuring in a jet dominated data sample the probability for a jet-like object to be reconstructed as a real lepton. This fake rate is then applied to events with 2 or 3 real leptons, obtaining the estimate for this background. The kinematic properties of interest for this analysis (in particular the 4 lepton invariant mass) for this background process are obtained from a Monte Carlo simulation of Z(→ll)γ.

Invariant mass distribution

The search for the Higgs boson signal in this final state is performed based on the four lepton invariant mass distribution. Comparing the Monte Carlo predicted distribution for the background processes to the data we can search for a narrow resonance, corresponding to events where the four reconstructed leptons come from the Higgs boson. These distributions are shown in the plot below, where added also is the prediction for the Higgs signal with different mass hypotheses. Since we see no evidence of a significant excess in the data with respect to the background prediction, we can set an upper limit on the Higgs production cross section.


Plots for several Higgs masses:

120 GeV/c²   130 GeV/c²   140 GeV/c²   150 GeV/c²   160 GeV/c²   170 GeV/c²   180 GeV/c²   190 GeV/c²   200 GeV/c²  
log   log   log   log   log   log   log   log   log  

95% C.L. Upper Limit

Using a Bayesian approach we set a 95% confidence level upper limit on the production cross section of the Standard Model Higgs boson. The results, for several Higgs mass hypotheses are summarazied in the plot and table below.
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 Updated by Matteo Bauce
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Last updated: July 18 2011.