Constraining the Higgs width in Higgs production associated with a top quark pair

Abstract: In this thesis we study the width of the Higgs boson in the process pp → t ̄t 4l at LO using MadGraph5_aMC@NLO to generate the events. The contributions including the Higgs signal, continuum background and interference were considered in order to calculate the expected number of events in a broad range of four-lepton invariant masses. Due to strongly enhanced off-shell contributions an upper bound on the Higgs width could be derived. By assuming that the coupling constants scale by a multiplicative constant, allowing the on-shell cross section to be compatible with the Standard Model for different Higgs widths, we calculate the expected number of events in the off-peak region using an integrated luminosity of 3000 fb−1. Considering the statistical uncertainty of the expected number of events for the process with the SM width we find an upper bound on the Higgs width Γ_H ≤ 3.54 Γ_H^SM at the 95% confidence level. This result translates into an upper limit on the branching ratio to invisible final states BRinv ≤ 0.47. We believe that the result can be improved by a more careful selection of the four-lepton invariant mass range. The major assumptions we use are that the top quarks can be perfectly reconstructed from their decay products and that we have an ideal detector. The cross section for this process is very small and therefore it is not particularly competitive to the limit on the Higgs width that can be derived from pp → H → 4l not including the top quarks. However, it still provides additional information which improves the understanding of the Higgs width.