Invariant mass reconstruction in a search for light charged Higgs bosons in $pp$ collisions at $\sqrt{s}$ = 7 TeV

An algorithm for invariant mass reconstruction in a search for light charged Higgs bosons (H⁺) produced in top quark decays and decaying to a tau lepton and neutrino, H⁺ -> τν, is presented. Here, 'light' means lighter than the top quark. The algorithm uses the top quark mass as a kinematical constraint to allow the calculation of the longitudinal momentum of the neutrinos. The invariant mass of the tau-and-neutrino system is then calculated using the missing transverse energy, the calculated longitudinal momentum of the neutrinos, and the measured momentum of the visible decay products of the tau lepton. Methods for resolving ambiguities and recovering unphysical results arising in the invariant mass reconstruction are presented. The invariant mass distribution could be used to extract a possible signal, replacing or complementing the transverse mass distribution that has been used so far in the analysis. In a preliminary data analysis using proton-proton collision data at sqrt(s) = 7 TeV corresponding to an integrated luminostity of 5.1 / fb recorded by the CMS experiment, it is shown that using invariant mass distribution obtained with the presented algorithm allows to set a more stringent upper limit on the signal branching fraction Br(t -> bH⁺) × Br(H⁺ -> τν) than does using the transverse mass distribution. An expected upper limit at the 95% confidence level between around 0.37% to 2.5% (transverse mass) and 0.13% to 1.9% (invariant mass) is found, depending on the H⁺ mass. These results suggest that using the invariant mass reconstructed with the new algorithm may improve the signal sensitivity of the search.