A New Mass Reconstruction Technique for Resonances Decaying to di-tau
Accurate reconstruction of the mass of a resonance decaying to a pair of $τ$ leptons is challenging because of the presence of multiple neutrinos from $τ$ decays. The existing methods rely on either a partially reconstructed mass, which has a broad spectrum that reduces sensitivity, or the collinear approximation, which is applicable only to the relatively small fraction of events. We describe a new technique, which provides an accurate mass reconstruction of the original resonance and does not suffer from the limitations of the collinear approximation. The major improvement comes from replacing assumptions of the collinear approximation by a requirement that mutual orientations of the neutrinos and other decay products are consistent with the mass and decay kinematics of a $τ$ lepton. This is achieved by minimizing a likelihood function defined in the kinematically allowed phase space region. In this paper we describe the technique and illustrate its performance using $Z/γ^*\toττ$ and $H\toττ$ events simulated with the realistic detector resolution. The method is also tested on a clean sample of data $Z/γ^*\toττ$ events collected by the CDF experiment at the Tevatron. We expect that this new technique will allow for a major improvement in searches for the Higgs boson at both the LHC and the Tevatron.