A mass reconstruction technique for a heavy resonance decaying to τ⁺τ^-

Abstract: For a resonance decaying to τ⁺τ^-, it is difficult to reconstruct its mass accurately because of the presence of neutrinos in the decay products of the τ leptons. If the resonance is heavy enough, we show that its mass can be well determined by the momentum component of the τ decay products perpendicular to the velocity of the τ lepton, p_⊥, and the mass of the visible/invisible decay products, m_vis/inv, for τ decaying to hadrons/leptons. By sampling all kinematically allowed values of p_⊥ and m_vis/inv according to their joint probability distributions determined by the MC simulations, the mass of the mother resonance is assumed to lie at the position with the maximal probability. Since p_⊥ and m_vis/inv are invariant under the boost in the τ lepton direction, the joint probability distributions are independent upon the τ's origin. Thus this technique is able to determine the mass of an unknown resonance with no efficiency loss. It is tested using MC simulations of the physics processes m pp→Z/h(125)/h(750)+X→ ττ+X at 13 TeV. The ratio of the full width at half maximum and the peak value of the reconstructed mass distribution is found to be 20%-40% using the information of missing transverse energy.