Kinematic reconstruction of $Z/H \rightarrow \tau\tau$ decay in proton-proton collisions.

The knowledge of $tau$ lepton kinematic and kinematic of the $tau$ pair in the decay $Z/H rightarrow tautau$ is essential for various analysis at LHC. However, the reconstruction of the whole kinematic of the $tau$ decay is a challenging task, since in every $tau$ decay at least one neutrino is present in the final state which escapes detection. In this paper a kinematic technique (Global Event Fit) to estimate the momentum escaped with neutrinos and hence the full momentum of the $tau$ lepton pair is described. The algorithm is based on iterative minimization of the likelihood with constrains derived from all available kinematic information on the decay. The method requires the direction of at least one $tau$ lepton to be well defined and therefore the method can be applied to the decays $Z/H rightarrow tautau rightarrow X + a_{1}nu$ with $a_{1}$ resonance decaying into three charged pions.