  The knowledge of $\tau$ lepton kinematic and kinematic of the $\tau$ pair in
the decay $Z/H \rightarrow \tau\tau$ 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 $Z/H \rightarrow \tau\tau \rightarrow X +
a_{1}\nu$ with $a_{1}$ resonance decaying into three charged pions.

High Energy Physics - Phenomenology;High Energy Physics - Experiment