  Boosted top quark tagging is one of the challenging, and at the same time
exciting, tasks in high energy physics experiments, in particular in the
exploration of new physics signals at the LHC. Several techniques have already
been developed to tag a boosted top quark in its hadronic decay channel.
Recently tagging the same in the semi-leptonic channel has begun to receive a
lot of attention. In the current study, we develop a methodology to tag a
boosted top quark ($p_T>$ 200 GeV) in its semi-leptonic decay channel with a
$\tau$-lepton in the final state. In this analysis, the constituents of the top
fatjet are reclustered using jet substructure technique to obtain the subjets,
and then $b$- and $\tau$- like subjets are identified by applying standard $b$-
and $\tau$-jet identification algorithms. We show that the dominant QCD
background can be rejected effectively using several kinematic variables of
these subjects, such as energy sharing among the jets, invariant mass,
transverse mass, Nsubjettiness etc., leading to high signal tagging
efficiencies. We further assess possible improvements in the results by
employing multivariate analysis techniques. We find that using this proposed
top-tagger, a signal efficiency of $\sim 77\%$ against a background efficiency
of $\sim 3\%$ can be achieved. We also extend the proposed top-tagger to the
case of polarized top quarks by introducing a few additional observables
calculated in the rest frame of the $b-\tau$ system. We comment on how the same
methodology will be useful for tagging a boosted heavy BSM particle with a $b$
and $\tau$ in the final state.
