  We present an analysis with improved sensitivity to the light charged Higgs
($m_{H^+} < m_t-m_b$) searches in the top quark decays $t \to b H^+ \to b
(\tau^+\nu_\tau) + ~{\rm c.c.}$ in the $t\bar{t}$ and single $t/\bar{t}$
production processes at the LHC. In the Minimal Supersymmetric Standard Model
(MSSM), one anticipates the branching ratio ${\cal B} (H^+ \to
\tau^+\nu_\tau)\simeq 1$ over almost the entire allowed $\tan \beta $ range.
Noting that the $\tau^+$ arising from the decay $H^+ \to \tau^+\nu_\tau$ are
predominantly right-polarized, as opposed to the $\tau^+$ from the dominant
background $W^+ \to \tau^+\nu_\tau$, which are left-polarized, a number of
$H^+/W^+ \to \tau^+\nu_\tau$ discriminators have been proposed and studied in
the literature. We consider hadronic decays of the $\tau^\pm$, concentrating on
the dominant one-prong decay channel $\tau^\pm \to \rho^\pm \nu_\tau$. The
energy and $p_T$ of the charged prongs normalised to the corresponding
quantities of the $\rho^\pm$ are convenient variables which serve as $\tau^\pm$
polariser. We use the distributions in these variables and several other
kinematic quantities to train a boosted decision tree (BDT). Using the BDT
classifier, and a variant of it called BDTD, which makes use of decorrelated
variables, we have calculated the BDT(D)-response functions to estimate the
signal efficiency vs. the rejection of the background. We argue that this chain
of analysis has a high sensitivity to light charged Higgs searches up to a mass
of 150 GeV in the decays $t \to b H^+$ (and charge conjugate) at the LHC. For
the case of single top production, we also study the transverse mass of the
system determined using Lagrange multipliers.

High Energy Physics - Phenomenology;High Energy Physics - Experiment