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Improved sensitivity to charged Higgs searches in Top quark decays $t \to bH^+ \to b (τ^+ν_τ)$ at the LHC using $τ$ polarisation and multivariate tecnniques

We present an analysis with improved sensitivity to the light charged Higgs ($m_{H^+} < m_t-m_b$) searches in the top quark decays $t \to b H^+ \to b (τ^+ν_τ) + ~{\rm c.c.}$ 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 ${\cal B} (H^+ \to τ^+ν_τ)\simeq 1$ over almost the entire allowed $\tan β$ range. Noting that the $τ^+$ arising from the decay $H^+ \to τ^+ν_τ$ are predominantly right-polarized, as opposed to the $τ^+$ from the dominant background $W^+ \to τ^+ν_τ$, which are left-polarized, a number of $H^+/W^+ \to τ^+ν_τ$ discriminators have been proposed and studied in the literature. We consider hadronic decays of the $τ^\pm$, concentrating on the dominant one-prong decay channel $τ^\pm \to ρ^\pm ν_τ$. The energy and $p_T$ of the charged prongs normalised to the corresponding quantities of the $ρ^\pm$ are convenient variables which serve as $τ^\pm$ 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.