Improved sensitivity to charged Higgs searches in top quark decays t→bH +→b(τ + ν τ ) at the LHC using τ polarisation and multivariate techniques

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→bH +→b(τ + ν τ )+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 ${\mathcal{B}} (H^{+} \to\tau^{+}\nu_{\tau})\simeq1$ over almost the entire allowed $\tan\beta$ range. Noting that the τ + arising from the decay H +→τ + ν τ are predominantly right-polarized, as opposed to the τ + from the dominant background W +→τ + ν τ , which are left-polarized, a number of H +/W +→τ + ν τ discriminators have been proposed and studied in the literature. We consider hadronic decays of the τ ±, concentrating on the dominant one-prong decay channel τ ±→ρ ± ν τ . The energy and p T of the charged prongs normalised to the corresponding quantities of the ρ ± are convenient variables which serve as τ ± 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→bH + (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.