Improved sensitivity to charged Higgs searches in Top quark decays $t   \to bH^+ \to b (\tau^+\nu_\tau)$ at the LHC using $\tau$ 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 $t \to b H^+ \to b (\tau^+\nu_\tau) + ~{\rm c.c.}$ 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. 
Ahmed Ali (DESY), Fernando Barreiro and Javier Llorente (UAM, Madrid)
hep-ph hep-ex