Improved sensitivity to charged Higgs searches in Top quark decays $t\to b{H}^{+}\to b({\tau }^{+}{\nu }_{\tau })$ at the LHC using $\tau$ polarisation and multivariate tecnniques

Abstract

We present an analysis with improved sensitivity to the light charged Higgs ($m_{H^{+}}<m_t-m_b$) searches in the top quark decays $t\to b{H}^{+}\to b({\tau }^{+}{\nu }_{\tau })+c.c.$ 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 $\mathcal{B}(H^{+}\to {\tau }^{+}{\nu }_{\tau })\simeq 1$ 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.

Comments: 32 pages, 21 figures; corrected numerical errors; added analysis for LHC@7 TeV; version accepted for publication in the Eur. Phys. J. C