Eur. Phys. J. C (2011) 71: 1737
https://doi.org/10.1140/epjc/s10052-011-1737-8

Special Article - Tools for Experiment and Theory

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

¹ Deutsches Elektronen-Synchrotron DESY, 22607, Hamburg, Germany
² Facultad de Ciencias C-XI, Departamento de Fisica, Universidad Autonoma de Madrid (UAM), Cantoblanco, Madrid, 28049, Spain

^* e-mail: ahmed.ali@desy.de

Received: 14 March 2011
Revised: 7 July 2011
Published online: 8 September 2011

Abstract

We present an analysis with improved sensitivity to the light charged Higgs () searches in the top quark decays t→bH ⁺→b(τ ⁺ ν _τ )+c.c. in the and single production processes at the LHC. In the Minimal Supersymmetric Standard Model (MSSM), one anticipates the branching ratio over almost the entire allowed 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.