Search for light charged Higgs boson in $t\rightarrow H^{\pm} + b (H^{\pm}\rightarrow cb)$ decays with the ATLAS detector at LHC

A search for light charged Higgs boson (60$\leq m_{H^{\pm}}\leq$160~GeV) in $t \rightarrow bH^{\pm}$ decay is presented. The analysis focuses on top-quark pair events in which one top quark decays to $Wb$, with the $W$ boson decaying leptonically, and the other top quark decays to $bH^{\pm}$, with $H^{\pm}$ subsequently decaying to a charm quark and a bottom quark ($H^{\pm}\rightarrow cb$). The search is based on $pp$ collisions at $\sqrt{s} = 13$~TeV recorded by the ATLAS detector at the LHC and uses an integrated luminosity of 139~fb$^{-1}$. The process results in the lepton-plus-jets final state, characterized by an isolated electron or muon and at least four jets. The search exploits the high b-jet multiplicity in signal events and employs a neural network discriminant that uses the kinematic differences between the signal and the background, which is dominated by a top-quark pair production.