Flavor changing top decays to charm and Higgs with $ττ$ at the LHC

Abstract: We investigate the prospects of discovering the top quark decay into a charm quark and a Higgs boson ($t \to c h^0$) in top quark pair production at the CERN Large Hadron Collider (LHC). A general two Higgs doublet model is adopted to study flavor changing neutral Higgs (FCNH) interactions. We perform a parton level analysis as well as Monte Carlo simulations using \textsc{Pythia}~8 and \textsc{Delphes} to study the flavor changing top quark decay $t \to c h^0$, followed by the Higgs decaying into $\tau^+ \tau^-$, with the other top quark decaying to a bottom quark ($b$) and two light jets ($t\to bW\to bjj$). To reduce the physics background to the Higgs signal, only the leptonic decays of tau leptons are used, $\tau^+\tau^- \to e^\pm\mu^\mp +\slashed{E}T$, where $\slashed{E}_T$ represents the missing transverse energy from the neutrinos. In order to reconstruct the Higgs boson and top quark masses as well as to effectively remove the physics background, the collinear approximation for the highly boosted tau decays is employed. Our analysis suggests that a high energy LHC at $\sqrt{s} = 27$ TeV will be able to discover this FCNH signal with an integrated luminosity $\mathcal{L} = 3$ ab$^{-1}$ for a branching fraction ${\cal B}(t \to ch^0) \agt 1.4 \times 10^{-4}$ that corresponds to a FCNH coupling $|\lambda{tch}| \agt 0.023$. This FCNH coupling is significantly below the current ATLAS combined upper limit of $|\lambda_{tch}| = 0.064$.