Search for heavy Higgs bosons with flavour-violating couplings in multi-lepton plus $b$-jets final states in $pp$ collisions at 13 TeV with the ATLAS detector

A search for new heavy scalars with flavour-violating decays in final states with multiple leptons and $b$-tagged jets is presented. The results are interpreted in terms of a general two-Higgs-doublet model involving an additional scalar with couplings to the top-quark and the three up-type quarks (${\rho }_{tt}$, ${\rho }_{tc}$, and ${\rho }_{tu}$). The targeted signals lead to final states with either a same-sign top-quark pair, three top-quarks, or four top-quarks. The search is based on a data sample of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139f b${}^{-1}$. Events are categorised depending on the multiplicity of light charged leptons (electrons or muons), total lepton charge, and a deep-neural-network-based categorisation to enhance the purity of each of the signals. Masses of an additional scalar boson $m_H$ between $200-630$ GeV with couplings ${\rho }_{tt}=0.4$, ${\rho }_{tc}=0.2$, and ${\rho }_{tu}=0.2$ are excluded at 95% confidence level. Additional interpretations are provided in models of $R$-parity violating supersymmetry, motivated by the recent flavour and $(g-2{)}_{\mu }$ anomalies.