A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle 𝑋 decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to $qX$, with $X\rightarrow b\bar{b}$, and the other top quark decays according to the Standard Model, with the $𝑊$ boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at

more »

... ast four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from $b$-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb$^{−1}$ of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. Observed (expected) 95% confidence-level upper limits between 0.019% (0.017%) and 0.062% (0.056%) are derived for the branching fraction $\mathscr{B}(t\rightarrow uX$) and between 0.018% (0.015%) and 0.078% (0.056%) for the branching fraction $\mathscr{B}(t \rightarrow cX)$, for masses of the scalar particle $X$ between 20 and 160 GeV.