Search for dijet resonances in events with an isolated lepton using $\sqrt{s} = 13$~TeV proton--proton collision data collected by the ATLAS detector

A search for dijet resonances in events with identified leptons is performed using $79.8~{\text{fb}}^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data recorded by the ATLAS detector. Dijet invariant mass distributions ranging from $0.22-6.3$ TeV in events with at least one isolated lepton ($e$ or $\mu$) are searched for excesses above background expectations. Selecting events based on the presence of a lepton can probe lower $m_{jj}$ than traditional inclusive dijet searches and targets a large range of new physics models with a final-state lepton. Signatures of such processes include associated production of a $W$ or $Z$ boson. No statistically significant deviation from the background hypothesis is found. In the absence of signals indicating the presence of new physics, limits are calculated based on generic Gaussian signals with a width ranging from that determined by the detector resolution to $15\%$ of the resonance mass. Contributions from a signal with effective cross-section ranging from $50$ fb to $0.1$ fb are excluded in the mass range between 0.25 TeV and 6 TeV.