  A search for dijet resonances in events with at least one isolated charged
lepton is performed using $139~{\text{fb}}^{-1}$ of $\sqrt{s}=13$ TeV
proton-proton collision data recorded by the ATLAS detector at the LHC. The
dijet invariant-mass ($m_{jj}$) distribution constructed from events with at
least one isolated electron or muon is searched in the region $0.22 < m_{jj} <
6.3$ TeV for excesses above a smoothly falling background from Standard Model
processes. Triggering based on the presence of a lepton in the event reduces
limitations imposed by minimum transverse momentum thresholds for triggering on
jets. This approach allows smaller dijet invariant masses to be probed than in
inclusive dijet searches, targeting a variety of new-physics models, for
example ones in which a new state is produced in association with a
leptonically decaying $W$ or $Z$ boson. No statistically significant deviation
from the Standard Model background hypothesis is found. Limits on contributions
from generic Gaussian signals with widths ranging from that determined by the
detector resolution up to 15% of the resonance mass are obtained for dijet
invariant masses ranging from 0.25 TeV to 6 TeV. Limits are set also in the
context of several scenarios beyond the Standard Model, such as the Sequential
Standard Model, a technicolor model, a charged Higgs boson model and a
simplified Dark Matter model.
