  Searches for new resonances are performed using an unsupervised
anomaly-detection technique. Events with at least one electron or muon are
selected from 140 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV recorded
by ATLAS at the Large Hadron Collider. The approach involves training an
autoencoder on data, and subsequently defining anomalous regions based on the
reconstruction loss of the decoder. Studies focus on nine invariant mass
spectra that contain pairs of objects consisting of one light jet or $b$-jet
and either one lepton ($e$, $\mu$), photon, or second light jet or $b$-jet in
the anomalous regions. No significant deviations from the background hypotheses
are observed.
