A search for pair production of vector-like leptons coupling to first- and second-generation Standard Model leptons is presented. The search is based on a dataset 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 140 fb$^{-1}$. Events are categorised depending on the flavour and multiplicity of leptons (electrons or muons), as well as on the scores of a deep neural network targeting particular signal topologies according to the decay modes of the vector-like leptons. In each of the signal regions, the scalar sum of the transverse momentum of the leptons and the missing transverse momentum is analysed. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No significant excess above the Standard Model background expectation is observed and limits are set at 95% confidence level on the production cross-sections of vector-like electrons and muons as a function of the vector-like lepton mass, separately for SU(2) doublet and singlet scenarios. The resulting mass lower limits are 1220 GeV (1270 GeV) and 320 GeV (400 GeV) for vector-like electrons (muons) in the doublet and singlet scenarios, respectively.

Comparison between data and the background prediction for the event yields in the (a) 10 VLLe SRs, (b) 10 VLLμ SRs, and (c) 9 CRs, after a background-only fit to data (`post-fit') in the (a) VLLe SRs and CRs and (b, c) VLLμ SRs and CRs. The background contributions in the CRs after the likelihood fit to data in the VLLe SRs and CRs are comparable to those in (c). Distributions for the VLLeD and VLLμD signal points for a VLL mass of 600 GeV are overlaid for comparison in (a) and (b), respectively. The lower panels show the ratio of data to the background estimate (`B.'), separately for post-fit background (black points) and pre-fit background (dashed blue line). The size of the combined statistical and systematic uncertainty in the background prediction is indicated by the blue hatched band.