Searches for BSM physics using challenging and long-lived signatures with the ATLAS detector

Various theories Beyond the Standard Model (BSM) predict new, Long-Lived Particles (LLP) with unique signatures which are difficult to reconstruct and for which estimating the background rates is also a challenge. LHC collaborations have been looking for such particles since Run-1, exploiting experimentally challenging signatures such as the ones from displaced and/or delayed decays, as well as those of new particles with fractional or multiple values of the charge of the electron or high mass stable charged particles. We will focus on the most recent results using 13 TeV $pp$ collision data collected by the ATLAS detector.