Comprehensive programs of searches for physics beyond the Standard Model (BSM) have been carried out by the experimental collaborations at the LHC, but so far discoveries are conspicuous by their absence. Most searches look for signs of BSM particles decaying promptly to SM particles, many with final states with large multiplicities of high-energy SM particles.
I will discuss a category of theoretical scenarios that could have avoided our searches so far due to new states with experimentally significant lifetimes, and a class of promising but challenging detector signatures that could facilitate their discovery. After an overview, I will present two recent searches for such long-lived particles with the ATLAS experiment in some detail.
Finally, the High-Luminosity LHC will deliver ~20 times more integrated luminosity than has so far been recorded, and I will highlight an opportunity which is tightly coupled to the Uppsala ATLAS group's detector upgrade involvement for how we could better explore signatures with long-lived particles to maximize our the chances to find BSM physics in the next decade.