A long-lived particle (LLP) is any hypothetical new particle that has a "macroscopic" lifetime, and can e.g. fly a macrocopic distance through a detector before decaying. New particles beyond the standard model might have long lifetimes for several reasons, including approximate symmetries that stabilize the LLP, small couplings between the LLP and lighter states, and suppressed phase space available for decays . LLP signatures at the LHC can include tracks with unusual ionisation and propagation properties; small, localized deposits of energy inside of the calorimeters without associated tracks; stopped particles that decay out of time with collisions; displaced vertices in the inner detector or muon spectrometer; and disappearing, appearing, and kinked tracks .
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