The observation of long-lived particles at the LHC would reveal physics beyond the Standard Model could
account for the many open issues in our understanding of our universe and conceivably point to a more
complete theory of the fundamental interactions. Such long-lived particle signatures are fundamentally
motivated and can appear in virtually every theoretical construct that addresses the Hierarchy Problem, Dark
Matter, Neutrino Masses and the Baryon Asymmetry of the Universe. We describe in this document a large detector, MATHUSLA, located on the surface above an LHC pp interaction point that could observe longlived
particles with lifetimes up to the Big Bang Nucleosynthesis limit of 0.1 s at the HL-LHC. We also
note that its large detector area allows MATHUSLA to make important contributions to cosmic ray physics.
Because of the potential for making a major breakthrough in our conceptual understanding of the universe,
long-lived particle searches should have the highest level of priority.