  Many extensions to the Standard Model of particle physics hypothesize the
existence of new low-mass particles. Typically there are few theoretical
constraints on the mass or lifetime of such particles. This requires the
experimentalist to perform a search in which both the mass and lifetime of the
particle are unknown. Such searches for low-mass particles are complicated by
the possible presence of resonances and other non-monotonic backgrounds. This
paper presents a simple and fast approach to assigning significance and setting
limits in such searches.
