Many theories of beyond Standard Model physics include new light, weakly-coupled particles, which can be challenging to search for experimentally. I will talk about two observational probes of such particles. The first is based on “stellar cooling”: if new particles are produced in the hot cores of stars, they can escape from the star and carry away energy, affecting its structure and evolution. I’ll describe how the plasma environment in stellar cores can parametrically alter the rates for these process, and how this can significantly change the constraints and discovery potential for some new particle candidates.

I will also discuss searches for light vectors at colliders. Unless these couple to a fully conserved SM current, the production rate for longitudinal modes is enhanced by (energy / vector mass)^2. This is true even if the current is only broken at loop level, as for anomalous vectors, and can result in significantly improved constraints on many models of phenomenological interest.