  Yes, the photon. While a nonzero photon mass has been under experimental and
theoretical study for years, the possible implication of a finite photon
lifetime lacks discussion. The tight experimental upper bound of the photon
mass restricts the kinematically allowed final states of photon decay to the
lightest neutrino and/or particles beyond the Standard Model. We discuss the
modifications of the well-measured cosmic microwave background spectrum of free
streaming photons due to photon mass and lifetime and obtain model-independent
constraints on both parameters---most importantly a lower direct bound of 3 yr
on the photon lifetime, should the photon mass be at its conservative upper
limit. In that case, the lifetime of microwave photons will be time-dilated by
a factor order 10^(15).
