Existing limits on the nonradiative decay of one neutrino to another plus a massless particle (e.g., a singlet Majoron) are very weak. The best limits on the lifetime to mass ratio come from solar neutrino observations and are $\tau /m\gtrsim {10}^{-4}\mathrm{s}/\mathrm{e}\mathrm{V}$ for the relevant mass eigenstate(s). For lifetimes even several orders of magnitude longer, high-energy neutrinos from distant astrophysical sources would decay. This would strongly alter the flavor ratios from the ${\varphi }_{{\nu }_e}:{\varphi }_{{\nu }_{\mu }}:{\varphi }_{{\nu }_{\tau }}=1:1:1$ expected from oscillations alone and should be readily visible in the near future in detectors such as IceCube.

• Received 19 November 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.181301