Cosmological and Astrophysical Bounds on Neutrino Masses and Lifetimes

The best upper bounds on the masses of stable and unstable light neutrinos derive from the upper bound on the total mass density, as inferred from the lower limit t0 > 13 Gyr on the dynamical age of the Universe: If the Universe is matterdominated, mν < 35(23)× max[1, (t0/τν) 1/2] eV, according as a cosmological constant is (is not) allowed. The best constraints on the radiative decay of light neutrinos derive from the failure to observe prompt gamma rays accompanying the neutrinos from Supernova 1987A: For any mν > 630 eV, this provides a stronger bound on the neutrino transition moment than that obtained from red giants or white dwarfs. For mν > 250 eV or τν < trec ∼ 7 × 10 12 sec, the upper limit on the radiative branching ratio is even smaller than that obtained from the limits on μ-distortion of the cosmic background radiation. Our results improve on earlier cosmological and radiative decay constraints by an overall factor twenty, and allow neutrinos more massive than 35 eV, only if they decay overwhelmingly into singlet majorons or other new particles. 1 Mass Limits on Stable and Unstable Neutrinos from the Age of the Universe The masses of stable neutrinos, Σmνi = 92Ω0νh 2 eV, are bounded by Ω0νh 2 < Ω0h , the total cosmological mass density in units of ρCRh −2 = 10.54 keV cm. The best constraint on Ω0h 2 does not come from poorly-known limits on Ω0 and the Hubble constant H0 = 100h km s −1 Mpc separately, but from the present dynamical age of the universe, believed to be t0 = (13 − 17) Gyr. Allowing the generous limits H0 = (50− 100) km s −1 Mpc, 0.66 < H0t0 < 1.7. ∗Contribution to the Franklin Institute Symposium honoring Fred Reines. An extension of an earlier paper appearing in Phys. Rev D45, 4720 (1992). Supported in part by DOE Contract No. DOE-AC02-76-ERO-3071.