Lower bound on e⁺e^- decay of massive neutrinos

Abstract

Astronomical observations of SN1987A, such as the light curve, spectral intensities of lines, the X-ray emission, etc., constrain the lifetime for the decay of a heavy neutrino 1≲ m_VH ≤50 MeV through V_H → µ₁+e⁺+e^- to exceed 4×10¹⁵ exp(-m_VH/5 MeV) s. Otherwise, resulting ionization energy deposits and stronger X-ray emission would have been observed. This coupled with traditional cosmological considerations argues that the lifetime of Τ-neutrinos probably exceeds the age of the universe. This in turn would imply the standard cosmological mass bound does apply to µ_Τ, namely m_νΤ ≲100h²eV (where h is the Hubble constant in units of 100 km s^-1Mpc^-1. The only significant loophole for these latter arguments would be if V_Τ primarily decays rapidly into particles having very weak interactions.