Bose-enhanced Neutrino Decays in a Thermal Medium

Abstract

We compute the decay width of neutrinos in a thermal medium using finite-temperature quantum field theory, focusing on non-standard decays into lighter neutrinos and a scalar or light vector boson. We derive general expressions for the thermal decay rate and show that finite-temperature effects can dramatically enhance neutrino decays when the parent and daughter states are nearly degenerate in mass. In this regime, the emitted boson is kinematically soft and undergoes strong Bose enhancement, leading to decay widths that can exceed their vacuum values by a couple of orders of magnitude. We demonstrate that this effect is largely insensitive to the Lorentz structure of the underlying interaction and arises generically from the interplay of thermal occupation factors and quasi-degenerate kinematics. Our results highlight a previously underappreciated feature of neutrino decay in thermal environments and provide a general framework applicable to a broad class of fermionic decay processes.