Why Neutrino Lines are Hypersharp

It was recently pointed out that mono-energetic neutrino lines from the 2-body decay of tritium (tau ~ 18-y) can be emitted, a significant fraction, with natural line width (~10-24 eV) for hypersharp resonance transitions 3H--> <--3He. The very long lifetime typical of a neutrino transition, encountered for the first time in resonance, is the key to this surprising effect which is not intuitive from perspectives of line broadening in resonances of short lived (tau ~ microsec) states.