When feebly interacting massive particles decay into neutrinos: The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math> story
Physical review. D/Physical review. D.