= Nuclear matrix element
{title2=$\mathcal M=\langle f|\widehat O|i\rangle$}
A transition-operator <matrix element> between specified nuclear states. It combines their many-nucleon wavefunctions with the relevant interaction operator. In <double beta decay>, different assumed mechanisms and nuclear-structure calculations give different <matrix elements>, so converting a half-life bound to a neutrino-mass bound requires that input. A measured <energy> endpoint alone does not determine it.
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