Upper limits for the branching fractions of such decay modes were measured in many experiments starting more than 50 years ago.

Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the coupling constants.

These branching fractions are important inputs to B meson measurements at BaBar and Belle.

The observations which limit the branching fraction of the neutron in this decay channel to less than are all done looking for appropriate decays of nuclei ( and ).

In particle physics and nuclear physics, the branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay.