JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Modern Physics

A free neutron is slightly heavier than a proton (about $939.6\text{MeV}/c^2$ against $938.3\text{MeV}/c^2$), so it carries surplus rest energy to share among the products in $n\to p+e^{-}+{\bar{\nu }}_e$, making the decay energetically allowed. The reverse change $p\to n+e^{+}+{\nu }_e$ would require the lighter proton to turn into a heavier neutron, which energy conservation forbids for a free proton. The decisive factor is that the neutron has the larger rest mass.