The weak interaction causes beta decay

Particles, Antiparticles & Photons — AQA A-Level Physics

Beta-minus decay and the W⁻ boson

$\beta^{-}$ decay: $n \rightarrow p + e^{-} + \bar{\nu}_e$
The $W^{-}$ boson mediates this interaction.
A neutron decays into a proton and a $W^{-}$ boson. The $W^{-}$ then decays into an electron and an anti-electron neutrino.

Beta-plus decay and the W⁺ boson

$\beta^{+}$ decay: $p \rightarrow n + e^{+} + \nu_e$
The $W^{+}$ boson mediates this interaction.
A proton decays into a neutron and a $W^{+}$ boson. The $W^{+}$ then decays into a positron and an electron neutrino.

Electron capture and electron-proton collisions

Both have the same equation: $p + e^{-} \rightarrow n + \nu_e$
Electron capture: a proton absorbs an orbital electron. Mediated by the $W^{+}$ boson (proton acts on electron).
Electron-proton collision: an electron collides with a proton. Mediated by the $W^{-}$ boson (electron acts on proton).
Same products, different exchange bosons, different Feynman diagrams.

Examiner Tips and Tricks

The sign of the W boson matches the type of beta decay: $W^{-}$ in $\beta^{-}$ decay, $W^{+}$ in $\beta^{+}$ decay.