A neutron (udd) decays to a proton (uud), an electron, and an antineutrino. This is called neutron beta decay. (The term beta ray was used for electrons in nuclear decays because they didn't know they were electrons!)

In frame 2, one of the d quarks is transformed into a u quark with the emission of a virtual W- boson. In frame 3, the new u quark rebounds away from the emitted W-. The neutron now has become a proton.

The intermediate stages of this process occur in less than almost a billionth of a billionth of a billionth of a second, and are not observable.

In frame 4, the electron and antineutrino emerge from the virtual W- boson. In frame 5, the proton, electron, and the antineutrino move away from one another.

Radioactivity