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!)

• Frame 1: The neutron (charge = 0) made of up, down, down quarks.
• Frame 2: One of the the down quarks is transformed into an up quark. Since the down quark has a charge of -1/3 and and the up quark has a charge of 2/3, it follows that this process is mediated by a virtual W- particle, which carries away a (-1) charge (thus charge is conserved!)
• Frame 3: The new up quark rebounds away from the emitted W-. The neutron now has become a proton.

• Frame 4: An electron and antineutrino emerge from the virtual W- boson.
• Frame 5: The proton, electron, and the antineutrino move away from one another.

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