A neutron (udd) decays to a proton (uud),
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-
Frame 5: The proton, electron, and the antineutrino move away from
The intermediate stages of this process occur in
about a billionth of
a billionth of a billionth of a second, and are not observable.