Do you remember how nuclei decay when the proportion of neutrons and protons isn't right? Let's
take a closer look at two of these decays, the beta and the positron decay.
You can see a beta decay in the animation on the right; a down quark turns
into an up quark and emits a W- particle in the
process. The W- in turn grabs an anti-neutrino coming from the future, turns it into an electron,
and smacks it the other way in time. Notice, that by turning
a down quark into an up quark, the neutron turned into a proton.
The W- is the force carrier particle of the weak force. The weak force is the only force which can
change the flavor of a quark or a lepton.
The weak force has three force carrier particles, the W-, the W+ and the electrically neutral Z.
Why three? Remember how electric charge is always conserved ? Depending on the process, the right
W or Z particle will take over the job of changing flavors, so that the electric charge will be
conserved. To make this clearer, let's take a look
at the positron decay.
On the left you can see a positron decay. An up quark turns into a down quark and emits a W+ in
the process. The overall electric charge stays the same. The positive charge
goes over from the W+ to the antielectron and is so conserved. Can you verify that the charge is
conserved in the beta decay too?
There are many other decays caused by the weak force. On the right you can see a few more.