Of the four known forces, one of them stands
out as having some unique properties. I’ve spoken in another video about the manner
in which the weak force only interacts with particles of a certain spin configuration
and antiparticles with the opposite spin configuration. But in this video, I want to talk about the
manner in which particles decay via the weak force. Now there is an extensive history of how this
has been observed that long predates our current understanding of the universe, but I will
explain the phenomenon strictly in terms of the Standard Model. So I made another video that introduces the
Standard Model, but let me remind you that there are three forces that are understood
in the quantum world. They are the strong nuclear force, the weak nuclear force and
electromagnetism. Each force is transmitted by one or more particles
called bosons. For the strong nuclear force, this particle is the gluon. For the weak nuclear
force, they are the W & Z bosons; and for electromagnetism, it is the photon. So let’s see what happens when a particle
emits another particle governed by the rules of each of these forces. So what would happen if a particle, say a
top quark, emitted a photon? Well, as we can see in this diagram, you start with a top
quark, it emits a photon and continues along as a top quark. The particle doesn’t change
its identity during the emission. Now if you are a physics minded individual,
you will probably be wondering about energy and momentum conservation in that interaction.
This requires that the top quark before the emission to have a mass that differs a little
from the number you can find in text books. This is one of those things that is allowed
under the laws of quantum mechanics and you’ll have to trust me on this. If you want to explore
this on your own, google the term “virtual particles.” Now getting back to the decay, I want to redirect
your attention to the fact that before and after the electromagnetic interaction, the
particle is the same. Now let’s look at what happens during a
strong force interaction. The same top quark might emit a gluon and would continue on as
a top quark. Because the gluon carries color, which is what physicists call the strong force
charge, the top quark will change its color, say from red to blue. And if you want to learn
more about those details, my videos on quantum color and the strong nuclear force will be
helpful. However, the key point here is that the particle
before and after the gluon emission is the same particle. But let’s see what happens when a particle
emission occurs that is governed by the weak force. There are two particles that transmit
the weak force: the W and the Z bosons. So let’s see what happens for the Z boson. When a top quark emits a Z boson, it stays
a top quark. Basically, it’s a lot like the electromagnetic emission of a photon.
Nothing weird there. But when the top quark emits a W particle,
something very different happens. Before the emission, we have a top quark and after the
emission, we see a bottom quark and a W boson. The weak force has changed the identity- what
physicists call the flavor- of the particle. This behavior is unique to the weak force.
And we call this phenomenon “flavor changing.” While this property is unique to the weak
force, it is not unique to the top quark. A bottom quark can emit a W boson and make
a charm quark; and a charm quark can emit a W boson and make a strange quark or a down
quark. On the lepton side, a tau lepton can emit
a W boson and make a tau neutrino. And a muon decays by emitting a W boson and making a
muon neutrino. There are many ways in which quarks and leptons
can change their flavor. And it only happens when the weak force is involved and only when
a W boson is emitted. So that last statement is interesting. Why
is it the only flavor changing weak interaction is the one with the W boson? Why is there
not one with the Z boson? Well let’s explore what that would look like. So the Z boson has zero electrical charge,
so that means when it’s emitted that the emitting particle can’t change its charge. The top, charm and up quarks all have the
same electric charge, specifically +2/3 that of a proton, while the bottom, strange, and
down quarks all have the same charge as well, this time with a charge of -1/3 that of a
proton. The three leptons, the tauon, the muon and the electron all have a charge of
-1. I already said that the interaction in which
the top quark emits a Z boson and stays a top quark is OK. And this is true for all
of the other particles as well. But what about a case like having a top quark emit a Z boson
and become a charm quark? That seems like it would be possible. But is it? Scientists have a name for this- they call
it a flavor changing neutral current, or FCNC for short. Flavor changing comes from the
fact that the particle identity is changed and neutral because the particle charge is
unchanged. So have FCNCs been observed? No. And that
was pretty mystifying until 1970 when Sheldon Glashow, John Illiopolous and Luciano Maini
were able to show that flavor changing neutral currents were forbidden if quarks came in
matched pairs, one with a charge of +2/3 and the other with a charge of -1/3. That’s
why scientists were pretty sure that the charm quark existed before it was discovered in
1974 and that the top quark existed before its discovery in 1995. Since flavor changing neutral currents are
essentially impossible in the Standard Model, there is a cottage industry looking for them.
If they are observed, that is a surefire signature that something new has been discovered. Unfortunately,
so far, no luck. But the flavor changing properties of the
weak nuclear force are very interesting and are probably telling us something profound
about the universe. The question, of course, is just what is that message? I don’t know
and, well, neither does anyone else at this point. It’s just one more mystery that needs
some bright young mind to solve. So what about it? Are you up for a challenge?
