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?