Video Transcript
Which of the following particles
have a baryon number of zero? Proton, top quark, electron, B
meson, down antiquark, tauon, mu neutrino, delta baryon, up quark.
To figure out which of these nine
particles has a baryon number of zero, let’s remember what gives something a baryon
number in the first place. This property comes from the kind
of quarks a particle does or does not possess. What we mean is that a quark has a
baryon number of positive one-third. An antiquark, on the other hand,
has a baryon number of negative one-third. We can determine the total baryon
number of a particle by considering what quarks and antiquarks it possesses.
For example, considering the first
item on our list, the proton, this is a particle that’s made of three quarks: two up
quarks and one down quark. Since each of these three quarks
has a baryon number of positive one-third, the total baryon number of this particle,
the proton, is equal to one-third plus one-third plus one-third. That’s one. So we see a proton does not have a
baryon number of zero, and we can cross it out.
What about a top quark? Well, we see that every quark has a
baryon number of positive one-third. So a top quark by itself will not
have a baryon number of zero. Rather, its baryon number is
positive one-third, so we cross this out as well.
Next, we consider an electron. An electron is a fundamental
particle. As far as we know, it’s not made of
anything smaller. As such, an electron possesses no
quarks. And since quarks are what give a
particle a baryon number, an electron, having none of them, has a baryon number of
zero. We know then that our answer to
this question will include the electron.
Next, let’s look at this particle
called the B meson. Though we may not be familiar with
this particular kind of meson, we can remember that mesons in general are particles
made of one quark and one antiquark. As an example of this, this is a
meson comprised of a charm quark and a down antiquark. Since the charm quark has a baryon
number of positive one-third and the down antiquark has a baryon number of negative
one-third, we can see that the overall baryon number for this particle is zero. And we can also see that this will
be true for all mesons because any quark has the same baryon number of positive
one-third and any antiquark has the same baryon number of negative one-third. So even though we may not exactly
recall what a B meson is made of, we know that since it’s a meson, it does have a
baryon number of zero.
Next, we consider a down
antiquark. All by itself, this antiquark, we
know, will have a baryon number of negative one-third, not zero. And so we’ll cross it off our list,
too.
Next, we consider this particle
called a tauon. A tauon is a fundamental particle
not made up of any particles smaller than itself. As such, it doesn’t possess any
quarks or antiquarks and, therefore, has nothing to contribute to giving it a
nonzero baryon number. So because it doesn’t possess any
quarks or antiquarks, a tauon will have a baryon number of zero.
We next consider this particle
called a mu neutrino, which is actually in the same class of particles as
tauons. They’re called leptons. Just like the tauon, the mu
neutrino is not comprised of any quarks. And so it, too, has a baryon number
of zero.
Next, there’s this particle called
the delta baryon. A baryon, we can recall, is a type
of particle that’s made of exactly three quarks. Our proton over here is an example
of a baryon. And just as the proton had a baryon
number of one, so all particles made of exactly three quarks, that is, all baryons,
have the same baryon number. This means a delta baryon does not
have a baryon number of zero, so we won’t choose this option.
Lastly, this brings us to the up
quark. As we’ve seen, any individual quark
has a baryon number of positive one-third. So the up quark has the same baryon
number and, therefore, does not have a baryon number of zero.
Of all these particles, it’s the
electron, the B meson, the tauon, and the mu neutrino that have a baryon number of
zero.
