Video Transcript
Which of the following particles
are hadrons? mu neutrino, lambda baryon, charm quark, top antiquark, pion, proton,
electron.
To figure out which of these
particles are and are not hadrons, let’s first recall what this term means. A hadron is a composite particle
made up of two or more quarks with a total relative charge equal to some integer
value. To understand hadrons then, we need
to understand quarks and their antiparticles, antiquarks. The six types of quark are up and
down, charm and strange, and top and bottom, where the three quarks in the top row
of this grid have a relative electric charge of positive two-thirds times the charge
of a single proton, while those in the bottom row have a relative charge of negative
one-third e, while each corresponding antiquark has a charge of the same magnitude,
but the opposite sign.
Hadrons then are made up of these
six types of quarks and their antiparticles. And so, the first test we can apply
to these particles to see if they’re hadrons is to determine whether they’re made up
of two or more quarks or not. Starting at the top of our list,
our first particle is called a mu neutrino. This is a fundamental particle. That is, it’s not made up of any
smaller particles as far as we know, and, therefore, it doesn’t meet the condition
of being made up of two or more quarks, as all hadrons are. This shows us that a mu neutrino is
not a hadron, so we’ll cross off that option.
Next, we come to the lambda
baryon. This is a particle that is made of
quarks. And in fact, the three quarks that
it’s comprised of are given to us. If we bring together an up, a down,
and a strange quark, then we have what’s called a lambda baryon. So this particle is made of two or
more quarks. And now we just need to know
whether its total relative charge is equal to an integer. To figure that out, we’ll add
together the relative electric charge of each of these three quarks. Based on our grid of quarks and
antiquarks, an up quark has a relative electric charge of positive two-thirds, and
we’ll leave off the e.
A down quark, on the other hand,
has a relative electric charge of negative one-third, and so, we see, does a strange
quark. If we add all these up, we get a
result of zero. Now zero is an integer value, and
so that means it satisfies the second condition of a hadron. Therefore, a lambda baryon is a
hadron, and we’ll box in this particle as part of our answer. Moving on, we come to the charm
quark. Just as a side note, if we saw this
without any context, we might think that this c refers to the speed of light in
vacuum. The only way we really know that
this refers to a charm quark is because we’re told that it is a particle. And therefore, we identify it with
this particular quark type.
As we’ve seen, a hadron is a
composite particle made up of two or more quarks. So one quark all by itself can
never make a hadron. The charm quark, then, is not a
hadron. Moving down to the top antiquark,
for the same reason, this also is not a hadron. A hadron needs to have at least two
quarks. This brings us to the pion which
we’re shown is comprised of an up quark and a down antiquark. If we draw that out, it looks like
this. And so this particle is made up of
two or more quarks. In particular, it’s made of
two. So once again, we need to test
whether its total relative charge is equal to an integer. The relative charge of an up quark
is positive two-thirds and that of a down antiquark we see is positive
one-third. This adds up to one, which is an
integer, which means that this particle, called a pion, is indeed a hadron.
We’ll box that particle then and
move on to consider the proton. A proton is made up of two up
quarks and one down quark. That would look like this. And once again, we’ll look at the
total charge for this particle. Each of the up quarks contributes a
relative charge of positive two-thirds, while the down quark, negative
one-third. And this adds up to three-thirds or
simply one. Just like we would expect, the
total charge for this proton is positive one. And since that’s an integer, it
means that a proton is a hadron.
The last particle we’re to consider
is an electron. Like a mu neutrino, an electron is
an elementary or fundamental particle. It’s not made of anything smaller
than itself, so it’s not made up of two or more quarks. This tells us that an electron is
not a hadron. So of this list of particles, the
lambda baryon, the pion, and the proton are hadrons.
