Video Transcript
In this video, our topic is
strange, charm, bottom, and top quarks. We’re going to learn what these
terms mean as well as some of the properties of these quarks.
Although the names strange, charm,
bottom, and top may not be familiar to us, we do know something about these
particles called quarks. If we think for a moment about an
atomic nucleus and say that in this nucleus the protons are the red dots and the
neutrons are the green ones, then we can recall that these particles are actually
not elementary. That is, they’re made up of
particles that are yet smaller. These smaller particles are called
quarks. And if we were to take an up-close
look at a single proton, we would see it’s comprised of two of what are called up
quarks and one down quark. One reason quarks are interesting
is that they have a fractional, that is a nonwhole number, relative electric
charge.
For example, the relative charge of
an up quark is positive two-thirds times the charge of a single proton. Sometimes this is written as
two-thirds 𝑒, or simply two-thirds. The relative charge of a down
quark, on the other hand, is negative one-third 𝑒 or just negative one- third. Notice that if we add together the
relative charges of these two up quarks and the one down quark, we get a result of
positive one. This is just what we would expect,
since this is the relative charge of a single proton. So, quarks, specifically up and
down quarks, group together to form nucleons, that is, protons and neutrons. If we were to make a table then
showing the different types of quarks, then we would include these two. And in addition to the shorthand
symbol that represents these types, we could add a second set of parentheses that
shows their relative charge.
Now, if all we ever looked at where
protons and neutrons, then the only quark types we would ever see are up and
down. These are by far the most common
types of quark. But it turns out that they’re not
the only types. Thanks to carefully performed
experiments involving high-energy collisions, four more types of quark have been
discovered. We can clear some space for them in
our table. And these four types are what our
lesson is about: charm, strange, top, and bottom quarks. The way we’ve arranged them, notice
that all the quarks in our top row have the same relative charge, positive
two-thirds. And likewise, in the bottom row,
there the down, strange, and bottom quarks all have a relative charge of negative
one-third.
Now, we mentioned that up and down
quarks are the elementary particles that make up protons and neutrons. As such, they’re very abundant. These other four types, though, are
much less common. As we said, they’re only formed in
high-energy interactions. And not only that, but the charm,
strange, and top and bottom quarks all have a very short lifetime as compared to up
and down quarks. These four less common types
typically exist for a few billionths of a second or even less before they decay into
another type of quark. The way these decays progress, the
shortest lifetime quarks, top and bottom, may decay into charm or strange types,
which, after a very short lifetime, may themselves decay into a longer lasting
type.
Interestingly, though, if we
consider another property of these six types of quark, their mass, we find that this
property trends in the opposite way. That is, it’s the up and the down
quarks which have the least mass of all six types and the top and bottom which have
the greatest. It’s worth pointing out that among
these three pairs — up, down, charm, strange, and top, bottom — on average, they
don’t share exactly the same lifetime or mass as one another. But compared to the types of quark
around them on either side of this table, generally the lifetime of a quark
increases as we go from right to left, while the quark mass gets bigger as we go
left to right.
Now, it’s a general rule that any
particle has a corresponding antiparticle, and quarks are no different. Each of the six types of quark has
a corresponding antiquark. And for each quark type, its
antiquark is a particle that has the same mass and lifetime and other properties,
but it has the opposite electric charge. So, for example, while the charm
quark has a relative electric charge of positive two-thirds, the charm antiquark has
a relative charge of negative two-thirds. Notice also that there’s a
difference in how we symbolize the quark and corresponding antiquark types.
The shorthand symbol for a given
type of quark is simply the first letter in that quark’s name, whereas for the
antiquark, we use that letter, but we put a bar over top of it. So far, we’ve said that there are
six types of quarks, but we may wonder, “do the antiquarks mean that there are
really 12?” Rather than thinking of the six
antiquarks as a separate type of quark, we can consider each one as a pair with the
given quark particle it’s associated with. So rather than saying, the up quark
and the up antiquark are two different quark types, we would say that there’re one
type, the up quark type, expressed with its antiparticle.
The antiquarks of each quark type
follow the same trends in terms of mass and lifetime. And once again, the charm, strange,
top, and bottom antiquarks are less common than the up and down antiquarks. This follows too from the
properties of those quark types. And just to be clear, the reason
that these four types are less common than the up and down quarks and that they have
a shorter lifetime is that they are less stable. That is, they’re more likely to
quickly decay into another type of quark. So when we say a quark type is less
common or that it has a shorter lifetime, we’re effectively saying that it’s less
stable.
Just like for other particles,
quarks interact via forces. Two quarks might exert a
gravitational force on one another or an electromagnetic force or a strong or weak
nuclear force. All four of the fundamental forces
are means by which quarks can interact with one another. Knowing all this about charm,
strange, top, and bottom quarks, let’s get some practice now with these ideas
through an example.
List the following particles in
order from greatest mass to least mass: top quark, strange quark, photon, tau
neutrino, electron, proton, up quark.
Okay, so, of this list of seven
particles, we want to put them in order from greatest to least mass. And as we look over these
particles, we see we have three quarks, a top quark, a strange quark, and an up
quark; a proton and an electron, those are particles that may be more familiar to
us; and then a photon and something called a tau neutrino. When we think about a photon, we
recall that this is a particle that has zero mass. So, whatever the order of the rest
of our list, we know that photon will come last because nothing can have less mass
than zero mass.
Now, this particle right below the
photon, the tau neutrino, this is known as a particle that does have mass, but it
has the least mass of all particles that do. In other words, its mass is not
zero but just barely so. So, working backwards then, we know
the last item on our list will be the photon, and the next to last will be the tau
neutrino. At this point, our remaining
choices are the three quarks and the proton and the electron. Protons we can recall are made up
of quarks. Specifically, one proton is made
from two up quarks and one down quark. And from this, we can reasonably
guess that a proton is more massive than any quark, and indeed this is true. And going further than that, we
know a proton is more massive than an electron, about 2000 times more massive. All this tells us that the particle
at the beginning of our list, the most massive one, is the proton.
That leaves us with our three
quarks and the electron. We want to find out how these
particles rank in terms of mass. One way to help us with this is to
recall, as we did earlier, that a proton is made of two up quarks and one down
quark. It turns out that only about 10
percent of a proton’s mass actually comes from the mass of the quarks that make it
up. But nonetheless, this suggests that
the mass of a quark is greater than the mass of an electron, which, as we recalled,
is about one 2000th that of a proton. And it turns out that our intuition
is correct. An electron is indeed less massive
than any type of quark, which means we can write it as the third to last item on our
ranked list. So now, the only question that
remains is of the top, strange, and up quarks, what is their mass ranking?
As we said, the up quark is part of
what makes up a proton, and it’s part of the lightest set of the quark pair
types. That is if we consider up, down;
charm, strange; and top, bottom, then the mass of each one of these types decreases
as we go from right to left. This shows us that an up or down
quark is less massive than a charm or strange quark, which is less massive than a
top or bottom quark.
And that shows us how we can write
in the last three options on our list, the top, strange, and up quarks. Of the three, the top quark has the
most mass, followed by the strange quark and then, lastly, the up quark. So now, we have our completed
list. In order from greatest to least
mass, we have the proton, the top quark, the strange quark, the up quark, the
electron, the tau neutrino, and the photon.
Let’s look now at a second example
exercise.
Which of the following particles
have a relative charge of positive two-thirds? Top antiquark, down quark, top
quark, up quark, electron, strange antiquark, charm quark.
Okay, one of the first things we
can notice here is that we’re looking for particles that have a fractional relative
charge, a relative charge that’s not a whole number. This is the hallmark of a
quark. And we see that our list is
composed entirely of quarks or antiquarks, with one exception. An electron is also listed
here. But we can recall that an electron
has a relative charge of negative one. And so, right away, we can cross
out this option because an electron does not have the relative charge we’re looking
for. That leaves us with the quark and
antiquark types listed.
At this point, let’s recall that
there are six types of quark: up and down, charm and strange, and top and
bottom. We’ve arranged these types in such
a way that all three in the top row have the same relative charge, positive
two-thirds, and all three in the bottom row also have the same relative charge of
negative one-third. This helps us answer this question
because we see that an up, a charm, or a top quark all will have the relative charge
we’re looking for. And in fact, on our list of
possible answers, we see all three of these quark types. And then from this table, we can
also say that if we find down, strange, or bottom quarks in our list of answer
options, we can cross those out because they don’t have the relative charge we
want. And we indeed see that the down
quark was an option, but we’ll cross that out, knowing now what we know about
it.
And this leaves us with these two
antiquarks. One property of the antiquark of a
given quark type is that it has the opposite relative charge, that is the same
magnitude but the opposite sign. This means that the relative charge
of the up antiquark, the charm antiquark, and the top antiquark is negative
two-thirds and then that of the other three antiquark types is positive
one-third. All this to say there are no
antiquarks that have the relative charge we’re looking for. None of them have a relative charge
of positive two-thirds, so we can cancel these options off of our list. Our answer then is that only the
top quark, the up quark, and the charm quark have a relative charge of positive
two-thirds.
Let’s look now at one last example
exercise.
Which of the following particles
have a relative charge with a magnitude of one-third? Proton, bottom antiquark, top
antiquark, bottom quark, down quark, up antiquark, strange antiquark.
All right, so we’re looking for all
the particles in this given list with a charge magnitude of one-third. We can see that this is a
fractional charge. And that is the hallmark of a quark
or an antiquark. Looking at our list, we see all of
the choices fall under this description except one. The proton, the first item listed,
has a relative charge of positive one. Right away then, we see that we can
cross off this answer choice.
So now, let’s look at the quark and
antiquark types listed. As we do, one important thing to
keep in mind is that we’re looking for particles that have a relative charge
magnitude of one-third. This means that any particle with a
charge of plus or minus one-third fits this description. To get started answering this
question, let’s recall the six types of quark. By pairs, there are the up and down
quark, the charm and strange quark, and the top and bottom quark. When they’re written out like this,
we know that all the quark types in our top row have the same relative charge,
positive two-thirds. And likewise, all the types of
quark in our bottom row have the same relative charge, negative one-third.
Recalling that we’re looking for
particles that have a charge magnitude of one-third, we can see that the magnitude
of this relative charge, negative one-third, is positive one-third. And therefore, any down, strange,
or bottom quarks we see on our list qualify. Looking once more at our answer
options, we see the bottom quark and the down quark listed here. And so, we know those are particles
that do have a relative charge with a magnitude of one-third. And now, let’s look at the
remaining particles, which are all antiquarks.
Just like with the quark types, we
can write out the antiquark types according to their pairs. And now, the types in the top row
have a relative charge of negative two-thirds and those in the bottom have a
relative charge of positive one-third. This means that if we find the down
antiquark, the strange antiquark, or the bottom antiquark in our list, then those
satisfy the condition we’re looking for. And indeed, we see the bottom
antiquark here and the strange antiquark here. The other two choices, the top and
the up antiquark, do not have the relative charge we want. So, we can cross out those options
and then select as part of our answer the bottom and strange antiquarks. So, of the particles in our
original list, the bottom antiquark, the bottom quark, the down quark, and the
strange antiquark, all have a relative charge with a magnitude of one-third.
Let’s summarize now what we’ve
learned about strange, charm, bottom, and top quarks. In this lesson, we saw that along
with up and down quarks, which we saw make up protons and neutrons, there are four
more types: charm, strange, top, and bottom. When we arranged those six types
like this, we saw that all three types on the top row have a relative charge of
positive two-thirds and the three on the bottom row have a relative charge of
negative one-third.
We saw further that while the mass
of these quark pairs increases as we go left to right, their lifetime decreases as
we go that way. Specifically, this means that
charm, strange, top, and bottom quarks are less stable than up and down. Along with this, we also learned
the symbols and relative charges of the six antiquark types. And lastly, we learned that quarks
interact with one another via all four of the fundamental forces, gravity,
electromagnetism, and the strong and weak force. This is a summary of strange,
charm, bottom, and top quarks.
