Video Transcript
Our topic in this video is color
charge. We’re going to learn that this sort
of charge is a property of particles called quarks. And we’ll also learn the rules that
govern what kinds of color charge quarks can have when they’re grouped together. Now, just the idea that there is a
kind of charge called color charge might seem strange. So, let’s start out by recalling
what we know about electric charge. Electric charge comes in two
varieties, positive and negative. And when two objects have a net
electric charge, that charge affects how they interact with one another.
When we talk about color charge,
we’re effectively building off of this idea. Except now, instead of two kinds of
charge, there are three, and those charges are represented by colors. One kind is red, another kind is
blue, and the third is green. One important thing to realize is
that if a particle does have, say, a blue-colored charge, that doesn’t mean the
particle is actually blue in color, but rather that it has that sort of charge. And we just happen to call these
three kinds red, blue, and green. Now, we might wonder if color
charge is like electric charge, why is it that one is so much more commonly known
than the other? A big part of the reason why is
that the only sorts of particles that can have a color charge are quarks.
We can recall that there are six
varieties of quark, up, down, charm, strange, and top, bottom, and it’s only
individual quark particles that can have a certain color of color charge. In general, any of the six quarks
can possess any of these three colors of color charge. Just as a few examples, we could
have, say, a charm quark with a blue color charge or a down quark, say, with a red
color charge or an up quark with a green color charge, or any quark with any
color. So, in this lesson, if we see a
quark — here, we have a strange quark — colored a particular color, that simply
refers to the sort of color charge it possesses.
Now, we know that when we combine
quarks, the particles formed out of these combinations are called hadrons. Hadrons themselves are commonly
divided into two categories. Mesons are hadrons that are formed
of two quarks, specifically one quark and one antiquark. And then baryons are hadrons that
are formed by combining exactly three quarks. And it turns out that for both
baryons and mesons, when we combine the color charges of the quarks that make up
these particles, that charge, we could say, nulls or cancels itself out. This is something that we’ve seen
with electric charges. If we combine two equal but
opposite charges, the total resulting charge is zero. When we combine color charges,
though, and this happens, we don’t say that the total charge is zero; instead, we
say the color is now white. This is due to the fact that red
and blue and green are what are called the three primary colors, where if we overlap
all three, then they result in white.
In the world of color charge then,
we could think of white or colorless as a neutral charge. And this brings us back to baryons
and mesons because, interestingly, for both of these types of particles, it’s a rule
that their overall charge in color terms must be white. That is, the color charge of every
baryon and every meson must be neutral or colorless. Here’s an example of how this might
work. Say that we have a baryon, a
particle made of exactly three quarks. We can see that in this case those
quarks are up, down, and down, which means that this baryon is a neutron. In any case, it’s necessary that
each one of these quarks has a particular color charge, and it can be red or blue or
green. There’s no such thing as a quark
with no color charge. So, let’s just assign some charges
to these quarks. Let’s just say that the up quark
has a green color charge, while the down quark has a blue color charge, and the
other one has a red color charge.
Now we might wonder, why did we
pick these particular colors for these particular quarks? Partly, it’s just because we had to
choose some color charge for each quark. Any one of these quarks could have
any of the three color charges. Not only that, but the color charge
of a given quark is not fixed over time. So, for example, our up quark,
which currently is green, might at some point change to a blue color charge or a red
one. But, and this is an important
caveat, at any given instant for a baryon, a particle made up of three quarks, at
all times one of the quarks must have a green color charge, one must have a blue
color charge, and one must have the red. And remember that these names and
labels don’t apply to the actual physical color of these quarks; they just describe
a kind of charge of which there are three varieties.
Now, the reason that every baryon
must at all times have one red, one green, and one blue is that, as we mentioned
earlier, a baryon must have a total color charge of white or neutral. When we’re working with three
quarks, we only get that when each one represents one of the different colors. So, if at some point, our up quark
changed from having a green color charge to a blue color charge, as part of that
same process, it will be necessary for our down quark that used to have a blue color
charge to change over to green. In this way, the overall neutrality
or colorlessness of this baryon is preserved.
Knowing this about baryons, we can
also recognize that mesons must follow the same rule of being overall white in their
color charge. But a meson, we recall, is made up
of a quark and an antiquark. So, how can this color balancing
occur then? Well, remember that for every
quark, there’s an antiparticle called an antiquark. We can identify the antiquarks by
saying that they have the same symbol as their corresponding quark, except for the
bar over top of the letter. That bar tells us that in this
case, we’re looking at an up antiquark. Just as quarks have what is called
a color charge, which could be red or green or blue, so antiquarks have what’s
called an anticolor charge.
To understand anticolor charges, we
can think of our three color charges in terms of their opposite colors. The opposite color of red, for
example, we could call it antired, is commonly known as cyan. When we’re thinking about color
charge then, the opposite charge of red is cyan. This means that for a quark with a
color charge of red as this down quark has in our neutron, the anticolor charge
corresponding to this is cyan. And then likewise, color charges of
blue and green have color opposites of yellow and magenta, respectively. Now, just as we said that there are
six types of quark and that each type has its own antiparticle, so we say that there
are three color charges, red, green, and blue, and that each type has its anticolor
charge. So, cyan, yellow, and magenta are
not three more kinds of color charges, rather their anticolor charges correspond to
the colors we’ve already seen.
We brought all this up because
we’re talking about mesons and how it would be possible for the overall color of a
meson to be white or neutral. Say that we’re working with this
example meson here; it’s made of one up quark and one down antiquark. For mesons, the rule for color
charge is this. Whatever the color charge of the
quark, and we know there will be just one of those in a meson; let’s just say that
in this case, the up quark has a color charge of green, then the antiquark in the
meson must possess the anticolor charge of the quark’s color charge. Looking at our color opposites, we
see the opposite of green is magenta, and so that is the necessary anticolor charge
for our down antiquark to possess.
And now, we can see that this color
charge and anticolor charge effectively cancel one another out. In other words, this is a white
meson. Note that if we had picked a
different color charge for our up quark, say we had picked red, then that would have
meant we needed to have a cyan down antiquark. The rule for mesons is that the
quark and the antiquark must have color opposites, that is, a corresponding color
charge and anticolor charge pair.
All right, so we’ve covered a
lot. We’ve learned about this new kind
of charge called color charge, of which there are three varieties. And we’ve also seen that it’s
quarks that possess color charges, while antiquarks possess what are called
anticolor charges. We further asserted that all
hadrons, all particles made up of quarks or antiquarks, must have a neutral color
charge. This is called white or
colorless. We’ve seen how this works for
baryons and mesons. And before we get to an example
exercise, let’s consider how this works for a particle called an antibaryon.
Knowing that a baryon is a particle
that’s made up of exactly three quarks, we might guess than an antibaryon is made up
of three antiquarks. And that’s correct. Say that we have an antibaryon made
up of an up antiquark, a down antiquark, and a strange antiquark. Just like all individual quarks
have a particular color charge, so all individual antiquarks have a particular
anticolor charge. We can assign these anticolor
charges randomly, knowing that they can change over time.
And let’s say that our up antiquark
has an anticolor charge of magenta, our down antiquark has an anticolor charge of
yellow, and then our strange antiquark has an anticolor charge of cyan. Knowing that our baryon with three
quarks had all three color charges represented, so our antibaryon with three
antiquarks has all three anticolor charges represented. This is how an antibaryon satisfies
the condition that all hadrons must meet of having an overall charge color, we could
say, of white.
Knowing all this about color
charge, let’s get some practice through an example exercise.
The diagram shows six baryons in
their quark content. The colors of the quarks correspond
to their color charge. Which baryons have color
configurations that are not possible?
Looking at these six baryons, we
see that they’re all made up of three quarks, as baryons must be. We’re told that the colors the
quarks take on, and we see that some are red, some are green, and some are blue
correspond to the color charge of the quarks. This means that in the diagram a
quark colored red, for example, this one, doesn’t indicate that that quark has a
positive electrical charge, but instead tells us that the color charge of that quark
is red. Along with this, we know that for
any color charge, whether red or green or blue, that doesn’t mean the particle with
that color charge actually has that color. Rather, this is just a convenient
way of talking about how a particle can have one of three kinds of charge.
This means that all of the
red-colored quarks we see in these diagrams have a red-color charge, all the green
ones have a green-color charge, and so on. In general, any of the six kinds of
quark, up, down, charm, strange, and top, bottom, can possess any of the color
charges. But, and this is important,
whenever we group quarks together to form particles called hadrons, the total or
overall color charge of these composite particles must be neutral. This is also called white.
From our knowledge of the primary
colors, we know that if we add together red and blue and green, then that gives us
the color white. And so, based on the rule that all
hadrons must be colorless or must have a total color charge of white, we can say
that for any baryon which is a particle made up of exactly three quarks, in order
for the total color charge of that Baryon to be colorless or white, it’s necessary
that one of the three quarks have a color charge of red, one have a color charge of
blue, and the last one have a color charge of green. This is the only way that the total
color charge of three quarks can add up to white.
And so, when our question asks
which baryons have color configurations that are not possible, we’re looking for
those configurations where we don’t have one red, one blue, and one green color
charge. We see two examples of that. The fourth baryon shown has two
blue color charges and no red, and the fifth one has two red, but no green. These baryons then will not have an
overall color charge of white and therefore are not possible color
configurations. All the other baryons do have one
green, one blue, and one red color charge and therefore are allowed.
Let’s look now at a second example
exercise.
The diagram shows six baryons and
their quark content. Which of the diagrams do not
correctly represent possible quark combinations? How the quarks are colored in the
diagrams does not represent their electric charges.
Among these six baryons, we see
quarks and antiquarks, and all of them are colored either red or blue or green. We’re told that these colors do not
represent electric charges, but they do represent another kind of charge called
color charge. Red is one type of color charge,
green is another, and blue is a third. In this question overall, we’re on
the lookout for any diagrams that do not correctly represent possible quark
combinations. Since this diagram is meant to
represent six baryons, let’s recall the conditions for quarks coming together to
form one of these particles.
The first condition that a baryon
must satisfy is that every baryon is made of exactly three quarks. And second, the total color charge
of a baryon must be what’s called white. This means that the color charges
of the three quarks that combine to form the baryon must add together to form
white. If we think about red, green, and
blue as primary colors, then we know that if we add them together in equal amounts,
the color we’ll end up with is white. For the total color charge of a
baryon to be white then, that means it must be made up of equal parts, red, green,
and blue color charge. And therefore, each baryon must
have one red, one blue, and one green color charge quark. It doesn’t matter which quarks have
which particular color charge, but only that they balance out this way.
Considering these two conditions a
baryon must satisfy, let’s look again at our six options. We see that choices (ii) and (vi)
are both made up of antiquarks rather than quarks. This means that these particles are
not technically baryons but instead would be called antibaryons. Since we want to identify which of
these diagrams do not correctly represent possible quark combinations, we’ll put
options (ii) and (vi) in boxes. And now, let’s look at the second
condition a baryon must satisfy, that its total color charge must be white. We can see that options (i), (iii),
and (iv) all satisfy this condition in that one of the quarks has a red color
charge, one has a blue color charge, and one has a green. But option (v) does not; all of the
quarks here have the same red color charge. This means the total color charge
of this particle cannot be white, so it does not represent a possible quark
combination. The remaining three baryons do show
possible combinations.
Let’s summarize now what we’ve
learned about color charge. In this lesson, we saw that color
charge is a property of quarks analogous to electric charge. There are three types of color
charge, red, green, and blue. And each color charge has its
corresponding anticolor charge. For red, that’s cyan; for blue,
it’s yellow; and for green, it’s magenta. Every quark has a color charge. And any of the six quark types can
have any of the three color charge colors. As an extension of this, every
antiquark has an anticolor charge.
And lastly, we saw that all
hadrons, that is, all particles made of quarks and antiquarks, have an overall color
of white. This means that the color charges
combined in that composite particle add up to white. For a baryon, this is accomplished
by having one red, one blue, and one green color charge, while a meson achieves an
overall color of white by pairing a quark with a given color charge with an
antiquark that has an opposite color anticolor charge.
