Welcome again from my den.
I hope you are staying safe and healthy.Did
you ever wonder what a quark is?
I mean, you might have thought that it was
the sound that an Australian duck makes.
Or maybe you thought it was an obscure phrase
from a very heavy and ponderous James Joyce
novel.
Well…yes it’s that.
But no…that’s not what I’m talking about.
Actually, a quark is one of the smallest building
blocks of matter and I’d like to tell you
more about it in this episode of Subatomic
Stories.
Quarks are one of the handful of fundamental
subatomic particles, which means that we don’t
know of anything smaller than them.
Most commonly, they can be found in the center
of atoms, in the nucleus.
There are actually six known quarks.
They have silly names.
They are up, down, charm, strange, top and
bottom.
The ones in the top row have positive electrical
charge, specifically two thirds that of the
proton.
The ones in the bottom row have negative electrical
charge, but only a third that of the electron.
Of the six quarks, only two of them are found
in nature – the up and down quarks, specifically,
inside the proton and the neutron.
The proton contains two up quarks and one
down quark.
The neutron contains two down quarks and one
up quark and you can actually do the math
to see the charges work out.
Those mixtures of quarks give you the right
charge for the proton and neutron.
The other quarks are not found in nature at
all, or at least not generally, but we can
make them easily inside large particle accelerators
like the Fermilab Tevatron, when it was running,
the CERN LHC, and now, the existing main injector
at Fermilab.
All of them can make quarks in huge amounts.
Quarks were first proposed in 1964 by Murray
Gell-mann.
When he proposed his theory, it had only three
quarks, the up, down, and strange ones.
He needed three to explain the huge number
of particles that were discovered in the 1940s
and 1950s.
The charm quark was discovered in October
of 1974 in what was called the October Surprise
by two groups, one in New York and the other
one in California.
The bottom quark was discovered in 1977 by
a group led by Leon Lederman who went on to
be the director of Fermilab, the second director
of Fermilab.
He was a great guy and was a lot of fun.
The top quark was discovered in March in 1995
by…well…me.
OK.
OK, it wasn’t just me.
It was me and 800 of my closest personal friends.
But, you know, don’t tell my mom.
Somehow, she got it in her head that I did
it all myself and, well, you know how moms
can be.
Quarks experience all of the subatomic forces,
but they are dominated by the strong force
because…well…it is the strongest.
I’ll talk about all of the forces in a few
episodes from now.
Quarks are the particles that I’ve probably
spent the most time studying so they may very
well be my favorite.
But they are not the only inhabitants of the
subatomic zoo.
In our next episode, I will tell you something
really interesting about another class of
subatomic particles – called the leptons.
Hi!
So, this is the portion of the video where
I answer viewer’s questions.
So first let me say that there were hundreds
of really, really interesting questions in
the last episode.
And I cannot answer them all.
So, let me offer to you the Fermilab website
which has a lot of interesting information.
We have YouTube videos on the Fermilab channel
that also provide information.
And, some of the books you see over my shoulder
by myself and some of my colleagues have a
lot of great things.
The Grand Nagus asks, “Who is your favorite
quark?”
Who’s my favorite quark?
Really did you have to ask that?
I’m embarrassed that you even asked that.
Who is my favorite quark?
Of course.
Our next question from CUCA_str asks, why
don’t all the quarks just clump up, instead
of forming groups of three, like the proton
and neutron?
That’s actually an excellent question and
the answer to it comes from the strong force
which we haven’t had time to talk about
yet.
We’ll talk about that in an upcoming episode.
In electromagnetism there are two charges,
the plus charges and the minus charges.
A plus charge and a minus charge can cancel
each other out to have something that’s
neutral.
For the strong force there are actually three
charges.
And the three charges add together to make
a neutral thing.
So, taking an analogy from classical physics: 
If you add red light, blue light and green
light together, it will be white light.
So we call the color force, or rather the
strong force “color” and we give each
of the charges, red, blue, or green even though
they have nothing to do with color.
You add them together and you get a neutral
object.
So, that’s why things come in threes because
inside a proton you have three quarks, one
red, one blue, and one green.
It has a color-neutral.
The same thing with neutrons.
That’s why they clump in threes and not
all clumped together.
Excellent question.
Our next question is from Jorge Torres Hernandez.
And his question is: Is there a particular
reason for quarks always to be in groups?
Can there be free quarks roaming around undetected?
That’s an excellent question.
People want to know if you can find a free
quark in the same way that you can pull an
electron off of an atom.
After all, that’s how electricity works.
But, it turns out that it’s not so easy
to pull a quark out of a proton.
And the reason is because of the nature of
the strong force.
In electromagnetism, if you take two magnets
and put them really close to one another,
the force is very, very, strong and it’s
weaker when they are apart.
But, that’s not how the strong force works.
The strong force is more like a rubber band
so that if you have the two ends of the rubber
band near one another, you don’t feel a
force.
But, as you pull them apart it gets stronger
and stronger and stronger.
And, if the force gets stronger, the energy
gets stronger.
So what happens is if you try to knock a quark
out of a proton, you get a stronger and stronger
force with more and more energy.
And when you have a lot of energy, eventually
what happens is you have, effectively, a little
lightning bolt of strong force and the force
converts into a matter and an anti-matter
quark.
And those matter and anti-matter quarks travel
along with the existing quark and the net
outcome of that is those particles, those
quarks, kind of connect to one another.
And where you had one quark shot outside of
the proton, instead, you have a whole bunch
of them, like a little shotgun blast, what
we call a jet of many particles travelling
in the same direction but the net outcome
of that is that you can’t find an isolated
quark.
So, that’s all the time we have today for
questions today.
If you liked our episode, please like it.
We hope you subscribe to the channel and share
us on social media.
In our next episode, we’re going to talk
about the leptons which are a simpler particle
than the quarks and ones that allow us to
probe the laws of physics at the subatomic
world with even more precision.
And who wouldn’t like that because, even
at home, physics is everything.
