What are Quarks?

To start with, there are six types of quarks (plus their six antiquarks),
which are coupled into three pairs. They are the up-down, the charm-strange, and
the top-bottom (sometimes known as truth-beauty). Another interesting fact about
quarks is that you can never find one by itself, as they are always with other
quarks arranged to form a composite particle. The name for these composite
particles is "hadrons". Quarks, like protons and electrons, have electric
charge. However, their electric charges are fractional charges, either 2/3 or
-1/3 (-2/3 and 1/3 for antiquarks), and they always arrange to form particles
with an integer charge (ie. -1, 0, 1, 2...).

Flavour

Mass
(GeV/c2)

Electric Charge
(e)

u

up

0.004

+2/3

d

down

0.08

-1/3

c

charm

1.5

+2/3

s

strange

0.15

-1/3

t

top

176

+2/3

b

bottom

4.7

-1/3

Because quarks join with eachother to form particles with integer charge,
not every kind of combination of quarks is possible. There are two basic types
of hadrons. They are baryons, which are composed of three quarks, and mesons
which are made up of a quark and an antiquark. Two examples of a baryon are the
neutron and the proton.

The proton is composed of two up quarks and one down
quark. As you can see, when the charges from the individual quarks are
added up, you arrive at the familiar charge of +1 for the
proton.

The neutron is made up of two down quarks and one up
quark. Again, adding the charges from the quarks up, we arrive at
zero.

An example of a meson is the pion. It is composed of an
up quark and a down antiquark. Because mesons are a combination of
particle and antiparticle, they tend to be very unstable and decay very
quickly.

So we've now talked about quarks, but there is still the other family of
elementary particles to talk about, the "leptons", which we will
now discuss.