Quarks possess a property to which has been given the name
Just as electric charge originates electromagnetic interactions, color originates
strong interactions. Thus color can also be called ``strong charge´´. Quarks come in
three different states of color, conventionally called
blue. A proton
or a neutron is made of three quarks of different colors. Since equal amounts of
red, green and blue result in white,
neutrons and protons can be said to be
In fact, according to the theory of strong interactions, only
white objects can exist
in isolation. Colored particles - such as quarks - can only exist as components of
white objects. This property is called
confinement. The region of space to which
quarks are confined to form a proton or a neutron - represented by the crystal ball
in the picture - is usually called a bag.
As far as is presently known, there exist six different
types of quarks. These types are known as flavors and have been
given the following names: up, down,
strange, charm, top and bottom.
One property which differs from one flavor to the other is mass.
The up and down quarks
have the smallest masses. These
are the only flavors which constitute ordinary stable matter. The other
flavors are present only in unstable particles produced in collisions.
In the present book, the up quark is represented
as an upper hemisphere and the
down quark is
represented as a lower hemisphere. This is a mere
visualization convention; in fact, quarks are ``point-like´´
in the standard theory of elementary particles.
The proton is made up of two up quarks and one
quark, while the neutron is made up of two down quarks and
one up quark.
The up quark has positive electric charge
equal in absolute value to 2/3 that of the electron.
The down quark has negative electric charge
equal in absolute value to 1/3 that of the eletron. Check that these
charges add up correctly to give, for the neutron, zero total charge and,
for the proton, positive charge equal in absolute value to that of the
Protons and neutrons are made of three quarks. Quarks,
like protons and neutrons, have spin
These three spins must add up to form the
spin of the proton or neutron. According to quantum mechanics, in order
to obtain a proton or neutron with spin up,
we must combine two quarks with spin up
with one quark with spin down.
Symmetrically, in order to obtain a proton or neutron with
spin down, we must combine two quarks
with spin down with one quark with
spin up. These rules are illustrated
in the pictures.