Elementary Particles, Quarks and Quantum Chromodynamics 251
mesons of a quark-antiquark pairs. The proton is made up of 2 up quarks
and one down quark so its charge is +2/3 e +2/3 e –1/3 e or +e and
the neutron is made up of one up and two down quarks so its charge is
+2/3 e –1/3 e –1/3 e = 0. The Σ+ is made up of 2 up quarks and one
strange quark so its charge is +2/3 e +2/3 e –1/3 e or +e and its
strangeness number is +1. The π+ is made up of an up quark and o
ne antidown quark so its charge is +2/3 e +1/3 e or +e. The K+ is made
up of an up quark and one antistrange quark so its charge is +2/3 e
+1/3 e or +e and its strangeness number is –1. The K- is made up of
an antiup quark and one strange quark so its charge is –2/3 e –1/3 e
or –e and its strangeness number is +1. All of the hadrons can be
accounted for in terms of quarks by making combinations similar to
these.
In addition to the families of 8, SU(3) also predicted that there
would be baryon families of ten particles consisting of the following
combinations of the up (u), down (d) and strange (s) quarks: uuu, uud,
udd, ddd, uus, uds, dds, uss, dss, and sss. The sss combination
corresponds to the Ω– and the uuu to the spin 3/2 N*++.
As scattering experiments were conducted at still higher energies
physicists encountered hadrons that were composed of other kinds of
quarks so that in addition to the first three quarks used to describe the
first generation of hadrons three more types of quarks were encountered,
named charm, bottom and top as well as their associated antiquarks. The
charm quark and the top quark each have a charge of +2/3 e while the
bottom quark has a charge of –1/3 e.
Although one can explain the SU(3) symmetry of hadrons in terms of
quarks, a quark has never been seen in isolation because of the
phenomenon of confinement, which is not totally understood. It is just an
empirical fact that quarks never can be separated from the hadrons in
which they exist. The closest physicists have come to seeing a quark is
when they are produced in high energy collisions and one see three jets
of many hadrons clustered together. The force between quarks that gives
rise to their confinement is due to the exchange of gluons. Gluons play
the same role in the quark-quark force that the photon plays in the force
between charged particles and mesons play in the force between
nucleons. Like photons gluons have spin 1 but they come in eight
different colour charges. The colour in colour charge is not the actual
colours of visible light but they are a metaphorical way of describing the