Background

Quantum Electrodynamics (QED) was a quantum field theoretic explanation of the interaction of electrons and photons. It was very successfull in giving precisely verfied predictions and the formulators were awarded the Nobel Prize in Physics. Around 1970 some of the top physicists were hard at work trying to develop an explanation for what was known as the nuclear Strong Force.

From 1933 when the neutron was discovered it was felt that nuclei consisted of collections of neutrons and protons held together by a force acting between them. Werner Heisenberg speculated that the neutron and the proton were the same particle and differed only in that a positive charge was turned on for the proton. Heisenberg coined the term nucleon to represent the common particle of the neutron and the proton. Heisenberg's conjecture proved not to be true, but the term nucleon continued to used to denote a partcle that is either a neutron or proton.

The conjectured Strong Force of attraction had to be strong enough at nuclear distances to offset the electrostatic repulsion between protons. It was conjectured that the hypothesized nuclear Strong Force acted with equal strength between any two nucleons. These conjectures explained the existence of stable nuclei but there was no other physical evidence for the validity of the conjectures. There is an alternate explanation for the existence of stable nuclei which has an empirical basis. See What Holds a Nucleus Together?.

So the efforts that went into the development of Quantum Chromodynamics were for explaining and justifying a model rather than a body of physical evidence. This came as an elaboration of the quark model of particle structure.

Before the quark model was proposed separately in 1964 by Murray Gell-Mann and G .Zweig physicists were searching for an explanation for the number of hadrons. Separately in 1961 Murray Gell-Mann and Yuval Ne'eman found that the hadron particles known at that time could be meaningfully classified by the representations of the Special Unitary Group of rank 3 (SU(3)), whose elements are 3by3 complex matrices whose determinants are equal to unity.

Others, F. Gürsey and L.A. Radicati, and A. Pais, in 1964 proposed to adjoin the rotation group SU(2) to SU(3), obtaining the equivalent of SU(6) to explain the known hadrons. Neither SU(3) nor SU(6) were successful in this endeavor. This led Gell-Mann and Zweig to propose that hadrons are composite objects made up of three entities and mesons are composite objects of two entities. These entities came to be known by Gell-Mann's name for them. quarks, rather than aces which was Zweig's name for them.

At the time only three quarks were needed to explain the known hadrons and these were given the names up, down and strangeness. Soon a fourth, charm, was added. Ultimately a fifth and sixth were added. For a time these were known as truth and beauty, but eventually propreity prevailed and they became known as top and bottom. These terms became known as the flavors of the quarks. Each quark had an antipartcle so altogether there are twelve quarks.

Baryons like the proton and neutron are composed of three quarks and mesons like the pion are composed of a quark and an antiquark. A proton consists of two up quarks and a down quark, whereas a neutron consists of two down quarks and one up quark. The electrostatic charges of a proton being +1 and a neutron charge of 0 are then accounted by charges of q_u and q_d for the up and down quarks, respectively, and hence

2q_u + q_d = 1
and
q_u + 2q_d = 0

The solution to these equations is

q_u = 2/3
q_d = −1/3

The charge of an antiquark is the negative of the charge of the quark it corresponds to. Thus the charge of a positive pion meson made up of an up quark and an antidown quark is (2/3)+[−(−(1/3)]=(2/3)+(1/3)=1. The charge of a negative pion made up of a down quark and an antiup quark is −(2/3)+(−(1/3)=−1.

The masses of hadrons cannot be accounted for so neatly. The mass of a baryon like the proton is on the order of 938.27 MeV/c² whereas that of meson like a pion is on the order of 139.57 MeV/c².

Gell-Mann initially questioned whether quarks were actual physical particles or merely a mathematical convenience. Now it is generally believed that quarks do exist as physical particles. Richard Feynman was one of the first to assert their physical existence, although he called them partons rather than quarks because they are parts of particles.

"Color"

The twelve quarks can account for the existence of a large number of baryons and mesons. Many plausible combinations do not correspond to physically observed particles. However one particle that does exist is an anomaly. It is the Δ⁺⁺ which is thought to consist of three up quarks. Such a particle would seem to violate Pauli's Principle that there cannot be two particles with identical quantum numbers in the same composite particle. The solution is to suppose that quarks have an additional quantum characteristic that has three possible values. This, probably unfortunately, has been labeled color although it has nothing to do with actual color. Calling it color suggests that it like a charge on the quark whereas it may equally well refer to a size attribute. For more on this point see A Sensible Model of Quark Confinement and Asymptotic Freedom.

(To be continued.)