12.6 Quarks

12.6 Quarks

In 1964, M. Gell-Mann and George Zweig proposed that none of the hadrons, not even the proton and neutron, are truly fundamental, but instead are made up of combinations of three more fundamental entities called quarks or quark flavours. Quarks are considered to be truly fundamental particles, like leptons. Three quarks originally proposed were named up, down, and strange, with abbreviations $u$, $d$ and $s$, respectively. Presently, we are aware of six quarks, just as there are six leptons based on a presumed symmetry in nature. The other three quarks are called charm, bottom, and top ($c$, $b$, $t$). These new quarks can be distinguished from the $3$ original quarks (Table 12.4). All quarks have a spin and an electric charge (a fraction of the previously thought smallest charge $e$ on an electron). Quarks are invisible. They never appear on their own.

All hadrons are considered to be made up of combinations of quarks (plus the gluons that hold them together), and their properties are described by looking at their quark content. Mesons consist of a quark–antiquark pair. For example, a $\pi^{+}$ meson is a combination of a $u$ and $\bar{d}$ quark.

Each baryon, on the other hand, consists of three quarks, such as:

The proton has a quark composition of $uud$ and so its charge quantum number is:

Neutron has a quark composition of $udd$ and its charge quantum number is:

Mesons are quark–antiquark pairs. Consider the meson $\pi^{+}$, which consists of an up-quark $u$ and an antidown quark $\bar{d}$. We see that the charge quantum number of the up quark is $+\tfrac{2}{3}$ and that of the antidown quark is $+\tfrac{1}{3}$. This adds nicely to a charge quantum number of $+1$ for the $\pi^{+}$ meson; that is, $q(u) + q(\bar{d}) = \tfrac{2}{3} + \tfrac{1}{3} = +1$.

Noting after the quark theory was proposed, it was suggested that quarks have another property (or quality) called colour, or "colour charge" (analogous to electric charge). According to this theory, each flavour of quark can have one of three colours, usually designated red, green, and blue. Note that the names "colour" and "flavour" have nothing to do with our senses, but are purely whimsical—as are other names, such as charm, in this new field. The anti-quarks are coloured antired, antigreen, and antiblue. Baryons are made up of three quarks, one of each colour. Mesons consist of a quark–anti-quark pair of a particular colour and its anti-colour. Both baryons and mesons are thus colourless or white. Each quark is assumed to carry a colour charge, analogous to an electric charge, and the strong force between quarks is referred to as the colour force. This theory of the strong force is called quantum chromodynamics or QCD, to indicate that the force acts between colour charges (and not between, say, electric charges). The strong force between two hadrons is considered to be a force between the quarks that make them up.

Table 12.4: Quark flavours