11.3 Quarks

In the 1960s, particle physicists began to realize that hadrons are not elementary particles but are made of particles called quarks . (The name ‘quark’ was coined by the physicist Murray Gell-Mann , from a phrase in the James Joyce novel Finnegans Wake .) Initially, it was believed there were only three types of quarks, called up ( u ), down ( d ), and strange ( s ). However, this number soon grew to six—interestingly, the same as the number of leptons—to include charmed ( c ), bottom ( b ), and top ( t ).

All quarks are spin-half fermions $\left(s=1\text{/}2\right),$ have a fractional charge $\left(1\text{/}3\text{or}2\text{/}3e\right)$ , and have baryon number $B=1\text{/}3.$ Each quark has an antiquark with the same mass but opposite charge and baryon number. The names and properties of the six quarks are listed in [link] .

Quark combinations

As mentioned earlier, quarks bind together in groups of two or three to form hadrons. Baryons are formed from three quarks. Sample baryons, including quark content and properties, are given in [link] . Interestingly, the delta plus ( ${\text{Δ}}^{+}$ ) baryon is formed from the same three quarks as the proton, but the total spin of the particle is 3/2 rather than 1/2. Similarly, the mass of ${\text{Δ}}^{+}$ with spin 3/2 is 1.3 times the mass of the proton, and the delta zero ( ${\text{Δ}}^0$ ) baryon with a spin 3/2 is 1.3 times the neutron mass. Evidently, the energy associated with the spin (or angular momentum) of the particle contributes to its mass energy. It is also interesting that no baryons are believed to exist with top quarks, because top quarks decay too quickly to bind to the other quarks in their production.

Mesons are formed by two quarks—a quark-antiquark pair. Sample mesons, including quark content and properties, are given in [link] . Consider the formation of the pion ( ${\pi }^{+}=u\stackrel{\text{−}}{d}$ ). Based on its quark content, the charge of the pion is

Both quarks are spin-half ( $s=\mathrm{½}$ ), so the resultant spin is either 0 or 1. The spin of the ${\pi }^{+}$ meson is 0. The same quark-antiquark combination gives the rho ( $\rho$ ) meson with spin 1. This meson has a mass approximately 5.5 times that of the ${\pi }^{+}$ meson.