3. Nucleons and Quarks

Binding forces and “massive” quarks

The binding forces carried by the gluons tend to be weak when quarks are close together. Within a proton (or other hadron), at distances of less than 10−15 metre, quarks behave as though they were nearly free. This condition is called asymptotic freedom. When one begins to draw the quarks apart, however, as when attempting to knock them out of a proton, the effect of the force grows stronger. This is because, as explained by QCD, gluons have the ability to create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an accelerated particle, the gluons utilize energy that they draw from the quark’s motion to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the effective binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines. This phenomenon is observed at high-energy particle accelerators in the production of “jets” of new particles that can be associated with a single quark.

The discovery in the 1970s of the “charm” (c) and “bottom” (b) quarks and their associated antiquarks, achieved through the creation of mesons, strongly suggests that quarks occur in pairs. This speculation led to efforts to find a sixth type of quark called “top” (t), after its proposed flavour. According to theory, the top quark carries a charge of 2/3e; its partner, the bottom quark, has a charge of −1/3e. In 1995 two independent groups of scientists at the Fermi National Accelerator Laboratory reported that they had found the top quark. Their results give the top quark a mass of 173.8 ± 5.2 gigaelectron volts (GeV; 109 eV). (The next heaviest quark, the bottom, has a mass of about 4.2 GeV.) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the Standard Model, the prevailing theoretical scheme of nature’s fundamental building blocks.

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