Quarks Have Their Ups and Downs

Quarks Have Their Ups and Downs

The quark model actually lost some of its early popularity because the original model with three quarks had to be modified. The up and down quarks seemed to compose normal matter as seen in this table, while the single strange quark explained strangeness. Why didn’t it have a counterpart? A fourth quark flavor called charm (c) was proposed as the counterpart of the strange quark to make things symmetric—there would be two normal quarks (u and d) and two exotic quarks (s and c). Furthermore, at that time only four leptons were known, two normal and two exotic. It was attractive that there would be four quarks and four leptons.

The problem was that no known particles contained a charmed quark. Suddenly, in November of 1974, two groups (one headed by C. C. Ting at Brookhaven National Laboratory and the other by Burton Richter at SLAC) independently and nearly simultaneously discovered a new meson with characteristics that made it clear that its substructure is \(c\stackrel{-}{c}\). It was called J by one group and psi (\(\psi \)) by the other and now is known as the \(J/\psi \) meson. Since then, numerous particles have been discovered containing the charmed quark, consistent in every way with the quark model. The discovery of the \(J/\psi \) meson had such a rejuvenating effect on quark theory that it is now called the November Revolution. Ting and Richter shared the 1976 Nobel Prize.

History quickly repeated itself. In 1975, the tau (\(\tau \)) was discovered, and a third family of leptons emerged as seen in this table). Theorists quickly proposed two more quark flavors called top (t) or truth and bottom (b) or beauty to keep the number of quarks the same as the number of leptons. And in 1976, the upsilon (\(\Upsilon \)) meson was discovered and shown to be composed of a bottom and an antibottom quark or \(b\stackrel{-}{b}\), quite analogous to the \(J/\psi \) being \(c\stackrel{-}{c}\) as seen in this table. Being a single flavor, these mesons are sometimes called bare charm and bare bottom and reveal the characteristics of their quarks most clearly. Other mesons containing bottom quarks have since been observed. In 1995, two groups at Fermilab confirmed the top quark’s existence, completing the picture of six quarks listed in this table. Each successive quark discovery—first \(c\), then \(b\), and finally \(t\) —has required higher energy because each has higher mass. Quark masses in this table are only approximately known, because they are not directly observed. They must be inferred from the masses of the particles they combine to form.