Abstract

Particle physicists, over the last 50 years, have discovered several hundred strongly interacting “elementary” particles. The list of such particles begins with the proton and the p meson and goes up from there. We now understand that all of these particles are actually composites, formed from various combinations of quarks. Such particles are normally classified in two types. Mesons (the name implies medium weight) are particles with masses that go up from 140 MeV/c 2 and have an intrinsic angular momentum or “spin” that is an integer in units of ħ (Planck’s constant over p). Baryons (the name implies heavy weight) are particles with masses that go up from 938 MeV/c 2 and have half-integer spin. This column is about the discovery of the pentaquark, a brand new form of matter that represents a new particle species, neither meson nor baryon, but a combination of both