Last of the normal mesons.

Sifting through the debris of 100 million high-energy collisions between protons and their antimatter counterparts, researchers at the Fermi National Accelerator Laboratory in Batavia, Ill., found 19 events that pointed to a new particle. Measurements of the particle's mass, lifetime, and production rate confirmed it as a long-sought member of the meson family. Shin-Hong Kim of Tsukuba University in Japan, a member of the Collider Detector at Fermilab collaboration, announced the discovery last month at Fermilab.

A meson is a particle made up of a quark paired with an antiquark. There are six types of quarks--up, down, strange, charm, bottom, and top. The top quark, however, doesn't survive long enough to allow meson formation (SN: 7/1/95, p. 10). That leaves 25 possible combinations, of which 10 are merely the antiparticles of another 10. Physicists had identified all but one of these mesons in previous experiments. The Fermilab discovery fills in the final blank: a meson consisting of a charm quark and an antibottom quark.

In addition to these normal mesons, theorists have proposed that other types of quark combinations, known as exotic mesons, may also exist (SN: 9/6/97, p. 148).