New stable meson has fifth quark flavor

Although the list of known mesons is long, the few that are stable against hadronic and electromagnetic decay form a rather exclusive club. Until recently, the catalogue of mesons stable against all but weak decay included only the $π+,$ $K+,$ $K0,$ $D+,$ $D0,$ $F+$ and their antiparticles. The stability of these particles attests to the conservation of their various quark flavors (up, down, strange and charmed). Only the weak interaction can transmute a quark of one flavor to another.