In the six decades since the discovery of the pion, more than a hundred other meson species have been found. Most decay by the strong nuclear interactions and therefore live less than 10−22 s. So one sees them only as resonant peaks in scattering cross sections or energy distributions. But until now they all had one thing in common: Every known meson was characterized by a combination of quantum numbers—charge, spin, parity, strangeness, charm, and the like—that could be accounted for simply by a quark–antiquark (q ) pair.
Now the Belle collaboration at the KEKB electron–positron collider in Tsukuba, Japan, has reported impressive evidence for the first manifestly exotic meson—a meson that clearly requires more quarks than just a q pair. 1 Dubbed Z± (4430), the new charged meson has a mass of 4.43 GeV, about four and a half times that of...
