Five easy pieces

Natural History, March, 2004 by Joomi Kim

For decades, physicists have been seeking exotic particles, fundamentally different from the particles that make up "ordinary" quark matter. Until last spring, all known assemblages of quarks occurred in twos and threes: the baryons (among them the familiar proton and neutron), made of three quarks; and the mesons (less familiar beasts such as the pion and the rho meson), made of one quark and one antiquark. But in March 2003 a team of physicists announced a new particle species in which five quarks join forces: the pentaquark. Other investigators soon confirmed the discovery. Now, proving the pentaquark was no fluke, another team of physicists, known as the NA49 Collaboration--working at CERN, the European Organization for Nuclear Research near Geneva, Switzerland--has found several new kinds of pentaquarks.

Quarks come in oddly named "flavors," and all the new species include two strange, one anti-up, and either an up or a down quark among the five (the pentaquarks discovered last spring all had two ups, two downs, and one anti-strange). These findings elucidate the deep structure of matter.

Coincidentally, the discoveries of the new affinities quarks have for one another come as physicists at the Brookhaven National Laboratory in Upton, New York, seem close to announcing the ultimate quark "soup"--a so-called quark-gluon plasma--which reproduces, on a small scale, the physical state of cosmic matter during the earliest moments of the big bang. (www.arXiv.org/abs/hep-ph/ 0401034; www.arXiv.org/ a bs/hep-ex/0310014)