Printed from: PhysLink.com; http://www.physlink.com/News/Index.cfm?ID=70
Original publication date: Wednesday July 9, 2003.
Evidence of Exotic 5-Quark Particle Found
For almost 40 years, all subatomic particles have fit neatly into two categories: three-quark baryons, like protons and neutrons; or mesons, made up of one quark and one anti-quark. The new particle spotted at Jefferson Lab is a sort of baryon-meson hybrid with five quarks - or, more precisely, four quarks and one anti-quark. The pentaquark is a member of the baryon family, but it's said to be ":exotic": because the anti-quark has a different ":flavor": to the other quarks.
There are six known flavors of quarks, three of which are studied at Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF). They are called up, down and strange quarks, with symbols u, d and s, respectively. The other three known flavors are charm (c), top (t) and bottom (b) quarks. Each of these six quarks also has a corresponding anti-quark.
With six quarks and six anti-quarks to choose from, one could think of many possible combinations of quarks. But not all of them can exist, according to the rules of Quantum Chromodynamics (QCD), the theory that describes the strong interactions between quarks. For instance, QCD forbids four-quark configurations, while the pentaquark that left its signature on the Jefferson Lab data is an allowed state. Physicists know from conservation laws that the only possible configuration for this new particle, dubbed Theta-plus, is two up quarks, two down quarks and an anti-strange quark (uudd s-bar).