A Perspective on Pentaquarks

Recent discoveries of manifestly exotic, narrow baryon resonances present a fundamental challenge for our theoretical understanding of low-energy QCD. This is a brief survey of their interpretation, emphasizing the possibility that diquark correlations are centrally involved. Many predictions and suggestions for future directions follow from that idea. Recently several experimental groups have reported observation of a new, manifestly exotic (B=1, S=1) baryon resonance Θ + (1540) [1, 2, 3, 4, 5, 6]. The resonance seems unusually narrow (Γ < 15 Mev) for a particle in this mass range with open channels to strong decay. Shortly after the conference the NA49 group at CERN announced evidence for an additional narrow “cascade ” exotic (B=1, Q=S=-2) Ξ − − and also Ξ −,Ξ 0 with masses close to 1860 Mev [7, 8]. (Particles of this kind were predicted in my talk, at the time it was delivered.) Since the simplest quark assignments consistent with the quantum numbers of Θ + and Ξ − − are (udud¯s) and (dsdsū) respectively, these particles have come to be called “pentaquarks”.