Pentaquarks: do they exist or not?

Pentaquarks, if they exist as narrow resonances, would provide a new testing ground for non-perturbative QCD. After an initial round of positive evidence for a strangeness S=+1 resonance at about 1535 MeV, called the $\Theta ^{+}$, there is a growing body of null results by high-statistics experiments. In addition, constraints from older KN scattering data suggest that the width of the $\Theta ^{+}$ resonance must be uncomfortably narrow ($<$1 MeV) if it has spin J=1/2. This has led some people to pronounce the $\Theta ^{+}$ to be ``dead''. However, the body of evidence is not yet complete. Several new experiments specifically designed to search for the $\Theta ^{+}$ in different reaction channels have yet to announce their results. Furthermore, it is possible that the $\Theta ^{+}$ could exist with spin J=3/2, as suggested by recent lattice gauge calculations, which allows for a more reasonable resonance width. In any case, convincing evidence for the $\Theta ^{+}$ is still lacking, and we should be skeptical about its existence at the present time.