Why pentaquarks are seen in some experiments and not in others
  The $\Theta^+$ Pentaquark is a Very narrow $\Gamma \approx 1$ MeV   KN Resonance. Why do some experiments see it and others do not? The lowest quark configuration that can describe it is exotic $uudd \bar s$.   Why have no exotics been seen before?   Is this the beginning of a new spectroscopy? Can it help toteach us about How QCD makes hadrons from quarks and gluons? 
Harry J. Lipkin