Understanding Pentaquark States in QCD

We estimate the mass of the pentaquark state with QCD sum rules and find that pentaquark states with isospin $I=0,1,2$ lie close to each other around $(1.55\pm 0.15)$ GeV. The experimentally observed baryon resonance $\Theta^+ (1540)$with$S=+1$canbeconsistentlyidentifiedasapentaquarkstateifits$J^P={1\over 2}^-$. Such a state is expected in QCD. If its parity is positive, this pentaquark state is really exotic. Now the outstanding issue is to determine its quantum numbers experimentally.