Pentaquark as a NK^* bound state with TJ^P=0{3/2}^-

Abstract

We have investigated negative-parity uudd\bar{s} pentaquarks by employing a quark model with the meson exchange and the effective gluon exchange as qq and q\bar{q} interactions. The system for the five quarks is dynamically solved; the effects of qq and q\bar{q} correlations on the wave function are taken into account. It is found that the masses of the pentaquarks are reasonably low. The lowest-mass state is found to be TJ^P=0{1/2}^-, and the next lowest one is 0{3/2}^-. The former is reported to have a large width. We argue the observed narrow peak corresponds to the latter state. It is still necessary to introduce an extra attraction to reduce the mass further by 140 -- 280 MeV to reproduce the observed Θ^+ mass. Since their level splitting is found to be less than 80 MeV, the lower level will not become a bound state below the NK threshold even after such an attraction is introduced. It is also found that the relative distance of two quarks with the attractive interaction is found to be by about 1.2 -- 1.3 times closer than that of the repulsive one. The two-body correlation seems important in the pentaquark systems.