Narrow Pentaquark States in a Quark Model with Antisymmetrized Molecular Dynamics

The exotic baryon Θ+(uudds̄) is studied with microscopic calculations in a quark model by using a method of antisymmetrized molecular dynamics(AMD). We predict narrow states, J = 1/2+(I = 0), J = 3/2+(I = 0), and J = 3/2(I = 1), which nearly degenerate in a low-energy region of the uudds̄ system. We discuss NK decay widths and estimate them to be Γ < 7 for the J = {1/2+, 3/2+}, and Γ < 1 MeV for the J = 3/2 state. The evidence of an exotic baryon Θ has recently been reported by several experimental groups. This discovery proved the existence of the multiquark hadron, whose minimal quark content is uudds̄ as given by the decay modes. The study of pentaquarks has become a hot subject in hadron physics. A chiral soliton model [1] predicted a narrow Θ(J = 1/2) state whose parity contradicts the naive quark model expectation. Theoretical studies were done to describe Θ by many groups [2,3]. The spin parity of Θ is not only a open problem but also a key property to understand the dynamics of pentaquark systems. In this paper we would like to clarify the mechanism of the existence of narrow pentaquark states. We try to extract a simple picture for the pentaquark baryon with levels, width, spinparity and structure from explicit calculation. In order to achieve this goal, we study the pentaquark with a flux-tube model [6,7] based on strong coupling QCD, by using a AMD method [4,5].