Five-body calculation of resonance and scattering states of pentaquark system

Abstract

Scattering problem of the $uudd{\bar s}$ system, in the standard non-relativistic quark model of Isgur-Karl, is solved for the first time, by treating the large five-body modelspace including the $NK$ scattering channel accurately with the Gaussian expansion method and the Kohn-type coupled-channel variational method. The two-body interaction that reproduces observed properties of ordinary baryons and mesons is applied to the pentaquark system with no additional adjustable parameter. The five-body wave function calculated has the correct asymptotic form in its the scattering-channel component and describes $qq$ and $q{\bar q}$ correlations properly. The $NK$ scattering phase shift calculated shows no resonance in the energy region of the reported pentaquark $\Theta^+(1540)$, that is, at 0-500 MeV above the $NK$ threshold ($1.4-1.9$ GeV in mass). The phase shift does show two resonances just above 500 MeV: a broad ${1/2}^+$ resonance with a width of $ \Gamma \sim~110$ MeV located at $\sim~520$ MeV ($\sim~2.0$ GeV in mass) and a sharp ${1/2}^-$ resonance with $\Gamma=$0.12 MeV at 540 MeV. Properties of these states are discussed.