Structure and Excitation Mechanism of theN*in Photoproduction γ +p→N*

Abstract

The photoproduction of nucleon resonances r+p-'7N* is theoretically examined in the framework of the fundamental triplet model of hadrons. The partial width of the decays N*-'7N+r and of the Breit-Wigner formula for the processes r+p-'7N*-'7N+rc are obtained in terms of the internal quantum numbers and the structure parameters of the Nand N* 's. The basic assumption is as follows: The nucleon and its resonance levels are well de­ scribed by the non-relativistic bound state wave functions of three constituent particles. Both the nucleon and the (3,3) -resonance, Na3* belong to the 56-dimensional representation of the SU6, and are further characterized by the S-wave internal space wave function. The other resonances are regarded as the single particle excitation mode in which one of constituents is in· a higher angular momentum state. The electromagnetic interaction is U-s pin invariant and its exchange current effect is neglected. First it is shown that the selection rule for electric and magnetic multipoles to induce the process r + N--'7 N* is completely fixed by the above assumption and an additional hypo­ thetical relation L+S=J which may be called the Kycia-Riley relation, where L, S and J are the internal orbital, spin and total angular momentum, respectively. Both the electric and magnetic multipoles (G'J-1/2 and 3JiJ+1/2) contribute to the excitation of the N 1,2J* 's with I= (1/2), while only the magnetic muhipole transition (3/iJ -1/2) is necessary for the excitation of the N 3,2J* 's with I= (3/2), I being the isospin of the resonance levels. Consequently the N 11* (1400) excitation would be much suppressed in agreement with experim~nt. The partial widths for various N* excitations are numerically obtained. It is shown that the existing experimental data on the Naa* and N1a* are consistent with them if the spatial extension of the bound state is put nearly equal to the nucleon Compton wave length. It is also noted that only the partial width of the N 33* photoexcitation is not sensitive to the core radius in the long photon wave-length approximation and in the su6 symmetric limit of the internal space wave function.