Two-Spin Asymmetry for ${\psi }^{\mathrm{\prime }}$ Photoproduction with Color-Octet Mechanism

We studied the photoproduction of ${\psi }^{\mathrm{\prime }}$ in the forward regions in polarized $\gamma p$ collisions at relevant HERA energies. We found that this reaction is very effective to test the color-octet mechanism which is based on the NRQCD factorization formalism. Furthermore we found that the value of the NRQCD matrix elements can be severely constrained by measuring the two-spin asymmetry, though the process depends on the polarized gluon distribution $\mathrm{\Delta }g(x)$.