Random matrix prediction of average entanglement entropy in non-Abelian symmetry sectors

We study the average bipartite entanglement entropy of Haar-random pure states in quantum many-body systems with global $\mathrm{S}\mathrm{U}(2)$ symmetry, constrained to fixed total spin $J$ and magnetization $J_z=0$. Focusing on spin-$\frac{1}{2}$ lattices and subsystem fractions $f<\frac{1}{2}$, we derive a asymptotic expression for the average entanglement entropy up to constant order in the system volume $V$. In addition to the expected leading volume law term, we prove the existence of a $\frac{1}{2}\log V$ finite-size correction resulting from the scaling of the Clebsch-Gordon coefficients and compute explicitly the $O(1)$ contribution reflecting angular-momentum coupling within magnetization blocks. Our analysis uses features of random matrix ensembles and provides a fully analytical treatment for arbitrary spin densities, thereby extending Page type results to non-Abelian sectors and clarifying how $\mathrm{S}\mathrm{U}(2)$ symmetry shapes average entanglement.