5. Given that the base $A B C$ of the regular tetrahedron $P-A B C$ is an equilateral triangle, and the center $O$ of the circumscribed sphere of the regular tetrahedron satisfies $\overrightarrow{O A}+\overrightarrow{O B}+\overrightarrow{O C}=\mathbf{0}$. Then the cosine value of the dihedral angle $A-P B-C$ is $(\quad)$.
(A) $\frac{1}{6}$
(B) $\frac{\sqrt{2}}{8}$
(C) $\frac{1}{5}$
(D) $\frac{\sqrt{3}}{3}$