From the equation, I can see that there is axial symmetry in the PAS because the matrix becomes 0 in all its terms except the diagonal, which its trace equals 0 too. And also because it doesn’t appear any sum term in the tensor, which means that the ή=0, and that indicates a symmetric situation.
From a visual inspection, I just would try to imagine the axis in which the system can rotate with a Cinfinite symmetry axis. This can only happen in the case of tetta=0, on that case all the particles remain at a same line, which we denominate z axis and is the PAS.