A particle with integer or half-integer spin \(s\) has \(2 s+1\) values of spin with respect to any arbitrary spin quantization axis \(\mathbf{n}\) in the particle's rest frame. If parity is a good symmetry, a spin eigenstate for a massless particle is a helicity eigenstate with helicity eigenvalues \(\pm s\). If parity is not conserved, then a single helicity eigenstate is possible with eigenvalue either \(+s\) or \(-s\). As succinctly as you can, summarize the key reasons for this result.