Past exam of the symmetry and relativity course of the University of Oxford / 2017 / 4 / c

For $P^{\mu }=(E/c,\mathbf{p})$, directly contract the supplied vector:Thus $\boxed{U_W\cdot P=0}$ whenever its stated normalization is defined, namely $m_0>0$ and $|\mathbf{s}|>0$. This is the Pauli-Lubanski orthogonality property.

The description as a unit vector, however, needs an additional assumption. From the given components and the mass shell,It equals minus one only for spin parallel to momentum, including the zero-momentum case. A concrete counterexample is $\mathbf{s}=(s,0,0)$ and $\mathbf{p}=(0,m_{0}c,0)$: the proposed normalized vector has square minus two. Orthogonality is still true. A genuine Pauli-Lubanski pseudovector constructed by boosting a fixed rest-frame spin has invariant norm $-m_0^2{c}^2|{\mathbf{s}}_{\mathrm{rest}}{|}^2$; the source's generic spatial expression and denominator do not supply that normalization for arbitrary $\mathbf{s}$.