Understanding the Covariance of the Spin Projection Operator in Rest Frame?

[LayMuon]

I cannot quite understand why expression \frac{1-\gamma_5 \slashed{s}}{2} is covariant? We defined it in the rest frame, and then said that because it is in the slashed expression, it's covariant, what does that mean? s is the direction of polarization, s \cdot s = -1

 

[LayMuon]

Figured it out, one should use the identity \hat{S}(\hat{a}) \gamma^\nu \hat{S}^{-1} (\hat{a}) = a_\mu ^{\text{ } \nu} \gamma^\mu, the rest is straightforward.