Meaning and interpretation of the frame-independent polarization

AbstractThis chapter discusses the importance of the invariant polarization observable $$\mathcal{F}$$ (or $$\tilde{\lambda}$$) in certain physics scenarios, where none of the adoptable polarization frames would provide a particularly simple picture in terms of $$\lambda_\vartheta$$, $$\lambda_\varphi$$ and $$\lambda_{\vartheta\varphi}$$. One such case is the production of Drell--Yan dileptons, where the polarization parameters, calculated including perturbative QCD corrections, satisfy the Lam--Tung identity, a frame-independent relation maintaining its seemingly surprising simplicity even when the polar and azimuthal anisotropies have strong dependences on the particle momentum. The notion of invariant polarization allows us to reinterpret this relation in a geometrical way, explaining it as a mere consequence of helicity conservation and rotational invariance.