On the rotational invariance and non-invariance of lepton angular distributions in Drell–Yan and quarkonium production

Several rotational invariant quantities for the lepton angular distributions in Drell–Yan and quarkonium production were derived several years ago, allowing the comparison between different experiments adopting different reference frames. Using an intuitive picture for describing the lepton angular distribution in these processes, we show how the rotational invariance of these quantities can be obtained. This approach can also be used to determine the rotational invariance or non-invariance of various quantities specifying the amount of violation for the Lam–Tung relation. While the violation of the Lam–Tung relation is often expressed by frame-dependent quantities, we note that alternative frame-independent quantities are preferred.