The coefficients determining the dilepton decay angular distribution of vector particles obey certain positivity constraints and a rotation-invariant identity. These relations are a direct consequence of the covariance properties of angular momentum eigenstates and are independent of the production mechanism. The Lam-Tung relation can be derived as a particular case, simply recognizing that the Drell-Yan dilepton is always produced transversely polarized with respect to one or more quantization axes. The dilepton angular distribution continues to be characterized by a frame-independent identity also when the Lam-Tung relation is violated. Moreover, the violation can be easily characterized by measuring a one-dimensional distribution depending on one shape coefficient.
@article{arxiv.1102.3946,
title = {Model-independent constraints on the shape parameters of dilepton angular distributions},
author = {Pietro Faccioli and Carlos Lourenco and Joao Seixas and Hermine K. Woehri},
journal= {arXiv preprint arXiv:1102.3946},
year = {2011}
}