Breit-Wigner phase is a fundamental property of a resonance
  In the course of devising a simple method for extraction of the S-matrix poles from the data, an additional fundamental resonance property emerged. It is a reaction invariant quantity, and since it is directly related to the Breit-Wigner parameters, we call it the Breit-Wigner phase beta. We propose that this beta is added in resonant data tables. 
S. Ceci, M. Vuk\v{s}i\'c and B. Zauner
hep-ph