It is well known that unconstrained single-energy partial wave analysis (USEPWA) gives many equivalent discontinuous solutions, so a constraint to some theoretical model must be used to ensure the uniqueness. It can be shown that it is a direct consequence of not specifying the angle-dependent part of continuum ambiguity phase which mixes multipoles, and by choosing this phase we restore the uniqueness of USEPWA, and obtain the solution in a model independent way.
Up to now, there was no reliable way to extract pole parameters from so obtained SE partial waves, but a new and simple single-channel method (Laurent + Pietarinen expansion) applicable for continuous and discrete data has been recently developed. It is based on applying the Laurent decomposition of partial wave amplitude, and expanding the non-resonant background into a power series of a conformal-mapping, quickly converging power series obtaining the simplest analytic function with well-defined partial wave analytic properties which fits the input. The generalization of this method to multi- channel case is also developed and presented.
Unifying both methods in succession, one constructs a model independent procedure to extract pole parameters directly from experimental data without referring to any theoretical model.