On the low energy behavior of Regge poles

We investigate the behavior of Regge poles in the low energy limit. With the use of small argument asymptotics of the spherical Hankel functions, we show that for a finite square well potential, the associated Regge poles tend to the spectral points of the limiting self-adjoint problem. This is generalized to a compactly supported potential by applying a resolvent argument to the difference of the nonzero and zero energy wavefunctions. Furthermore, by an integral equation method we prove analogous results for a potential such that |(1+r)U(r)| is integrable. This confirms the experimental results which show that Regge poles formed during low energy electron elastic scattering become stable bound states.