Deep Minimum and a Vortex for Positronium Formation in Low-Energy Positron-Helium Collisions

We find a zero in the positronium formation scattering amplitude and deep minimum logarithm of corresponding differential cross section for positron–helium collisions an energy just above threshold. Corresponding to zero, there is vortex extended velocity field that associated with this when one treats both magnitude momentum incident positron angle scattered as independent variables. Using complex Kohn variational method, we determine accurately two-channel K-matrices Ore gap. fit these using polynomials Watanabe Greene’s multichannel effective range theory taking into account explicitly polarization potential Ps-He+ channel. fitted show it rotates anticlockwise around amplitude. valley includes also forward direction.