Deep minimum in the Ps-formation differential cross section for positron-helium collisions in the Ore gap*

Using the {\it s-}, {\it p-}, {\it d-} and {\it f-}wave complex Kohn K-matrices for positron-helium collisions in the Ore gap [1] we have computed the Ps-formation differential cross section. We found a deep minimum in the cross section that corresponds to a zero in the Ps-formation scattering amplitude and a vortex in the extended velocity field [2] that is associated with this amplitude. Using the Watananbe and Greene's multichannel effective range theory [3] and polynomial fits of the K-matrices we are exploring the importance of the polarization potential in the Ps-He$^+$ channel \item{[1.]} P.~Van~Reeth and J.~W.~Humberston, J.~Phys.~B {\bf 32} 3651 (1999). \item{[2.]} A.~W.~Alrowaily, S.~J.~Ward and P.~Van Reeth, J.~Phys.~B {\bf 52} 205201 (2019). \item{[3.]} S.~Watanabe and C.~H.~Greene, Phys.~Rev.~A {\bf 22} 158 (1980).