Energy loss rate for guiding-center antihydrogen atoms

Collisional drag between a bound positron and a background positron plasma is considered as a mechanism for guiding-center antihydrogen atoms to relax to deeply bound states. Contrary to previous assessment, an adiabatic cutoff to the drag is predicted at deep binding, when the bound positron's ExB drift speed vd exceeds the plasma positron thermal speed. In this regime, small-impact parameter collisions neglected in the drag calculation become the dominant 3-body recombination mechanism. At shallow binding, when ξ=vd/v-bar<<1, the atom's energy loss rate due to drag scales like ξ3/2 log2 ξ. When ξ>>1 the adiabatic cutoff takes over and the rate scales as ξ7/6 exp(-(3/2)(2ξ)2/3). The adiabatic cutoff implies that collisional drag can only assist positron-antiproton recombination up to a finite binding energy