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 $E×B$ 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 $ξ=vd/v̄≪1,$ 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 $ξ7/6 exp(−32(2ξ)2/3).$ The adiabatic cutoff implies that collisional drag can only assist positron–antiproton recombination up to a finite binding energy.