UC San Diego

Recombination of antiprotons with positrons (into neutral antihydrogen) in a strong magnetic field is investigated with classical models. A single antiproton in a pure- positron plasma is considered, given the very-low antiproton density in antihydrogen experiments. In these cryogenic experiments, three-body recombination dominates charge capture and produces highly-excited atoms with binding energy near the thermal level. Such atoms exist in a "guiding-center" state, characterized by ExB drift motion and adiabatically invariant cyclotron action in the positron orbit. De-excitation occurs by collisional and radiative relaxation. Radiation is small in guiding-center atoms. At deeper binding or low angular momentum, the orbit is chaotic and radiation becomes significant. Collisional de-excitation is investigated with a classical Monte-Carlo code and analytic theory. The code gives the probability w([epsilon],[epsilon]') of an atom making a collisional transition from energy [epsilon] to energy [epsilon]'. The component from collisions with large impact parameters is confirmed with analytic theory using integrals over unperturbed trajectories. A direct calculation of the drag exerted by the plasma on the bound positron orbit confirms the energy-loss rate from such collisions. It peaks when the drift velocity equals the thermal velocity. The transition rates from the Monte- Carlo simulation are used in a numerical solution of the master equation to calculate the rate of antihydrogen formation from a thermal plasma. Deep binding energies become populated as states trickle in from an invariant reservoir of thermal equilibrium atoms at shallow binding. A steady-state distribution forms at shallow binding, then propagates to deep binding over thousands of collision times. We estimate the number of atoms that collisionally de-excite to the chaotic regime for typical experimental parameters. A classical estimate of the radiation rate is made by averaging the Larmour power over phase-space surfaces defined by fixing the two conserved atomic variables: azimuthal angular momentum and energy. A small fraction of low-angular-momentum atoms will radiate rapidly to the ground state. We estimate the number of such atoms from the distributions calculated before