Collisional and Radiative Relaxation of Antihydrogen.

Antihydrogen is produced in high-magnetic-field Penning traps by introducing antiprotons into a pure-positron plasma at cryogenic temperature $T$.\footnote{G. Gabrielse et al., {\it Phys. Rev. Lett.} {\bf 89}, 213401 (2002).}$^,$\footnote{M. Amoretti et al., {\it Nature} {\bf 419}, 456 (2002).} In the experimental regime, three-body recombination forms highly-excited atoms which exhibit classical guiding-center drift orbits.\footnote{M.E. Glinsky and T.M. O'Neil, {\it Phys. Fluids B} {\bf 3}, 1279 (1991).}$^,$\footnote{F. Robicheaux and J.D. Hanson, {\it Phys. Rev. A} {\bf 69}, 010701 (2004).} Using energy transition rates obtained from a Monte-Carlo simulation, we track the collisional evolution of a distribution of atoms from binding energies near $T$ to $U_c = e^{2} (B^{2}/m_e c^{2})^{1/3}$, where atom dynamics is chaotic. While the flux through the kinetic bottleneck $(U = 4 T)$ is proportional to $T^{-9/2}$, data suggest that the flux at $U_c$ (at a fixed time) does not scale strongly with $T$ or magnetic field $B$. At $U_c$, radiation begins to take over as the principle energy-loss mechanism. Evolution due to radiation is tracked for a typical collisionally-evolved energy distribution to show that a small number of low-angular-momentum atoms radiate to the ground state rapidly, while others drop into slowly-radiating, circular orbits at intermediate energies.