Atomic collisions at ultrarelativistic energies

At ultrarelativistic energies, atomic collisions between heavy, highly-charged atoms produce extremely intense, rapidly varying electromagnetic fields that, in turn, give rise to very large ionization cross sections, diminished normal three-body electron capture cross sections, and copious lepton pair formation. Availability of beams of ultrarelativistic heavy ions at the CERN SPS enabled experimental studies of a variety of basic atomic collision phenomena in gases and solids, including radiative electron capture and electron capture from pair production, total energy loss, ionization and electron loss to continuum, and ion channeling. Results of some of these measurements and comparisons with theoretical treatments will be presented, as well as a number of remaining puzzles presented by the data.