  Beam halo is one of the crucial issues limiting the machine performance and
causing radioactivation in high-intensity accelerators. A clear picture of
beam-halo formation is of great importance for successful suppression of the
undesired beam loss. We present numerical and experimental studies of
transverse and longitudinal halos in the KEK Accelerator Test Facility. The
fair accordance between predictions and observations in various conditions
indicates that the Touschek scattering is the dominant mechanism forming the
horizontal and momentum halos.
