  In circular colliders, as well as in damping rings and synchrotron radiation
light sources, beam halo is one of the critical issues limiting the performance
as well as potentially causing component damage and activation. It is
imperative to clearly understand the mechanisms that lead to halo formation and
to test the available theoretical models. Elastic beam-gas scattering can drive
particles to large oscillation amplitudes and be a potential source of beam
halo. In this paper, numerical estimation and Monte Carlo simulations of this
process at the ATF of KEK are presented. Experimental measurements of beam halo
in the ATF2 beam line using a diamond sensor detector are also described, which
clearly demonstrates the influence of the beam-gas scattering process on the
transverse halo distribution.
