  We investigate the effects of pitch-angle scattering on the efficiency of
particle heating and acceleration by MHD turbulence using phenomenological
estimates and simulations of non-relativistic test particles interacting with
strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering
rate, which is meant to approximate the effects of high frequency plasma waves
and/or velocity space instabilities. We focus on plasma parameters similar to
those found in the near-Earth solar wind, though most of our results are more
broadly applicable. An important control parameter is the size of the particle
mean free path lambda_{mfp} relative to the scale of the turbulent fluctuations
L. For small scattering rates, particles interact quasi-resonantly with
turbulent fluctuations in magnetic field strength. Scattering increases the
long-term efficiency of this resonant heating by factors of a few-10, but the
distribution function does not develop a significant non-thermal power-law
tail. For higher scattering rates, the interaction between particles and
turbulent fluctuations becomes non-resonant, governed by particles heating and
cooling adiabatically as they encounter turbulent density fluctuations. Rapid
pitch-angle scattering can produce a power-law tail in the proton distribution
function but this requires fine-tuning of parameters. Moreover, in the
near-Earth solar wind, a significant power-law tail cannot develop by this
mechanism because the particle acceleration timescales are longer than the
adiabatic cooling timescale set by the expansion of the solar wind. Our results
thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin
of the v^{-5} tail in the proton distribution function observed in the solar
wind.
