  Hot accretion flows contain collisionless plasmas that are believed to be
capable of accelerating particles to very high energies, as a result of
turbulence generated by the magnetorotational instability (MRI). We conduct
unstratified shearing-box simulations of the MRI turbulence in ideal
magnetohydrodynamic (MHD), and inject energetic (relativistic) test particles
in simulation snapshots to conduct a detailed investigation on particle
diffusion and stochastic acceleration. We consider different amount of net
vertical magnetic flux to achieve different disk magnetizations levels at
saturated states, with sufficiently high resolution to resolve the gyro-radii
($R_g$) of most particles. Particles with large $R_g$ ($\gtrsim0.03$ disk scale
height $H$) show spatial diffusion coefficients of $\sim30$ and $\sim5$ times
Bohm values in the azimuthal and poloidal directions, respectively. We further
measure particle momentum diffusion coefficient $D(p)$ by applying the
Fokker-Planck equation to particle momentum evolution. For these particles,
contribution from turbulent fluctuations scales as $D(p)\propto p$, and shear
acceleration takes over when $R_g\gtrsim0.1H$, characterized by $D(p)\propto
p^3$. For particles with smaller $R_g$ ($\lesssim0.03H$), their spatial
diffusion coefficients roughly scale as $\sim p^{-1}$, and show evidence of
$D(p)\propto p^2$ scaling in momentum diffusion but with large uncertainties.
We find that multiple effects contribute to stochastic
acceleration/deceleration, and the process is also likely affected by
intermittency in the MRI turbulence. We also discuss the potential of
accelerating PeV cosmic-rays in hot accretion flows around supermassive black
holes.
