  The magnetorotational instability (MRI) plays a crucial role in regulating
the accretion efficiency in astrophysical accretion disks. In low-luminosity
disks around black holes, such as Sgr A* and M87, Coulomb collisions are
infrequent, making the MRI physics effectively collisionless. The collisionless
MRI gives rise to kinetic plasma effects that can potentially affect its
dynamic and thermodynamic properties. We present 2D and 3D particle-in-cell
(PIC) plasma simulations of the collisionless MRI in stratified disks using
shearing boxes with net vertical field. We use pair plasmas, with initial
$\beta=100$ and concentrate on sub-relativistic plasma temperatures ($k_BT
\lesssim mc^2$). Our 2D and 3D runs show disk expansion, particle and magnetic
field outflows, and a dynamo-like process. They also produce magnetic pressure
dominated disks with (Maxwell stress dominated) viscosity parameter $\alpha
\sim 0.5-1$. By the end of the simulations, the dynamo-like magnetic field
tends to dominate the magnetic energy and the viscosity in the disks. Our 2D
and 3D runs produce fairly similar results, and are also consistent with
previous 3D MHD simulations. Our simulations also show nonthermal particle
acceleration, approximately characterized by power-law tails with temperature
dependent spectral indices $-p$. For temperatures $k_BT \sim 0.05-0.3\, mc^2$,
we find $p\approx 2.2-1.9$. The maximum accelerated particle energy depends on
the scale separation between MHD and Larmor-scale plasma phenomena in a way
consistent with previous PIC results of magnetic reconnection-driven
acceleration. Our study constitutes a first step towards modeling from first
principles potentially observable stratified MRI effects in low-luminosity
accretion disks around black holes.
