  The streaming instability is a promising mechanism to drive the formation of
planetesimals in protoplanetary disks. To trigger this process, it has been
argued that sedimentation of solids onto the mid-plane needs to be efficient
and therefore that a quiescent gaseous environment is required. It is often
suggested that dead-zone or disk-wind structure created by non-ideal
magnetohydrodynamical (MHD) effects meets this requirement. However,
simulations have shown that the midplane of a dead zone is not completely
quiescent. In order to examine the concentration of solids in such an
environment, we use the local-shearing-box approximation to simulate a
particle-gas system with an Ohmic dead zone including mutual drag force between
the gas and the solids. We systematically compare the evolution of the system
with ideal or non-ideal MHD, with or without back-reaction drag force from
particles on gas, and with varying solid abundances. Similar to previous
investigations of dead zone dynamics, we find that particles of dimensionless
stopping time $\tau_s=0.1$ do not sediment appreciably more than those in ideal
magneto-rotational turbulence, resulting in a vertical scale height an order of
magnitude larger than in a laminar disk. Contrary to the expectation that this
should curb the formation of planetesimals, we nevertheless find that strong
clumping of solids still occurs in the dead zone when solid abundances are
similar to the critical value for a laminar environment. This can be explained
by the weak radial diffusion of particles near the mid-plane. The results imply
that the sedimentation of particles to the mid-plane is not a necessary
criterion for the formation of planetesimals by the streaming instability.
