  Electron and proton acceleration in three-dimensional electric and magnetic
fields is studied through test particle simulations. The fields are obtained by
a three-dimensional magnetohydrodynamic simulation of magnetic reconnection in
slab geometry. The nonlinear evolution of the system is characterized by the
growth of many unstable modes and the initial current sheet is fragmented with
formation of small scale structures. We inject at random points inside the
evolving current sheet a Maxwellian distribution of particles. In relatively
short time (less than a millisecond) the particles develop a power law tail.
The acceleration is extremely efficient and the electrons absorb a large
percentage of the available energy in a small fraction of the characteristic
time of the MHD simulation, suggesting that resistive MHD codes, used
extensively in the current literature, are unable to represent the full extent
of particle acceleration in 3D reconnection.
