  We present large scale 3D particle-in-cell (PIC) simulations to examine
particle energization in magnetic reconnection of relativistic
electron-positron (pair) plasmas. The initial configuration is set up as a
relativistic Harris equilibrium without a guide field. These simulations are
large enough to accommodate a sufficient number of tearing and kink modes.
Contrary to the non-relativistic limit, the linear tearing instability is
faster than the linear kink instability, at least in our specific parameters.
We find that the magnetic energy dissipation is first facilitated by the
tearing instability and followed by the secondary kink instability. Particles
are mostly energized inside the magnetic islands during the tearing stage due
to the spatially varying electric fields produced by the outflows from
reconnection. Secondary kink instability leads to additional particle
acceleration. Accelerated particles are, however, observed to be thermalized
quickly. The large amplitude of the vertical magnetic field resulting from the
tearing modes by the secondary kink modes further help thermalizing the
non-thermal particles generated from the secondary kink instability.
Implications of these results for astrophysics are briefly discussed.
