  We investigate shock structure and particle acceleration in relativistic
magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell
simulations. We explore a range of inclination angles between the pre-shock
magnetic field and the shock normal. We find that only magnetic inclinations
corresponding to "subluminal" shocks, where relativistic particles following
the magnetic field can escape ahead of the shock, lead to particle
acceleration. The downstream spectrum in such shocks consists of a relativistic
Maxwellian and a high-energy power-law tail with exponential cutoff. For
increasing magnetic inclination in the subluminal range, the high-energy tail
accounts for an increasing fraction of particles (from ~1% to ~2%) and energy
(from ~4% to ~12%). The spectral index of the power law increases with angle
from -2.8+-0.1 to -2.3+-0.1. Particle energization is driven by the Diffusive
Shock Acceleration process for nearly parallel shocks, and switches to
Shock-Drift Acceleration for larger subluminal inclinations. For "superluminal"
shocks, the downstream particle spectrum does not show any significant
suprathermal tail. As seen from the upstream frame, efficient acceleration in
relativistic (Lorentz factor gamma0 > 5) magnetized (sigma > 0.03) flows exists
only for a very small range of magnetic inclination angles (< 34/gamma0
degrees), so relativistic astrophysical pair shocks have to be either nearly
parallel or weakly magnetized to generate nonthermal particles. These findings
place constraints on the models of AGN jets, Pulsar Wind Nebulae and Gamma Ray
Bursts that invoke particle acceleration in relativistic magnetized shocks.
(Abridged)
