  Magnetic reconnection in relativistic collisionless plasmas can accelerate
particles and power high-energy emission in various astrophysical systems.
Whereas most previous studies focused on relativistic reconnection in pair
plasmas, less attention has been paid to electron-ion plasma reconnection,
expected in black hole accretion flows and relativistic jets. We report a
comprehensive particle-in-cell numerical investigation of reconnection in an
electron-ion plasma, spanning a wide range of ambient ion magnetizations
$\sigma_i$, from the semirelativistic regime (ultrarelativistic electrons but
nonrelativistic ions, 0.001<<$\sigma_i$<<1) to the fully relativistic regime
(both species are ultrarelativistic, $\sigma_i$>>1). We investigate how the
reconnection rate, electron and ion plasma flows, electric and magnetic field
structures, electron/ion energy partitioning, and nonthermal particle
acceleration depend on $\sigma_i$. Our key findings are: (1) the reconnection
rate is about 0.1 of the Alfvenic rate across all regimes; (2) electrons can
form concentrated moderately relativistic outflows even in the
semirelativistic, small-$\sigma_i$ regime; (3) while the released magnetic
energy is partitioned equally between electrons and ions in the
ultrarelativistic limit, the electron energy fraction declines gradually with
decreased $\sigma_i$ and asymptotes to about 0.25 in the semirelativistic
regime; (4) reconnection leads to efficient nonthermal electron acceleration
with a $\sigma_i$-dependent power-law index, $p(\sigma_i) \simeq $const$+0.7
{\sigma_i}^{-1/2}$. These findings are important for understanding black hole
systems and lend support to semirelativistic reconnection models for powering
nonthermal emission in blazar jets, offering a natural explanation for the
spectral indices observed in these systems.
