  Electron acceleration during magnetic reconnection is thought to play a key
role in time-variable high-energy emission from astrophysical systems. By means
of particle-in-cell simulations of trans-relativistic reconnection, we
investigate electron injection and acceleration mechanisms in low-$\beta$
electron-proton plasmas. We set up a diversity of density and field structures
(e.g., X-points and plasmoids) by varying the guide field strength and choosing
whether to trigger reconnection or let it spontaneously evolve. We show that
the number of X-points and plasmoids controls the efficiency of electron
acceleration, with more X-points leading to a higher efficiency. Using
on-the-fly acceleration diagnostics, we also show that the non-ideal electric
fields associated with X-points play a critical role in the first stages of
electron acceleration. As a further diagnostic, we include two populations of
test particles that selectively experience only certain components of electric
fields. We find that the out-of-plane component of the parallel electric field
determines the hardness of the high-energy tail of the electron energy
distribution. These results further our understanding of electron acceleration
in this regime of magnetic reconnection and have implications for realistic
models of black hole accretion flows.
