  Particle-in-cell (PIC) simulations have shown that relativistic collisionless
shocks mediated by the Weibel instability accelerate about 1% of incoming
particles, while the majority are transmitted through the shock and become
thermalized. The microphysical processes that determine whether an incoming
particle will be transmitted or reflected are poorly understood. We study the
microphysics of particle reflection in Weibel-mediated shocks by tracking a
shell of test particles in a PIC simulation of a shock in pair plasma. We find
that electrons in positron-dominated filaments and positrons in
electron-dominated filaments efficiently reflect off of strong magnetic
structures at the shock. These reflected particles headed towards the upstream
must then find filaments of the same sign of current as the current carried by
the reflected particles in order to successfully move with the shock and
participate in diffusive shock acceleration (DSA). The final injection
efficiency on the order of about 1% thus results from the effectiveness of the
initial reflection at the shock and the reflected particles' probability of
survival in the upstream post-reflection. We develop a model that predicts the
fraction of high-energy particles as a function of the properties of Weibel
filamentation.
