  Collisionless shocks generated by two colliding relativistic
electron-positron plasma shells are studied using particle-in-cell (PIC)
simulations. Shocks are mediated by the Weibel instability (WI), and the
kinetic energy of the fastest accelerated particles is found to be
anisotropically modified by WI-induced electric fields. Specifically, we show
that all particles interacting with the shock bifurcate into two groups based
on their final relativistic Lorentz factor $\gamma$: slow ($\gamma <
\gamma_{bf}$) and fast ($\gamma > \gamma_{bf}$), where $\gamma_{bf}$ is the
bifurcation Lorentz factor that was found to be approximately twice the initial
(upstream) Lorentz factor $\gamma_0$. We have found that the energies of the
slow particles are equally affected by the longitudinal and transverse
components of the shock electric field, whereas the fast particles are
primarily accelerated by the transverse field component.
