  Ab-initio numerical study of collisionless shocks in electron-ion
unmagnetized plasmas is performed with fully relativistic particle in cell
simulations. The main properties of the shock are shown, focusing on the
implications for particle acceleration. Results from previous works with a
distinct numerical framework are recovered, including the shock structure and
the overall acceleration features. Particle tracking is then used to analyze in
detail the particle dynamics and the acceleration process. We observe an energy
growth in time that can be reproduced by a Fermi-like mechanism with a reduced
number of scatterings, in which the time between collisions increases as the
particle gains energy, and the average acceleration efficiency is not ideal.
The in depth analysis of the underlying physics is relevant to understand the
generation of high energy cosmic rays, the impact on the astrophysical shock
dynamics, and the consequent emission of radiation.
