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Espresso is a novel acceleration model for Ultra-High-Energy Cosmic Rays(UHECRs), where lower-energy CRs produced in supernova remnants experience aone-shot reacceleration in the relativistic jets of powerful Active GalacticNuclei (AGNs) to reach energies up to $10^{20}$ eV. To test the espressoframework, we follow UHECR acceleration bottom-up from injection to the highestenergies by propagating 100,000 particles in realistic 3D magneto-hydrodynamic(MHD) simulations of ultra-relativistic jets. We find that simulations agreewell with analytical expectations in terms of trajectories of individualparticles. We also quantify that $\sim 10\%$ of CR seeds gain a factor of$\sim\Gamma^2$ in energy, where $\Gamma$ is the jet's effective Lorentz factor;moreover, about $0.1\%$ of the particles undergo two or more shots to achievegains in excess of $\Gamma^2$. Particles are generally accelerated up to thejet's Hillas limit, indicating that the espresso mechanism should boostgalactic CRs to UHECRs in typical AGN jets. Finally, we discuss how espressoacceleration in AGN jets is consistent with UHECR spectra and chemicalcomposition, and also with the UHECR arrival directions measured by Auger andTelescope Array.

Espresso Acceleration of Ultra-High-Energy Cosmic Rays up to the Hillas Limit in Relativistic MHD Jets