  Relativistic magnetized jets from active galaxies are among the most powerful
cosmic accelerators, but their particle acceleration mechanisms remain a
mystery. We present a new acceleration mechanism associated with the
development of the helical kink instability in relativistic jets, which leads
to the efficient conversion of the jet's magnetic energy into nonthermal
particles. Large-scale three-dimensional ab initio simulations reveal that the
formation of highly tangled magnetic fields and a large-scale inductive
electric field throughout the kink-unstable region promotes rapid energization
of the particles. The energy distribution of the accelerated particles develops
a well-defined power-law tail extending to the radiation-reaction limited
energy in the case of leptons, and to the confinement energy of the jet in the
case of ions. When applied to the conditions of well-studied bright knots in
jets from active galaxies, this mechanism can account for the spectrum of
synchrotron and inverse Compton radiating particles, and offers a viable means
of accelerating ultra-high-energy cosmic rays to $10^{20}$ eV.
