Demonstration of momentum-space diffusive particle acceleration in kinetic simulations of relativistic plasma turbulence

We present a statistical study of nonthermal particle acceleration (NTPA) in 3D particle-in-cell (PIC) simulations of driven turbulence in relativistic pair plasmas. NTPA is an important topic for astrophysical systems such as pulsar wind nebulae and black hole accretion flows. However, the diffusive nature of particle energy evolution assumed by Fokker-Planck models of NTPA has not yet been rigorously tested by self-consistent simulations. We develop a novel methodology to measure energy diffusion using the time evolution of tracked particle samples in simulations of relativistic plasma turbulence. We find that the standard deviation of energies for particles with the same initial energy indeed grows in time close to t^1/2, i.e., diffusively. The energy diffusion and advection coefficients (D_ε and A_ε) for the Fokker-Planck equation are measured as functions of particle energy ε. We find D_ε to be proportional to ε² in the nonthermal power-law tail, in line with theoretical expectations. However, we observe a much shallower scaling at lower energies. We also investigate the dependence of D_ε and A_ε on the magnetization. These results support models of NTPA in which particles stochastically gain energy from scattering by turbulent fluctuations.