Particle acceleration and entropy production in relativistic plasma turbulence

Abstract: Turbulence has long been considered as a candidate process for accelerating particles to relativistic energies in the collisionless plasmas associated with space and astrophysical systems. There is a long line of stochastic particle acceleration theories spanning back to Enrico Fermi in 1949; these models are widely used for describing cosmic rays and radiative signatures from high-energy astrophysical systems, but have remained essentially unvalidated over the decades. Particle-in-cell (PIC) simulations of relativistic turbulence have recently opened this topic to rigorous, first-principles numerical scrutiny. I will describe the latest progress on understanding relativistic particle energization by turbulence. PIC simulations of turbulence demonstrate efficient particle acceleration and confirm its diffusive nature, while also yielding new insights into the electron-to-ion heating ratio and intermittent kinetic beaming. However, several deep mysteries remain, such as the prevalence and properties of power-law particle energy distributions. I suggest that a complete understanding will require us to confront the age-old problem of entropy production in collisionless plasmas. I will describe new theoretical efforts to model particle acceleration with generalized maximum entropy principles, which explain several numerical results and may extend to phenomena beyond turbulence (such as magnetic reconnection). The next several years promise to bring new, fundamental breakthroughs into these problems.