## Interconnections between the Physics of Plasmas and Self-gravitating Systems

The long-range nature of the inverse square law governs the key physics of both dilute electromagnetic plasmas (i.e. collections of charged particles) and self-gravitating systems (i.e. collections of massive point-like objects in star clusters). This physics is central to understanding many key problems in heliophysics and astrophysics, including the origin of the solar wind, accretion disks around black holes, and star clusters around massive black holes. |

The crucial similarity in plasma physics and self-gravitating systems arises from the fact that inter-particle interactions in both systems are primarily governed by coherent forces from distant particles, as opposed to quasi-random forces from violent collisions with nearby particles. This implies they must be described in six-dimensional phase space using kinetic theory. They also exhibit many equivalent processes, such as Landau damping, dynamical friction, resonant relaxation, quasi-periodic orbits, and polarization effects. |

However, with a few notable exceptions, the two research communities have remained separate, explaining different phenomena using different languages, across different scales, from different observational data sets. For this program, we aim to stimulate conversation between these two groups, with a particular focus on fundamental kinetic theory such as collisionless/collisional relaxation and phase-space dynamics. The goals are to establish a common language for the kinetic theory of plasmas and self-gravitating systems, to foster a fruitful exchange of ideas and methods between our two communities, and to tackle the fundamental physics of phase-space dynamics in new and creative ways. |

Some specific topics and questions for consideration include: + Collisionless relaxation – how do systems relax in phase space on timescales much shorter than the particle collision time? Do there exist generic relaxed states? + Collisional relaxation – how do we predict, use, measure, and understand collision operators? What is the impact of finite-N effects? + Reduced models – how do we formulate and use closures and reduced models (e.g., gyrokinetics or orbit-averaged methods)? + Stability & Landau damping – how can we compute the dispersion relation and stability of a general kinetic equilibrium? + Species/mass distributions – how do distributions of particle mass and/or charge change the response of collisionless systems? + Numerical methods and diagnostics – how can we use numerical simulations to advance theoretical frameworks and to understand their potential pitfalls? |

**Coordinators**: J.-B. Fouvry, M. Kunz, J. Squire and A. L. Varri
**Scientific Advisors**: A. Schekochihin and S. Tremaine