# Fractional Bhatnagar-Gross-Krook kinetic equation

## TL;DR

This paper introduces a fractional generalization of the Bhatnagar-Gross-Krook kinetic equation to model superdiffusive Levy walk transport in external fields, incorporating fractional scattering rates and retardation effects.

## Contribution

It formulates a novel fractional kinetic equation that extends classical models to include Levy walk superdiffusion and compares it with existing fractional Fokker-Planck approaches.

## Key findings

- Derivation of a fractional Bhatnagar-Gross-Krook kinetic equation.
- Identification of the importance of retardation effects in superdiffusive regimes.
- Demonstration of the model's viability as an alternative to fractional Kramers-Fokker-Planck equations.

## Abstract

The linear Boltzmann equation approach is generalized to describe fractional superdiffusive transport of the Levy walk type in external force fields. The time distribution between scattering events is assumed to have a finite mean value and infinite variance. It is completely characterized by the two scattering rates, one fractional and a normal one, which defines also the mean scattering rate. We formulate a general fractional linear Boltzmann equation approach and exemplify it with a particularly simple case of the Bohm and Gross scattering integral leading to a fractional generalization of the Bhatnagar, Gross and Krook kinetic equation. Here, at each scattering event the particle velocity is completely randomized and takes a value from equilibrium Maxwell distribution at a given fixed temperature. We show that the retardation effects are indispensable even in the limit of infinite mean scattering rate and argue that this novel fractional kinetic equation provides a viable alternative to the fractional Kramers-Fokker-Planck (KFP) equation by Barkai and Silbey and its generalization by Friedrich et al. based on the picture of divergent mean time between scattering events. The case of divergent mean time is also discussed at length and compared with the earlier results obtained within the fractional KFP.