The motion of twisted particles in a stellar gravitational field

A twisted particle possesses intrinsic orbital angular momentum (OAM), and the dynamics of such a particle may challenge the Einstein Equivalence Principle. In this study, we disregard the spin characteristic of the twisted particle, modeling it as a massless complex twisted scalar wave packet to simplify its interaction with gravitational fields. Building on this simplification, we investigate the gravitational birefringence of the twisted particle by analyzing the center of its energy density. We demonstrate that the gravitational birefringence induced by OAM can potentially exceed that induced by spin by several orders of magnitude, significantly enhancing its detectability. Furthermore, we examine the influence of the nonminimal coupling term $\lambda R|\phi|^2$ on the propagation of the twisted particle through the internal gravitational field of a star. Contrary to the predictions of the Mathisson-Papapetrou-Dixon equations, our findings show that the trajectory of the twisted particle under nonminimal coupling differs from that in the minimal coupling scenario. Specifically, we find that for a positive nonminimal coupling constant, the trajectory of the twisted particle is expected to deviate away from the stellar center, compared to the minimal coupling scenario, and this deviation is independent of the particle's OAM. These findings could provide new avenues for testing the Einstein Equivalence Principle.