Spin-Hair Induced Chaos of Spinning Test Particles in Rotating Hairy Black Holes

TL;DR

This study explores how spin-curvature coupling and hairy black hole geometries influence the finite-time instability of spinning particles, revealing complex, localized regions of chaos that differ from Kerr black hole behavior.

Contribution

It demonstrates that hairy black hole geometries reorganize the phase space of spinning particle dynamics, showing non-monotonic instability regions influenced by hair parameters.

Findings

Small spins and geodesic trajectories remain regular.

Large spins induce stronger finite-time growth and chaos.

Instability regions depend on hair parameters and particle spin.

Abstract

We investigate the finite-time instability of massive spinning test particles around a rotating hairy black hole generated through gravitational decoupling. The particle motion is described by the full Mathisson-Papapetrou-Dixon equations with the Tulczyjew spin supplementary condition, and the sensitivity to initial conditions is measured using a ZAMO-projected finite-time Lyapunov analysis. The hairy deformation is controlled by two parameters: $\alpha$, which sets the deviation from Kerr, and $\beta$, which changes the radial localization of the deformation. We show that spin-curvature coupling and the hairy geometry can shift the evolved orbit away from the requested seed parameters, making the empirical orbital map essential for interpreting the dynamics. Small-spin and geodesic trajectories remain close to regular behavior, whereas large-spin trajectories show stronger finite-time growth. A scan of the $(S,\beta)$ plane shows that the instability does not grow monotonically, but appears in localized regions where the particle spin and the radial profile of the hair act cooperatively. Thus, the hairy background does not simply rescale the Kerr result; it reorganizes the strong-field phase-space region sampled by spinning particles.