Weak chaos detection in the Fermi-Pasta-Ulam-$\alpha$ system using $q$-Gaussian statistics

TL;DR

This study investigates weak chaos in the Fermi-Pasta-Ulam-$\alpha$ system by analyzing the statistical distributions of orbit sums, revealing $q$-Gaussian states in weakly chaotic regimes and contrasting behaviors based on initial mode excitation.

Contribution

It demonstrates the emergence of $q$-Gaussian quasi-stationary states in weakly chaotic regimes and compares the dynamics of energy sharing for different initial excitations.

Findings

$q$-Gaussian distributions characterize weak chaos at low energies.

Different initial modes lead to distinct routes to equipartition.

Transition to Gaussian statistics occurs at different energy thresholds depending on initial mode.

Abstract

We study numerically statistical distributions of sums of orbit coordinates, viewed as independent random variables in the spirit of the Central Limit Theorem, in weakly chaotic regimes associated with the excitation of the first ($k=1$) and last ($k=N$) linear normal modes of the Fermi-Pasta-Ulam-$\alpha$ system under fixed boundary conditions. We show that at low energies ($E=0.19$), when the $k=1$ linear mode is excited, chaotic diffusion occurs characterized by distributions that are well approximated for long times ($t>10^9$) by a $q$-Gaussian Quasi-Stationary State (QSS) with $q\approx1.4$. On the other hand, when the $k=N$ mode is excited at the same energy, diffusive phenomena are \textit{absent} and the motion is quasi-periodic. In fact, as the energy increases to $E=0.3$, the distributions in the former case pass through \textit{shorter} $q$-Gaussian states and tend rapidly to a Gaussian (i.e. $q\rightarrow 1$) where equipartition sets in, while in the latter we need to reach to E=4 to see a \textit{sudden transition} to Gaussian statistics, without any passage through an intermediate QSS. This may be explained by different energy localization properties and recurrence phenomena in the two cases, supporting the view that when the energy is placed in the first mode weak chaos and "sticky" dynamics lead to a more gradual process of energy sharing, while strong chaos and equipartition appear abruptly when only the last mode is initially excited.