Stability of Neutral Fermi Balls with Multi-Flavor Fermions

TL;DR

This paper investigates the stability of neutral Fermi balls with multiple fermion flavors, revealing conditions under which they remain stable or marginally stable, and how flavor number influences their stability region.

Contribution

It demonstrates that multi-flavor Fermi balls are generally stable against fragmentation and explores how flavor number and higher-order corrections affect their stability boundaries.

Findings

Fermi balls are stable against fragmentation in most cases.

Stability is marginal in some cases and depends on higher-order corrections.

Larger number of flavors broadens the stable parameter region.

Abstract

A Fermi ball is a kind of non-topological soliton, which is thought to arise from the spontaneous breaking of an approximate $Z_2$ symmetry and to contribute to cold dark matter. We consider a simple model in which fermion fields with multi-flavors are coupled to a scalar field through Yukawa coupling, and examine how the number of the fermion flavors affects the stability of the Fermi ball against the fragmentation. (1)We find that the Fermi ball is stable against the fragmentation in most cases even in the lowest order thin-wall approximation. (2)We then find that in the other specific cases, the stability is marginal in the lowest order thin-wall approximation, and the next-to-leading order correction determines the stable region of the coupling constants; We examine the simplest case where the total fermion number $N_i$ and the Yukawa coupling constant $G_i$ of each flavor $i$ are common to the flavor, and find that the Fermi ball is stable in the limited region of the parameters and has the broader region for the larger number of the flavors.