Instability of coherent states of a real scalar field

TL;DR

This paper analyzes the stability of localized and uniform coherent states in a real scalar field with logarithmic nonlinearity, deriving a stability chart and showing that pulsons are generally unstable but can be long-lived, and that condensates may decay into pulsons.

Contribution

It extends the Lindemann-Stieltjes method to analyze the stability of solutions to the Klein-Gordon equation with a logarithmic nonlinearity, providing a new stability chart and insights into decay processes.

Findings

All pulsons are unstable over time.

Nodeless pulsons can be long-lived in narrow amplitude ranges.

Oscillating condensates can decay into nodeless pulsons.

Abstract

We investigate stability of both localized time-periodic coherent states (pulsons) and uniformly distributed coherent states (oscillating condensate) of a real scalar field satisfying the Klein-Gordon equation with a logarithmic nonlinearity. The linear analysis of time-dependent parts of perturbations leads to the Hill equation with a singular coefficient. To evaluate the characteristic exponent we extend the Lindemann-Stieltjes method, usually applied to the Mathieu and Lame equations, to the case that the periodic coefficient in the general Hill equation is an unbounded function of time. As a result, we derive the formula for the characteristic exponent and calculate the stability-instability chart. Then we analyze the spatial structure of the perturbations. Using these results we show that the pulsons of any amplitudes, remaining well-localized objects, lose their coherence with time. This means that, strictly speaking, all pulsons of the model considered are unstable. Nevertheless, for the nodeless pulsons the rate of the coherence breaking in narrow ranges of amplitudes is found to be very small, so that such pulsons can be long-lived. Further, we use the obtaned stability-instability chart to examine the Affleck-Dine type condensate. We conclude the oscillating condensate can decay into an ensemble of the nodeless pulsons.