Non-BPS exact solutions and their relation to bions in ${\mathbb C}P^{N-1}$ models

TL;DR

This paper explores non-BPS exact solutions in ${\mathbb C}P^{N-1}$ models, revealing their connection to multi-bion configurations, and discusses their role in resurgence theory, stability, and transitions between different instanton states.

Contribution

It introduces a method to construct and analyze non-BPS solutions, linking them to multi-bion configurations and elucidating their significance in resurgence and stability analysis.

Findings

Non-BPS solutions correspond to multi-bion configurations with fractional instantons.

Transitions between multi-instanton configurations occur via moduli changes.

Non-BPS solutions contribute to the resurgent trans-series as special multi-bion cases.

Abstract

We investigate non-BPS exact solutions in ${\mathbb C}P^{N-1}$ sigma models on ${\mathbb R}^1 \times S^{1}$ with twisted boundary conditions, by using the Din-Zakrzewski projection method. We focus on the relation of the non-BPS solutions to the ansatz of multi-instanton (bion) configurations and discuss their significance in the context of the resurgence theory. We find that the transition between seemingly distinct configurations of multi-instantons occur as moduli changes in the non-BPS solutions, and the simplest non-BPS exact solution corresponds to multi-bion configurations with fully-compressed double fractional instantons in the middle. It indicates that the non-BPS solutions make small but nonzero contribution to the resurgent trans-series as special cases of the multi-bion configurations. We observe a generic pattern of transitions between distinct multi-bion configurations (flipping partners), leading to the three essential properties of the non-BPS exact solution: (i) opposite sign for terms corresponding to the left and right infinities, (ii) symmetric location of fractional instantons, and (iii) the transition between distinct bion configurations. By studying the balance of forces, we show that the relative phases between the instanton constituents play decisive roles in stability and instability of the muli-instanton configurations. We discuss local and global instabilities of the solutions such as negative modes and the flow to the other saddle points, by considering the deformations of the non-BPS exact solutions within our multi-instanton ansatz. We also briefly discuss some classes of the non-BPS exact solutions in Grassmann sigma models.