Phenomenological model of decaying Bose polarons

TL;DR

This paper presents a phenomenological model for Bose polarons that captures experimental observations of quasiparticle formation and decay in Bose-Einstein condensates, emphasizing impurity-boson correlations and decay processes.

Contribution

It introduces a minimal variational wave function with complex interaction strength to model Bose polaron decay, aligning with recent experimental results.

Findings

Model recovers main experimental spectral features

Highlights importance of impurity-boson correlations

Provides an intuitive framework for data analysis

Abstract

Cold atom experiments show that a mobile impurity particle immersed in a Bose-Einstein condensate forms a well-defined quasiparticle (Bose polaron) for weak to moderate impurity-boson interaction strengths, whereas a significant line broadening is consistently observed for strong interactions. Motivated by this, we introduce a phenomenological theory based on the assumption that the most relevant states are characterized by the impurity correlated with at most one boson, since they have the largest overlap with the uncorrelated states to which the most common experimental probes couple. These experimentally relevant states can however decay to lower energy states characterised by correlations involving multiple bosons, and we model this using a minimal variational wave function combined with a complex impurity-boson interaction strength. We first motivate this approach by comparing to a more elaborate theory that includes correlations with up to two bosons. Our phenomenological model is shown to recover the main results of two recent experiments probing both the spectral and the non-equilibrium properties of the Bose polaron. Our work offers an intuitive framework for analyzing experimental data and highlights the importance of understanding the complicated problem of the Bose polaron decay in a many-body setting.