# Quasiparticle energy in a strongly interacting homogeneous Bose-Einstein condensate

## TL;DR

This study uses two-photon Bragg spectroscopy to measure quasiparticle energies in a strongly interacting homogeneous Bose-Einstein condensate, revealing significant deviations from existing theories and highlighting the need for further theoretical development.

## Contribution

It provides experimental data on quasiparticle energies in strongly interacting BECs, showing deviations from Bogoliubov theory and comparing results with Feynman and Wilson operator product expansion models.

## Key findings

- Sign change in excitation resonance shift at a specific scattering length.
- Breakdown of Bogoliubov theory at high interaction strengths.
- Better agreement with Wilson operator product expansion at strong interactions.

## Abstract

Using two-photon Bragg spectroscopy, we study the energy of particle-like excitations in a strongly interacting homogeneous Bose-Einstein condensate, and observe dramatic deviations from Bogoliubov theory. In particular, at large scattering length $a$ the shift of the excitation resonance from the free-particle energy changes sign from positive to negative. For an excitation with wavenumber $q$, this sign change occurs at $a \approx 4/(\pi q)$, in agreement with the Feynman energy relation and the static structure factor expressed in terms of the two-body contact. For $a \gtrsim 3/q$ we also see a breakdown of this theory, and better agreement with calculations based on the Wilson operator product expansion. Neither theory explains our observations across all interaction regimes, inviting further theoretical efforts.