# Efimov physics beyond three particles

## TL;DR

This paper reviews recent theoretical advances in Efimov physics extending beyond three particles, exploring conditions for Efimov effects in larger systems and identifying where such phenomena occur or are absent.

## Contribution

It provides new insights into Efimov physics in systems with more than three particles, including critical mass ratios and the existence of five-body Efimov effects.

## Key findings

- The (4+1) system exhibits a pure five-body Efimov effect.
- No Efimov physics is found in six- and seven-body systems.
- The critical mass ratios for Efimov effects vary with system size.

## Abstract

Efimov physics originally refers to a system of three particles. Here we review recent theoretical progress seeking for manifestations of Efimov physics in systems composed of more than three particles. Clusters of more than three bosons are tied to each Efimov trimer, but no independent Efimov physics exists there beyond three bosons. The case of a few heavy fermions interacting with a lighter atom is also considered, where the mass ratio of the constituent particles plays a significant role. Following Efimov's study of the (2+1) system, the (3+1) system was shown to have its own critical mass ratio to become Efimovian. We show that the (4+1) system becomes Efimovian at a mass ratio which is smaller than its sub-systems thresholds, giving a pure five-body Efimov effect. The (5+1) and (6+1) systems are also discussed, and we show the absence of 6- and 7-body Efimov physics there.