The origin of three-body forces

In our usual understanding of macroscopic matter, two-body forces describe most physical phenomena. In particular, the Coulomb interaction between electrons, and between electrons and nuclei, is in principle sufficient to understand, to a very high degree of accuracy, all common matter. On the other hand, if we go down to the scale of a few Fermi, three- and many-body forces appear to be indispensable, as is evident in the calculation of binding energies obtained from nuclear forces in doubly-magic nuclei [1]. On the other hand, there is no reason to suspect bosonic atoms in the ultracold regime to show any evidence of a three-body force. However, recent experimental results have shown the need of a phenomenological three-body interaction in 85Rb Bose-Einstein condensates [2] as well as dipolar gases [3]. In this talk, I will take a non-technical tour to the origins of three-body forces, and show how some of these naturally arise from our ignorance of the microscopic details of two-body forces.

[1] Ring and Schuck, The Nuclear Many-Body Problem, Springer (1980).
[2] Everitt et al., arXiv:1509.06844 (2015).
[3] Ferrier-Barbut, Kadau, Schmitt, Wenzel and Pfau, arXiv:1601.03318 (2016).