Spin-dependent exotic interactions

TL;DR

This review discusses theoretical models and experimental searches for spin-dependent fifth forces mediated by exotic bosons, which could explain dark matter and violate fundamental symmetries.

Contribution

It provides a comprehensive overview of interaction potentials, experimental methods, and current constraints on exotic spin-dependent interactions beyond the standard model.

Findings

Experimental techniques like atomic comagnetometers and torsion balances set constraints on exotic interactions.

Derived interaction potentials based on quantum field theory with minimal assumptions.

Summarized current experimental constraints and identified promising research directions.

Abstract

Novel interactions beyond the four known fundamental forces in nature (electromagnetic, gravitational, strong and weak interactions), may arise due to "new physics" beyond the standard model, manifesting as a "fifth force". This review is focused on spin-dependent fifth forces mediated by exotic bosons such as spin-0 axions and axionlike particles and spin-1 Z' bosons, dark photons, or paraphotons. Many of these exotic bosons are candidates to explain the nature of dark matter and dark energy, and their interactions may violate fundamental symmetries. Spin-dependent interactions between fermions mediated by the exchange of exotic bosons have been investigated in a variety of experiments, particularly at the low-energy frontier. Experimental methods and tools used to search for exotic spin-dependent interactions, such as atomic comagnetometers, torsion balances, nitrogen-vacancy spin sensors, and precision atomic and molecular spectroscopy, are described. A complete set of interaction potentials, derived based on quantum field theory with minimal assumptions and characterized in terms of reduced coupling constants, are presented. A comprehensive summary of existing experimental and observational constraints on exotic spin-dependent interactions is given, illustrating the current research landscape and promising directions of further research.