Particle Spin Reveals Clues to Dark Matter’s Hidden Interactions

Dmitry Budker and colleagues at Johannes Gutenberg-Universitat Mainz in collaboration with Helmholtz Centre, University of California, University of Michigan, PSI Center for Neutron and Muon Sciences, Institute for Particle Physics and Astrophysics and Indiana University  present a thorough review of spin’s key role over the last century, tracing its influence from the foundations of quantum mechanics to contemporary challenges in particle physics. Their work highlights the continuing significance of spin-dependent measurements in testing core symmetries and pursuing elusive phenomena such as dark matter and potential connections between this mysterious substance and the structure of spacetime. This analysis establishes that spin remains a vital and new approach to addressing some of the most challenging questions in modern physics.

Precision measurements relied heavily on Penning traps, devices utilising strong magnetic and electric fields to confine charged particles. Electrons, or other particles, are held in cyclotron orbits within the magnetic field, enabling extremely accurate determination of their magnetic moment. A key innovation was the use of image currents, tiny electrical signals induced by the particle’s motion on the trap’s electrodes, to non-destructively monitor and cool the particle’s energy.

Budker and colleagues employed Penning traps, utilising strong magnetic and electric fields to confine single electrons and antiprotons for extended periods. These devices radially confine particles in cyclotron orbits, enabling precise determination of their magnetic moment via induced image currents. Initial experiments achieved resolutions at the parts-per-billion level, and subsequent refinements, including coherent spectroscopy and double-trap techniques, pushed precision to approximately 0.1 parts per billion and even below 20 parts per trillion.

Muon anomaly measurements surpass electron precision via Penning trap refinements

Measurements of the muon’s magnetic moment anomaly now reach a precision of 0.1 parts per billion, a substantial improvement over the electron’s anomaly which currently stands at 23 parts per million. This increased precision allows for increasingly stringent tests of the Standard Model of particle physics, probing for discrepancies that could signal new physics beyond our current understanding. Previously, limitations in both theoretical calculations and experimental techniques prevented such high-resolution analysis of muon behaviour, with the muon’s short lifespan and complex decay pathways presenting significant challenges.

Techniques utilising Penning traps, devices employing strong magnetic and electric fields to confine charged particles, have been key in achieving these results, alongside coherent spectroscopy and double-trap methods. The BASE collaboration recently measured the antiproton magnetic moment with a fractional resolution reaching 10⁻⁹. Advances in electron magnetic moment measurements have refined accuracy to 27 parts per billion, establishing corrections to the theoretical g-factor in the 1950s using atomic and unbound electron experiments. These developments demonstrate the power of quantum electrodynamics in calculating interactions, but current limitations in lattice QCD calculations prevent similar precision in predicting the muon’s magnetic moment. Hadronic vacuum polarization loops present the greatest challenge and largest uncertainties in calculations, hindering definitive identification of new physics beyond the Standard Model. The muon’s magnetic moment anomaly, expressed as g-2, benefits from QED and electroweak corrections calculated to high precision.

Spin, gravity and the search for physics beyond the Standard Model

For a century, the intrinsic angular momentum of particles, known as spin, has underpinned our understanding of the universe, revealing subtle details of quantum mechanics and the Standard Model of particle physics. Despite this success, a clear picture of how spin interacts with gravity remains elusive, prompting consideration of connections between spin and the very geometry of spacetime. Alternative approaches to detecting dark matter, such as axion searches utilising sensitive cavities and lumped-circuit resonators, offer competing avenues for investigation.

Nevertheless, understanding how spin interacts with spacetime could reveal new physics beyond the Standard Model, offering a complementary approach to identifying dark matter candidates. Active research utilising precision measurements of magnetic and electric dipole moments is refining techniques to detect subtle links between spin and gravity. Spin has been an indispensable probe of fundamental laws for a century, and these measurements continue to provide a fresh perspective on profound questions in physics.

Spin’s century-long influence extends beyond establishing quantum mechanics and validating the Standard Model, now providing tools to investigate phenomena beyond our current understanding. Continued precision measurements of particle properties, like magnetic moments, refine tests of fundamental symmetries including CP and CPT invariance. These sensitive techniques, alongside atomic magnetometry detecting minuscule magnetic fields, are not only probing the nature of dark matter but also exploring a potential connection between a particle’s spin and the structure of spacetime itself.

The research confirms that spin, a fundamental property of particles, remains central to investigating the laws of physics. Understanding how spin interacts with gravity may offer a new approach to identifying dark matter and exploring physics beyond the Standard Model. Researchers are currently refining precision measurements of magnetic and electric dipole moments to detect subtle connections between spin and spacetime geometry. These ongoing investigations build upon a century of utilising spin to probe fundamental questions in physics and validate existing theories.