# Spin tune mapping as a novel tool to probe the spin dynamics in storage rings

## TL;DR

This paper introduces spin tune mapping, a new experimental method to precisely probe spin dynamics and determine the stable spin axis in storage rings, crucial for high-precision electric dipole moment measurements.

## Contribution

The paper presents a novel spin tune mapping technique to measure the stable spin axis with unprecedented accuracy in storage rings.

## Key findings

- Successfully determined the stable spin axis at two locations in COSY.
- Achieved measurement accuracy better than 2.8 microradians.
- Demonstrated the method's potential for background signal analysis in EDM experiments.

## Abstract

Precision experiments, such as the search for electric dipole moments of charged particles using storage rings, demand for an understanding of the spin dynamics with unprecedented accuracy. The ultimate aim is to measure the electric dipole moments with a sensitivity up to 15 orders in magnitude better than the magnetic dipole moment of the stored particles. This formidable task requires an understanding of the background to the signal of the electric dipole from rotations of the spins in the spurious magnetic fields of a storage ring. One of the observables, especially sensitive to the imperfection magnetic fields in the ring is the angular orientation of stable spin axis. Up to now, the stable spin axis has never been determined experimentally, and in addition, the JEDI collaboration for the first time succeeded to quantify the background signals that stem from false rotations of the magnetic dipole moments in the horizontal and longitudinal imperfection magnetic fields of the storage ring. To this end, we developed a new method based on the spin tune response of a machine to artificially applied longitudinal magnetic fields. This novel technique, called \textit{spin tune mapping}, emerges as a very powerful tool to probe the spin dynamics in storage rings. The technique was experimentally tested in 2014 at the cooler synchrotron COSY, and for the first time, the angular orientation of the stable spin axis at two different locations in the ring has been determined to an unprecedented accuracy of better than $2.8\mu$rad.