First Experimental Limit on the Deuteron Electric Dipole Moment using a Storage Ring

Abstract

Permanent electric dipole moments (EDMs) provide a sensitive probe of physics beyond the Standard Model and are directly linked to additional sources of CP violation that could explain the matter-antimatter asymmetry of the universe. EDM measurements of charged particles in storage rings rely on detecting a small tilt of the invariant spin axis with respect to the ring plane. In this work, we present the experimental determination of the invariant spin axis in the COoler SYnchrotron (COSY), a conventional magnetic storage ring, using a combination of an radio-frequency Wien filter, a superconducting Siberian snake and an electron-cooler solenoid. The measurements reveal tilts of a few milliradians, which are dominated by systematic effects. From the observed tilts, we derive the first experimental limit on the deuteron EDM, $|d^d|< 2.5\cdot10^{-17}\,e\cdot\mathrm{cm} \; (95\%\,\text{ C.L.})$. This result demonstrates the feasibility of using storage rings for EDM searches and provides a foundation for future dedicated facilities.

Figures (4)

• Figure 1: Influence of the EDM on the invariant spin axis $\vec{n}$. The gray line marks the beam trajectory. Assuming an idealized storage ring and no EDM, the spin is precessing in the $x$-$z$ plane. A non-zero EDM tilts the spin-precession plane around the $z$-axis by an angle $\xi_{\text{EDM}}$ indicated by the blue plane. The situation shown here corresponds to a particle with $G<0$, e.g. a deuteron. The spin $\vec{s}$ precesses in opposite direction to the momentum $\vec{p}$. For a positive EDM, i.e. $\eta >0$, the angle $\xi_{\text{EDM}}$ defined in Eq. (\ref{['eq:eta_edm']}) is negative.
• Figure 2: Sketch of the COSY ring indicating the position of the relevant installations: electron coolers, rf Wien filter, Siberian snake and the JEDI polarimeter (JePo).
• Figure 3: The resonance strength for Map 2/bunch 1 is shown as a function of the rf Wien filter rotation angle and the spin rotation in the Siberian snake. The minimum of the resulting two-dimensional paraboloid indicates the orientation of the invariant spin axis at the location of the rf Wien filter.
• Figure 4: Perpendicular contributions to the main (vertical) magnetic field as a function of a rotation of the beam axis in the horizontal plane ($x^\prime$), illustrating the sensitivity of the field orientation to uncertainties in the beam parameters. The plot resembles a projection of the 16-parameter space onto the $x^\prime$ axis and is based on 800 full-wave simulations, further processed using a machine-learning-based sparse polynomial chaos expansion as discussed in Ref. Slim:2016dct. Expected uncertainties in $x^\prime$ are in the order of [range-phrase=--, range-units=single]12mrad. The colors indicate the probability for the $H_{\perp}$ contribution given at a certain $x'$.