New Limit on Lorentz- and $CPT$-Violating Neutron Spin Interactions

We performed a search for neutron spin coupling to a Lorentz- and $CPT$-violating background field using a magnetometer with overlapping ensembles of K and ${}^3\mathrm{He}$ atoms. The comagnetometer is mounted on a rotary platform for frequent reversal of its orientation. We measure sidereal oscillations in the signal to search for anomalous spin coupling of extra-solar origin. We determine the equatorial components of the background field interacting with the neutron spin to be ${\tilde{b}}_X^n=(0.1\pm 1.6)\times {10}^{-33}\mathrm{GeV}$ and ${\tilde{b}}_Y^n=(2.5\pm 1.6)\times {10}^{-33}\mathrm{GeV}$, improving on the previous limit by a factor of 30. This measurement represents the highest energy resolution of any spin anisotropy experiment.

Figure 1

Experimental setup of the rotating comagnetometer.

Figure 2

Amplitude of the comagnetometer signal for a 180° rotation as a function of the initial platform rotation angle. The Lorentz violation data are collected at four points indicated by crosses.

Figure 3

Amplitude of the comagnetometer signal for N-S and E-W reversals as a function of sidereal time with a sinusoidal fit. Each data point represents seven orientation reversals.

Figure 4

Top panel: Summary of the data as a function of time. Each data point represents a several-day run. Upward and downward triangles represent direction of ${}^3\mathrm{He}$ spin. Bottom panel: Systematic comparisons: SU/SD - spin up vs spin down (from winter data only), SM/WN - summer vs winter data, EW/NS - amplitudes from E-W vs N-S reversals only.