Testing neutrality of matter using the cryogenic helium ABS

The potential existence of minicharges challenges the principle of charge conservation and, consequently, the neutrality of neutrons (or sum of charges of proton and electron), and by extension, the neutrality of matter itself. The neutron’s charge has been measured with great precision and remains consistent with zero: $q_n = (-0.2\pm0.8)\times10^{-21}\mathbbm{e}$ [Phys. Rev. A {\bf 83}, 052101 (2011)]. This measurement comes from testing for neutrality of SF$_6$ molecules using acoustic oscillations (sound) upon the application of an oscillating electric field. But, deflection of a beam of cold neutrons or atoms, under the influence of an electric field have also set leading constraints of $q_n = (-0.4\pm1.1)\times10^{-21}\mathbbm{e}$ [Phys. Rev. D {\bf 37}, 3107 (1988)] or $q_n = (0.4\pm1.5)\times10^{-19}\mathbbm{e}$ [Z. Phys. D {\bf 10}, 145 (1988)], respectively. Other techniques for testing neutrality of matter in the past have involved suspended macro-particles, and by measuring potential differences generated upon gas efflux.

The sensitivity of atomic beam deflection technique scales inversely with the beam temperature. The previous best measurement using this technique employed an oven source of Cesium and Potassium atoms at 480 K. The Helium ABS, originally designed for the cryogenic neutron electric dipole moment measurement, produces a highly collimated beam at $\sim1~$K. Furthermore, using $^4$He atoms suppresses the systematic effects arising from the combination of atomic spin and magnetic fields due to $\bm{v}\times\bm{E}$, $\bm{\nabla}\cdot\bm{E}$, and Earth's magnetic field. With these advantages, we estimate a sensitivity of $\sigma_{q_n} \approx 5 \times10^{-20}\mathbbm{e}/(30~\text{kV/cm})/\sqrt{\text{Day}}$.