Testing neutrality of matter by measuring magnetic fields*

Possible existence of minicharges questions charge conservation, thereby also the neutrality of neutrons, and by extension the neutrality of matter. The charge of the neutron is extremely well measured, and is consistent with zero: q_n = (-0.2+-0.8)*10^{-21} e. This measurement comes from testing for neutrality of SF6 molecules using acoustic oscillations (sound). Furthermore, charge conservation in conjunction with the asymmetry between the proton and electron charges further confirms the neutrality of the neutron: (q_p + q_e) <10^{-21} e. Other techniques for testing neutrality of matter in the past has involved, suspended macro-particles falling, deflection of beams of neutrons, atoms, and molecules, both under the influence of electromagnetc fields, and by measuring potential differences upon gas efflux.

In the presence of small minicharges, a flowing cryogenic fluid generates measurable constant magnetic field. Current state of the art DC-SQUID magnetometers have achieved a precision of better than 100 aT, and can already provide a pathway to testing neutrality of matter at the level of 10^{-21} e. The technique, pioneered by the ABRACADABRA collaboration, of characterizing oscillating magnetic fields with the help of SQUIDs, provides robust background noise rejection. An oscillating density mimics an alternating flow-direction, both of which generate oscillating magnetic fields in the presence of minicharges. Alternating flow-direction can be generated by alternating pumping, and oscillating density can be generated by thermal acoustic oscillation. The sensitivity of both these techniques towards testing neutrality of matter will be presented.