New constraints on the axion's coupling to nucleons from a Spin Mass Interaction Limiting Experiment (SMILE)

Abstract

Long range anomalous spin-mass interactions are a generic feature of many theories beyond the Standard Model. They are typically mediated by a light pseudoscalar boson with CP-violating couplings such as an axion or axion-like particle that is the pseudo Nambu-Goldstone boson of some spontaneously broken U(1) symmetry. In this work, we present the results of a Spin-Mass Interaction Limiting Experiment (SMILE) where we search for possible long range spin-mass interactions using a continuously pumped alkali-noble gas co-magnetometer and two 250 kg Pb source masses and report new constraints on g^N_s g^n_p, the product of the axion's scalar and pseudoscalar coupling to nucleons and neutrons, that represent an order of magnitude improvement over existing laboratory limits over two decades of axion mass range from 0.01-1 micro eV. Slightly improved limits on g^N_s g^e_p, the product of the axion's scalar and pseudoscalar coupling to nucleons and electrons are also presented.

Analysis of correlation data and noise spectrums indicate that significant improvement is possible if unforeseen systematic thermal effects correlated to the position of the source masses at the level of 1 mK are controlled and if the low frequency performance of the co-magnetometer is further improved by decreasing optical rotation and pump deflection noise. We discuss various analyses that pinpoint these problems and suggest strategies to overcome them. The dynamics of hybrid pumping used in this work, in which a dense alkali vapor is polarized via spin-exchange collisions with another sparse optically pumped alkali species, and its implication on co-magnetometer operation are also discussed.

Finally, we present simulation and experimental results of a new pulsed alkali-noble gas co-magnetometer in which alkali atoms are optically pumped by short intense laser pulses and anomalous fields are measured during the alkali's decay in the dark. Unlike its continuously pumped counterpart, the pulsed co-magnetometer can potentially suppress noise due to pump beam deflections while retaining suppression of ordinary magnetic fields. Moreover, it also possesses simultaneous dual axis sensitivity which we demonstrate by measuring the gyroscopic effect from Earth's rotation.