Search for ultralight axion dark matter in a side-band analysis of a ¹⁹⁹Hg free-spin precession signal

Ultra-low-mass axions are a viable dark matter candidate and may form a coherently oscillating classical field. Nuclear spins in experiments on Earth might couple to this oscillating axion dark-matter field, when propagating on Earth's trajectory through our Galaxy. This spin coupling resembles an oscillating pseudo-magnetic field which modulates the spin precession of nuclear spins. Here we report on the null result of a demonstration experiment searching for a frequency modulation of the free spin-precession signal of \magHg in a 1μT magnetic field. Our search covers the axion mass range 10⁻¹⁶ eV ≲ m_a ≲ 10⁻¹³ eV and achieves a peak sensitivity to the axion-nucleon coupling of g_aNN ≈ 3.5 × 10⁻⁶ GeV⁻¹.ISSN:2542-465

Search for an interaction mediated by axion-like particles with ultracold neutrons at the PSI

We report on a search for a new, short-range, spin-dependent interaction using a modified version of the experimental apparatus used to measure the permanent neutron electric dipole moment at the Paul Scherrer Institute. This interaction, which could be mediated by axion-like particles, concerned the unpolarized nucleons (protons and neutrons) near the material surfaces of the apparatus and polarized ultracold neutrons stored in vacuum. The dominant systematic uncertainty resulting from magnetic-field gradients was controlled to an unprecedented level of approximately 4 pT cm⁻¹ using an array of optically-pumped cesium vapor magnetometers and magnetic-field maps independently recorded using a dedicated measurement device. No signature of a theoretically predicted new interaction was found, and we set a new limit on the product of the scalar and the pseudoscalar couplings gₛgₚλ² < 8.3 × 10⁻²⁸ m² (95% C.L.) in a range of 5 µm < λ < 25 mm for the monopole–dipole interaction. This new result confirms and improves our previous limit by a factor of 2.7 and provides the current tightest limit obtained with free neutrons.ISSN:1367-263

Mapping of the magnetic field to correct systematic effects in a neutron electric dipole moment experiment

Experiments dedicated to the measurement of the electric dipole moment of the neutron require outstanding control of the magnetic-field uniformity. The neutron electric dipole moment (nEDM) experiment at the Paul Scherrer Institute uses a Hg199 co-magnetometer to precisely monitor temporal magnetic-field variations. This co-magnetometer, in the presence of field nonuniformity, is, however, responsible for the largest systematic effect of this measurement. To evaluate and correct that effect, offline measurements of the field nonuniformity were performed during mapping campaigns in 2013, 2014, and 2017. We present the results of these campaigns, and the improvement the correction of this effect brings to the neutron electric dipole moment measurement.ISSN:1094-1622ISSN:0556-2791ISSN:1050-294