New Limit on Axion-Like Dark Matter using Cold Neutrons

Abstract

We report on a search for dark matter axion-like particles (ALPs) using a Ramsey-type apparatus for cold neutrons. A hypothetical ALP-gluon-coupling would manifest in a neutron electric dipole moment signal oscillating in time. Twenty-four hours of data have been analyzed in a frequency range from 23 $μ$Hz to 1 kHz, and no significant oscillating signal has been found. The usage of present dark-matter models allows to constrain the coupling of ALPs to gluons in the mass range from $10^{-19}$ to $4 \times 10^{-12}$ eV. The best limit of $C_G$/$f_a m_a = 2.7 \times 10^{13}$ GeV$^{-2}$ (95\% C.L.) is reached in the mass range from $2 \times 10^{-17}$ to $2 \times 10^{-14}$ eV.

Figures (4)

• Figure 1: (Color online) Schematic of the experimental setup where a neutron beam enters from the left, polarized along the $B_0$-field. It shows the 6 m-long mu-metal shield around the interaction region and the two 40 cm-long RF spin-flip coils for the $\pi/2$-flips in green. The electrodes and the electric field direction are shown in red and the magnetic field direction is indicated in blue. The spin analyzer (purple) reflects one spin state and transmits the other. The neutrons are detected using a 2D pixel detector with a sensitive area of $10 \times 10$ cm$^2$ with $16 \times 16$ pixels. The vacuum beam pipe surrounding the electrodes is not shown.
• Figure 2: (Color online) Calibration factor $S_B / S_A$ as a function of frequency. The measured data are shown as dots, whereas the red dashed line is a least-squares fit of a Butterworth-filter function stephen_butterworth_theory_1930 which is used for the data analysis. For instance, typical neutron asymmetry signals of the order $10^{-5}$ correspond to a pseudo-magnetic field of $14$ pT for frequencies smaller than 5 Hz using Eq. (\ref{['eq:magneticFieldFromAsymmetry']}).
• Figure 3: (Color online) The data for the top beam (blue $\blacktriangle$), the bottom beam (yellow $\blacktriangledown$), and the difference between the two beams (red $\bullet$) are shown for various stages in the data processing. (a) Measured neutron asymmetry for a time window of 5 seconds. (b) Frequency spectrum between 1 Hz and 60 Hz. The highly significant signal at 50 Hz from the power line frequency are canceled out by analyzing the beam difference. (c) Discrete peaks appear in the spectrum due to the data structure. (d) After applying the calibration as shown in Fig. \ref{['fig:calibration']}, the neutron asymmetry spectrum translates into a pseudo-magnetic field spectrum. Error bars were omitted for reasons of readability but are of order 3 pT in the most sensitive central range. Note that the plots of (b), (c), and (d) are based on the whole 12 hour data set and that only a fraction of the data points is shown for legibility in all sub-figures.
• Figure 4: (Color online) Limits on the ALP-gluon-coupling are shown as a function of the mass or frequency. The shaded areas are exclusion regions from cosmology and astrophysical observations (blue: Galaxies corasaniti_constraints_2017, BBN blum_constraining_2014stadnik_can_2015, SN1987A raffelt_astrophysical_1990graham_new_2013) and laboratory experiments (orange: nEDM abel_search_2017, HfH roussy_experimental_2021). The black outlines with the pink area mark the exclusion region of this publication (labeled Beam EDM). The solid and dotted lines correspond to the deterministic and stochastic dark-matter models, respectively. The green line shows the canonical QCD axion.