A new of way of searching for dark-matter candidate particles called axions has produced the tightest constraint yet on how they can interact with normal matter. Using a two-city network of quantum sensors based on nuclear spins, physicists in China narrowed the possible values of a parameter known as axion-nucleon coupling below a limit previously set by astrophysical observations. As well as insights on the nature of dark matter, the technique could aid investigations of other beyond-the-Standard-Model physics phenomena such as axion stars, axion strings and Q-balls.

Dark matter is thought to make up over 25% of the universe’s mass, but it has never been detected directly. Instead, we infer its existence from its gravitational interactions with visible matter and its effect on the large-scale structure of the universe.

While the Standard Model of particle physics does not incorporate dark matter, several physicists have proposed ideas for how to bring it into the fold. One of the most promising involves particles called axions. First hypothesized in the 1970s as a way of explaining unresolved questions about charge-parity violation, axions are chargeless and much less massive than electrons. This means they interact only weakly with matter and electromagnetic radiation.

According to theoretical calculations, the Big Bang should have produced axions in abundance. During phase transitions in the early universe, these axions would have formed topological defects – defects that study leader Xinhua Peng of the University of Science and Technology of China (USTC) says should, in principle, be detectable. “These defects are expected to interact with nuclear spins and induce signals as the Earth crosses them,” Peng explains.

A new axion search method

The problem, Peng continues, is that such signals are expected to be extremely weak and transient. She and her colleagues therefore developed an alternative axion search method that exploits a different predicted behaviour.

When fermions (particles with half-integer spin) interact, or couple, with axions, they should produce a pseudo-magnetic field. Peng and colleagues looked for evidence of this interaction using a network of five quantum sensors, four in Hefei and one in Hangzhou. These sensors combined a large ensemble of polarized rubidium-87 (⁸⁷Rb) atoms with polarized xeon-129 (¹²⁹Xe) nuclear spins.

“Using nuclear spins has many advantages,” Peng explains. “These include a higher energy resolution detection for topological dark matter (TDM) axions thanks to a much smaller gyromagnetic ratio of nuclear spins; substantial spin amplification owing to the high ensemble density of noble-gas spins; and efficient optimal filtering enabled by the long nuclear-spin coherence time.”

The USTC researchers’ setup also has other advantages over previous laboratory-based TDM searches, including the Global Network of Optical Magnetometers for Exotic physics searches (GNOME). While GNOME operates in a steady-state detection mode, the USTC researchers use a detection scheme that probes transient “free-decay oscillating” signals generated on spins after a TDM crossing. The USTC team also implemented a dual-phase optimal filtering algorithm to extract TDM signals with a signal-to-noise ratio at the theoretical maximum.

Peng tells Physics World that these advantages enabled the team to explore regions of TDM parameter space well beyond limits set by astrophysical searches. The transient-state detection scheme also enables sensitive searches for TDM in the region where the axion mass exceeds 100 peV – a region that GNOME cannot access.

Most stringent constraints

The researchers have not yet recorded a statistically significant topological crossing event using their setup, so the dark matter search is not over. However, they have set more stringent constraints on axion-nucleon coupling across a range of axion masses from 10 peV to 0.2 μeV. Notably, they calculated that the coupling strength must be greater than 4.1 x 10¹⁰ GeV at an axion mass of 84 peV. This limit is stricter than those obtained from astrophysical observations, though Peng notes that these rely on different assumptions.

Peng says the technique developed in this study, which is published in Nature, could lead to the development of even larger, more sensitive networks for detecting transient spin signals such as those from TDM. It also opens new avenues for investigating other physical phenomena beyond the Standard Model that have been theoretically proposed, but have so far lacked a pathway for experimental exploration.

The researchers now plan to increase the number of sensor stations in their network and extend their geographical baselines to intercontinental and even space-based scales. Peng explains that doing so will enhance the network’s detection sensitivity and boost signal confidence. “We also want to enhance the sensitivity of individual sensors via better spin polarization, longer coherence times and advanced quantum control techniques,” she says. Switching to a ³He–K system, she adds, could boost their current spin-rotation sensitivity by up to four orders of magnitude.

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