Ultra-cold neutrons aid the search for dark matter21/11/17Science & Technology
Since the start of its operation in 1985, the experimental installation ‘Physique Fondamentale 2’ (PF2) at the Institut Laue-Langevin (ILL) in Grenoble, France, has been the only user facility for ultra-cold neutron (UCN) research in the world.
Ultra-cold neutrons play an important role in addressing key questions of particle physics at the low-energy, high-precision frontier, complementary to the high-energy frontier probed at particle accelerators.
An unusual property of UCNs is that their kinetic energy is so small that they can become trapped in material and/or magnetic bottles, hence are observable for long times.
It is unsurprising then, that over the last 30 years since its inception, data generated on PF2 is still important today and leads to regular and steady publications in refereed journals. One most recent international study published in Physical Review X, is a great example of this.
It has led to setting some of the tightest limits to date on the strength of interactions between axions (hypothetical elementary particle originally postulated to solve the strong Charge Conjugation Parity (CP) problem but they also would explain dark matter in a rather natural manner) and gluons (exchange particles for the strong force) and nucleons.
A collaboration between the Paul Scherrer Institute (PSI) and the Institute for Particle Physics and Astrophysics at ETH Zurich, Switzerland, searched for axions in the nano- to milli-hertz range, using the data taken in their experiment to measure a non-vanishing electric dipole moment of the neutron (nEDM).
Measurements were conducted using an improved and upgraded apparatus previously employed at the ILL by the RAL-Sussex-ILL EDM collaboration between 1998 and 2002.
The experiment was a room temperature neutron EDM one using UCNs on PF2 at the ILL reactor.
The results of the measurement were published in 1999, giving an upper limit on the neutron EDM of 6.3×10−26 ecm. New measurements published in 2006 improved this to 2.9×10−26 ecm – the current world limit until now.
“It’s the high quality of the data generated by our RAL-Sussex-ILL EDM experiment at ILL’s UCN source PF2 almost 20 years ago which has now enabled a re-analysis with a different and new goal, namely to search for axion-like dark matter”, says Peter Geltenbort of the ILL.
If the gluons and nucleons in the neutrons were interacting with axions during the nEDM experiments, the data should reveal harmonic oscillations with frequencies ranging from nano- to millihertz, depending on the axion mass.
Analysis of the ILL data covered oscillation periods of the order of days and longer, whereas the analysis of the PSI data provided information for oscillation periods down to minutes.
For both frequency ranges the search ended with a null result; no signal consistent with the presence of axions was spotted.
The lack of oscillation signals allowed the teams to set the first laboratory limits on the interaction strength of axions with gluons, improving those obtained from astrophysical observations of helium-4 by a factor of 1,000.
They also derived the best limit on the axion-nucleon coupling strength for laboratory-based experiments, notably in an energy range that has so far been unexplored in laboratory experiments.