Amplified signal and extreme sensitivity: on the trail of light dark matter particles

New nuclear magnetic resonance technique is five orders of magnitude more sensitive

15 December 2021

An international team of researchers with participation of the PRISMA+ Cluster of Excellence  of Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM) has successfully advanced a laboratory method to search for extremely light axion-like particles (ALPs), which are possible candidates for being the elusive dark matter. The researchers use nuclear magnetic resonance techniques in their experiments: by using a new setup, they have now been able to increase the sensitivity by five orders of magnitude compared to previous experiments, as they show in their article in Nature Physics, a leading journal in the field.

Little is known about the exact nature of dark matter. Today, extremely light bosonic particles, such as the so-called axions, axion-like particles, and dark photons, are considered to be promising candidates. These can be regarded as a classical field oscillating at a certain frequency. How large this frequency – and consequently the mass of the particles – is, is not yet known. That is why the researchers are systematically searching different frequency ranges with their experiments for evidence of dark matter. "There is still a lot of work to be done, because we have not yet checked a large mass range for ALPs," said Professor Dr. Dmitry Budker, a principal investigator at PRISMA+ and Section Leader at HIM, an institutional cooperation of Johannes Gutenberg University Mainz and the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt. "In doing so, we continue to rely on the principle of nuclear magnetic resonance, i.e., the fact that nuclear spins respond to magnetic fields that oscillate at a certain resonance frequency. We determine the strength of this resonance signal with a sensitive magnetometer."

The basic premise of the experiments is that a dark matter field also affects the nuclear spins of a sensor in this way. As the Earth moves through this field, the nuclear spins in the sensor behave exactly as they would in an oscillating magnetic field. The result is a nuclear spin signal caused by dark matter.

Combination of two effects: amplified signal and more sensitive measurement

The Mainz scientists and their colleagues at the University of Science and Technology of China (USTC) use the noble gas xenon, or more precisely the isotope xenon-129, as a sensor. The magnetometer, which measures potential signals, is based on the element rubidium. There are two main special characteristics here: "We set up the experiment in such a way that the xenon atoms first amplify an oscillating field. So the effect triggered by a potential ALP field would be a factor of 100 larger," described co-author Antoine Garcon, a PhD student at HIM. "Moreover, our magnetometer – that is, the readout unit – is located in the same cell as the sensor gas, xenon. The stronger contact between the two, in addition to the stronger signal, increases the sensitivity of the measurement."

"This is more or less the same principle underlying our Cosmic Axion Spin Precession Experiment research program – CASPEr for short –, a collaboration of PRISMA+ and HIM with Boston University in the USA. However, the details of the technical implementation are quite different," explained Professor Dmitry Budker.

In the current work, the cooperation partners first showed that their idea basically works: they apply a weak oscillating magnetic field to simulate an ALP field and can thus detect the predicted signals exactly. In the next step, they determine the sensitivity of their experimental setup. As a result, it is five orders of magnitude better than in previous experiments. After successful proof-of-principle, the scientists started the first series of measurements to search for dark matter. They were able to survey the mass range from a few femtoelectronvolts (feV) to almost 800 feV.

Although they have not yet been able to find an ALP signal in this range, the much higher sensitivity has enabled them to formulate new and stringent limits with respect to the strength of the ALP interaction with normal matter. In addition, they were able to extend the search range by an order of magnitude towards higher masses compared to the earlier CASPEr experiments, thus further narrowing the search range for ALPs after the exclusion procedure. The setup could also be used for the search for dark photons. And here, too, the research team has succeeded in setting appropriate limits. Longer measurement times could further improve the sensitivity of their method, as the authors explain in Nature Physics.

Similar setup – different research project

A very similar experimental setup is described in another paper recently published in Science Advances. Again, Professor Dmitry Budker is involved: "We use essentially the same spin amplifier, but for a different purpose. Instead of looking for the dark matter field, we are looking for a possible exotic interaction between a mass source and nuclear spins – a 'fifth force,' so to speak. The exotic interactions would arise from the existence of 'new' particles, which in turn might have a connection to dark matter." In any case, in the search for new physics beyond the Standard Model, the new method offers exciting new approaches and perspectives.