There is strong evidence that dark matter exists and permeates the cosmos, yet all searches for the hypothetical particles that may make up this invisible form of matter have drawn a blank so far. In light of these null results, researchers have started to spread a wider net in their searches, exploring as many types of particle as possible, new regions in which the particles may lie hidden and new ways to probe them. The NA64 experiment collaboration has now widened the scope of its searches with a search for axions and axion-like particles – hypothetical particles that could mediate an interaction between dark matter and visible matter or comprise dark matter itself, depending on their exact properties.

The NA64 team targeted an unexplored area for axions and axion-like particles, a gap in the two-dimensional area of possible values of their mass and interaction strength with a pair of photons. This gap doesn’t include the regions where axions and axion-like particles could make up dark matter, but it includes an area where axions could explain the long-puzzling symmetry properties of the strong force, for which axions were originally proposed, as well as an area where axion-like particles could mediate an interaction between dark matter and visible matter.

To explore this gap, the NA64 team used an electron beam of 100 GeV energy from the Super Proton Synchrotron and directed it onto a fixed target. They then searched for axions and axion-like particles that would be produced in interactions between high-energy photons generated by the 100 GeV electrons in the target and virtual photons from the target’s atomic nuclei. The researchers looked for the particles both through their transformation, or “decay”, into a pair of photons in a detector placed right after the target and through the “missing energy” that the particles would carry away if they decayed downstream of the detector.

The NA64 team analysed data that was collected over the course of three years, between 2016 and 2018. Together, these data corresponded to some three hundred billion electrons hitting the target. The NA64 researchers found no sign of axions or axion-like particles in this dataset, but the null result allowed them to set limits on the allowed values of the interaction strength of axions and axion-like particles with two photons for particle masses below 55 MeV.

“We’re very excited to have added NA64 to the list of experiments that are hunting for axions as well as axion-like particles, which are a popular candidate for a mediator of a new force between visible and dark matter”, says NA64 collaboration spokesperson Sergei Gninenko. “Little by little, and together, these experiments are narrowing down the regions of where to look for, and perhaps find, these particles.”