Superconducting sensors can detect single low-energy photons. UZH researchers have now used this capability to search for light dark matter particles in the universe.

About 80 percent of the universe’s mass is thought to consist of dark matter. And yet, little is known about the composition and structure of the particles that make up dark matter, presenting physicists with some fundamental questions. To explore this elusive matter, researchers are attempting to capture photons, or light particles, which are produced when dark matter particles collide with the visible matter we are familiar with.

Most experiments to date have focused on dark matter particles with masses that more or less overlap with those of known elementary particles. If the particles are lighter than an electron, however, it is unlikely they would be detectable with the current standard, namely detectors based on liquid xenon. So far, no experiment has succeeded in directly detecting dark matter. Yet this in itself is an important finding, as it shows that dark matter particles do not exist within the mass range and interaction strength tested.

New device sensitive to lower-energy events

An international team led by Laura Baudis, Titus Neupert, Björn Penning and Andreas Schilling from UZH’s Department of Physics has now been able to probe the existence of dark matter particles across a wide mass range below one mega electron volt (MeV). Using an improved superconducting nanowire single-photon detector (SNSPD), the researchers reached a sensitivity threshold of about one-tenth the mass of an electron, above which dark matter particles are highly unlikely to exist. “This is the first time we’ve been able to search for dark matter particles in such a low mass range, made possible by a new detector technology,” says first author Laura Baudis.

In a 2022 proof of concept, the researchers had tested the first SNSPD device that’s highly sensitive to lower-energy photons. When a photon strikes the nanowire, it heats it up slightly and causes it to instantly lose its superconductivity. The wire briefly becomes a regular conductor, and the resulting increase in electrical resistance can be measured.

Detecting smallest dark matter particles

For their latest experiment, the UZH scientists optimized their SNSPD for dark matter detection. In particular, they equipped it with superconducting microwires instead of nanowires to maximize its cross section. They also gave it a thin, planar geometry that makes it highly sensitive to changes in direction. Scientists assume that the Earth passes through a “wind” of dark matter particles, and the particle’s direction therefore shifts over the course of the year depending on relative velocity. A device capable of picking up directional changes can help to filter out non-dark-matter events.

“Further technological improvements to the SNSPD could enable us to detect signals from dark matter particles with even smaller masses. We also want to deploy the system underground, where it will be better shielded from other sources of radiation,” Titus Neupert says. Below the mass range of electrons, current models to describe dark matter face considerable astrophysical and cosmological constraints.