CRESST utilizes crystals of calcium tungstate cooled to approximately -273 Celsius – close to absolute zero. Should a particle of dark matter collide with an atomic nucleus in the crystal, it would increase the temperature at that point by about one millionth of a degree. This minute change can be measured by a highly sensitive thermometer in the detector. The collision also generates light flashes, which are recorded by another sensor. This second signal reveals the type of particle involved, allowing the scientists to distinguish potential dark matter events from unrelated events.
"We have been continually improving our experimental setup over the past few years," says Federica Petricca, group leader at the Max Planck Institute for Physics and CRESST spokesperson. As a result, CRESST covers the lowest mass range of the approximately 20 existing dark matter experiments (*). "We are currently the only group able to search in this range. If dark matter manifests itself as a very light particle, CRESST has the best chance of detecting it," Petricca adds.
The new results come from the last measurement run from May 2016 to February 2018, before which the CRESST detectors had been comprehensively upgraded. At 25 grams, the new modules are significantly smaller than in the previous setup, making them sensitive to extremely light particles. The scientists weren't successful in detecting dark matter. "However, we only employed 10 detectors," explains Petricca. “This is set to change. Starting in 2020, we plan to install 90 more detectors."
Why dark matter exists
Matter attracts matter, a phenomenon we are all familiar with as gravity. As early as the 1930s, the renowned astronomer Fritz Zwicky coined the term "dark matter" after he investigated the motion of individual galaxies in galaxy clusters and found that mass was, apparently, missing. This effect can also be observed in spiral galaxies. The orbital velocity of stars far from the center of these galaxies is significantly higher than might be expected from the mass of the visible stars. The conclusion is that the gravitation can only be explained by non-luminous, "dark" matter.
Since then, there has been repeated evidence for dark matter to exist; one notable method involves precise observation of the cosmic microwave background, the reverberation of the Big Bang. Gravitational lensing, in which light bends around non-luminous matter, and investigations of collisions of galaxies provide further evidence.
(*) The mass range of CRESST is between 0.16 and 1.8 gigaelectronvolts/c2 (GeV/c2), a mass unit commonly employed in physics, where c stands for the speed of light. The XENON experiment can detect the most massive dark matter particles (up to several 100 GeV/c2). For comparison, the mass of the electron is about 0.00051 gigaelectronvolts/c2, which corresponds to 9.1 x 10-28 grams.