Studying the universe remains an exhilarating and humbling experience. All the particles and elements ever discovered, all the atoms and neutrinos, comprise a very small fraction of our universe’s content. This is one of the great scientific surprises of recent decades. Indeed, today astronomers believe that the overwhelming majority of the universe consists of “Dark Matter” and “Dark Energy”, entities that have never been detected on Earth. We look up into a universe made of substances utterly foreign to our world.
But scientists are searching for Dark Matter here on Earth. One Vatican Observatory cosmologist is part of that search. Maria Elena Monzani, a lead scientist at the SLAC National Accelerator Laboratory, in Menlo Park, California and the Kavli Institute for Particle Astrophysics and Cosmology of Stanford University—and also an adjunct scholar at the Vatican Observatory—is part of a team of over 200 co-authors whose paper “First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment” was recently accepted for publication in Physical Review Letters, one of the most prestigious journals in the field of physics.
Hints of Dark Matter were first observed by the astronomer Fritz Zwicky in the 1930s. He noticed that galaxies within a galaxy cluster seemed to be moving as though there was more matter in the cluster than was visible. Forty years later, Vera Rubin found that the orbits of stars within a galaxy are governed by an invisible mass. (Rubin is the namesake of the new Vera Rubin Telescope, expected to significantly advance what we know about the dark universe; she taught at the first Vatican Observatory Summer School in 1986.) Over the following decades, work by other cosmologists has indicated that Dark Matter is five times more abundant in mass than the ordinary atoms in the universe.
The LZ experiment hunts for hypothetical particles known as “weakly interacting massive particles” (or WIMPs) that have been proposed to account for Dark Matter. The name involves some humor, because “wimp” is the English equivalent of “fifone”. As befits the humorous name, WIMPs are supposed to have mass, but to do little else, interacting rarely and weakly with the normal matter we see. They are therefore “dark”—almost but not quite, impossible to see.
LZ uses a ten-ton tank of liquid xenon to try to detect rare interactions between WIMPS and xenon atoms that might produce light flashes or loose electrons which could then be recorded. To reduce interference from other sources in the Earth’s atmosphere, the experiment is located 1.5 km underground, in a former gold mine in South Dakota in the USA.
Monzani is the deputy operations manager for software and computing of LZ. She led simulated “dress rehearsals”, or “Mock Data Challenges”, to prepare for the early science phase of the experiment. Her team developed powerful analysis software to search out signals from a handful of possible dark matter interactions from petabytes of data collected by the detector. The LZ computing infrastructure employs supercomputers hosted at the National Energy Research Center (NERSC) in Berkeley, California.
The authors of “First Dark Matter Search Results” report a non-detection, while establishing the world record sensitivity for a search of WIMP dark matter particles. “Unfortunately,” says Monzani, “we did not discover the Dark Matter particle in this search.” However, the work so far has been oriented toward testing the experiment as much as toward detecting particles. “We knew that we would not have sufficient exposure for a discovery,” she says. “We pivoted immediately to a longer data taking campaign, which will take us into 2028 and will accumulate 20 times more exposure, allowing us to probe interesting scenarios describing the nature of the Dark Matter particle.” The search for the unknown in the universe continues.