Hidden beneath a nearly mile-thick rock in Abruzzo, Italy, scientists are hard at work unraveling the mysteries of the universe’s tiniest bits of matter. When a radioactive process called beta decay occurs, it usually releases two particles: a negatively charged electron and version of the tiny, neutrally charged neutrino. The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND-200) at Gran Sasso National Laboratory is designed to find out if the process can occur without a neutrino at the end. The answer could shape our understanding of how matter came to be.
The process of “neutrinoless double beta decay” if it does occur, is very rare. Noticing when the decay results in electrons but not neutrinos can be tricky, especially since neutrinos are everywhere — billions pass through your body every second — and they’re often produced when background radiation reacts with machine components.
That’s why scientists focus on “choosing really low-radioactivity materials to start with and then coming up with lots of clever ways to reject the background [particles]- says Drexel University particle physicist Michelle Dalinsky, who is not involved in the project.
The LEGEND-200 is equipped with slightly radioactive germanium crystals that act as both a beta decay source and a neutrino detector. To screen out particles from the environment, the entire setup is immersed in a cryogenic tank protected by water and liquid argon. This core is surrounded by green optical fibers and a reflective film that bounces off stray particles.
If LEGEND-200 observes double beta decay without a neutrino, it would mean that unlike protons, electrons, and other elementary particles, which each have an “antiparticle” that destroys them on contact, neutrinos are their own antiparticles and can destroy each other. If this is the case, then when double beta decay occurs, two neutrinos are produced, which immediately annihilate, leaving none behind. “This is an important ingredient in trying to understand why matter dominated antimatter in the early universe and why the universe exists as it does today,” says Dalinsky.
LEGEND collaborator Laura Baudis, an experimental physicist at the University of Zurich, is looking forward to what the experiment will reveal when data collection begins later this year. “There are so many things we don’t know about neutrinos,” she says. “They really are still full of surprises.”
