How did we come to live in a universe full of matter? The laws of physics as we currently understand them would predict complete annihilation with antimatter just after the Big Bang. That we are here to ask such a question is proof that our understanding is not yet complete. Ironically, the key to this mystery likely involves the tiniest of things—a subatomic particle called the neutrino. 

Neutrino physics research at PNNL is supported by the Department of Energy (DOE) Office of Nuclear Physics, which supports experimental and theoretical efforts to learn how visible matter came into being and evolved, how it organizes itself, and how it interacts. This fundamental research provides solid foundations for many other areas of science. The more we learn about neutrinos, the better we understand matter and the universe. The quest to learn such things often leads to technologies and applications that cannot be foreseen.

Currently, some basic neutrino properties, such as their absolute mass, and even their intrinsic particle nature are unknown. Even if individual neutrinos are extremely light, they are so abundant that they might collectively account for a significant amount of all the mass of normal matter in the universe. Furthermore, if they are the right kind of particle, they might have played a critical role in tipping our universe in favor of matter over antimatter. These properties are crucial for understanding the history of the universe. At PNNL, researchers work with international collaborations of scientists and engineers to address the unknown mass and particle nature of neutrinos through some of the world’s leading particle physics experiments.

Finding missing energy

The neutrino’s mass can be revealed through a small amount of “missing” energy in the decay of tritium, a radioactive form of hydrogen. The energy of the electrons emitted in those decays can be very precisely determined by observing their motions in a magnetic trap. Any missing energy must have been carried away as the mass of the unobserved neutrino, also emitted in the decay. 

The Project 8 collaboration was the first to measure individual electrons like this; the information about electron motion is carried by a signal with just a quadrillionth of a watt of power. Project 8 is currently working to grow its prototype demonstrations into the next generation of neutrino mass experiment.

The particle nature of the neutrino can be determined by observing an extremely rare type of radioactive decay: neutrinoless double beta decay. The “MAJORANA” Demonstrator Project, is searching for evidence using PNNL technology that produced the most radiopure copper in the world, thereby reducing background signals from natural radiation in the detector materials. PNNL scientists and engineers are now working toward the next generation of experiments to increase sensitivity to a quadrillion times the age of the universe by exposing tons of material for several years. In particular, they are working with an international team to create the next Enriched Xenon Observatory (nEXO) consisting of several tons of liquid xenon, enriched in the rare isotope xenon-136.