Lab in a gold mine looks at matter-antimatter imbalance

There's a problem with our understanding of the universe: We don't know why it has enough matter to make it interesting.

The Standard Model of particle physics encompasses three of nature's four fundamental forces—all but gravity—yet it tells us the Big Bang should have produced matter and antimatter in equal measure. Therefore, the two should have cancelled each other out, leaving a universe with energy but essentially no matter: no galaxies, no planets, no us.

Clearly the Standard Model is missing something; figuring out what that might be is one of the great challenges of modern science.

"The fact that we're here talking about it means that that didn't happen," noted ORNL physicist David Radford, "So there must have been some imbalance bigger than what was predicted by the equations—quite a bit bigger."

A team co-led by Radford created a lab 4,850 feet underground at South Dakota's Sanford Underground Research Facility to help them find the glitch. There, in a retired gold mine, they will focus on nature's smallest known particles: neutrinos.

Neutrinos are so unassuming that trillions produced by the sun go through your body every second without notice. They may explain the universe's matter-antimatter imbalance because they may be their own antiparticles, a theory put forward by Italian physicist Ettore Majorana 80 years ago. If he was right, neutrinos could have tipped the balance of matter over antimatter after the Big Bang.

Radford's team is working to verify Majorana's theory by establishing the conditions necessary to confirm a theoretical nuclear reaction called neutrinoless double beta decay.

When an atom goes through regular double beta decay, two neutrons in its nucleus spontaneously transform into protons, accompanied by the emission of two electrons and two neutrinos. Majorana's theory, however, says that if neutrinos are their own antiparticles, then double beta decay would sometimes happen without the emission of neutrinos. If we can prove the existence of neutrinoless double beta decay, we will demonstrate that neutrinos are their own antiparticles.

The team knows exactly what a neutrino-free decay would look like in its detectors. Nevertheless, such reactions, if they exist at all, are so rare that identifying them will be much more difficult than finding a needle in a haystack.

The South Dakota project—known as the Majorana Demonstrator Project— houses 40 kilograms of detectors made from germanium, primarily the isotope germanium-76. Its primary goal is to show that other nuclear reactions—those caused by cosmic rays, for instance—can be minimized in the detectors.

This is necessary because neutrinoless double beta decays are rare. Germanium-76 has a radioactive half-life more than 100 billion times longer than the age of the universe, so even though the next experiment will contain 1 ton of germanium detectors, researchers expect to see the neutrino-free decay in only about one atom per year. Over the course of a 10-year experiment, then, they hope to see about 10 or so neutrinoless double beta decays total.

The demonstrator's detectors have been operating since October, and so far researchers are confident they will be able to sufficiently minimize background radiation.

According to DOE's Nuclear Science Advisory Committee, the experiment might answer one of the universe's most compelling mysteries.

"A ton-scale instrument designed to search for this as-yet-unseen nuclear decay will provide the most powerful test of the particle-antiparticle nature of neutrinos ever performed," the committee said in its 2015 long-range plan.

Radford agrees.

"These are perhaps the most compelling questions in all of fundamental physics right now: 'Are neutrinos their own antiparticle, and what gives rise to the excess matter in the universe?'"