Underground Experiment Asks Why We're Not Antimatter

The Majorana Demonstrator experiment
The Majorana Demonstrator experiment, being built in the tunnel on the left inside the Sanford Underground Research Facility in South Dakota, will search for a rare particle decay process that could illuminate the nature of antimatter. (Image credit: Matt Kapust/LBNL)

A new experiment buried deep underground in a South Dakota mine aims to detect rare particle decays that could explain the mystery of antimatter.

Scientists don't know why the universe is made of matter and not antimatter, but they hope to find differences in the way these two types of stuff behave that could explain the discrepancy. Antimatter particles have the same mass as their normal-matter counterparts, but opposite charge and spin.

The South Dakota effort, called the Majorana Demonstrator, aims to observe a theorized-but-never-seen process called neutrinoless double beta decay.

Unstable atomic nuclei (the cores of atoms containing protons and neutrons) will often let go of a neutron in a process known as beta decay. The neutron transforms into a proton by releasing an electron and a tiny particle called a neutrino. [5 Elusive Particles Beyond the Higgs]

Sometimes, two neutrons are lost in a process called double beta decay, which usually releases two electrons and two antineutrinos (the antimatter partner particles of neutrinos). But scientists have also theorized that two neutrons could convert into two protons and two electrons, without producing any antineutrinos — a process dubbed neutrinoless double beta decay.

If such a transformation were possible, it would mean that neutrinos and antineutrinos are the same particle. Scientists call particles like these, which are their own antimatter counterparts, Majorana particles.

Any new clues about the nature of antimatter could help elucidate why the universe contains so little of it.

"It might explain why we're here at all," David Radford, a physicist at Oak Ridge National Laboratory in Tennessee who works on the Majorana Demonstrator project, said in a statement. "It could help explain why the matter that we are made of exists."

The Majorana Demonstrator, a collaboration between scientists from the United States, Russia, Japan and Canada, aims to search for evidence of neutrinoless double beta decay in atoms of germanium-76, a slightly radioactive version of germanium. The experiment will eventually include 30 germanium detectors, each weighing 2.2 lbs (1 kilogram).

Building these detectors is a complex effort. For starters, the scientists had to obtain 93.7 lbs (42.5 kg) of 86-percent enriched white germanium oxide powder from a Russian enrichment facility — a sample worth $4 million. This power had to be processed, purified and refined into metal germanium bars that could then be turned into the separate cylindrical detectors that make up the experiment.

Furthermore, the material has to be carefully stored and shielded to protect it against charged particles from space called cosmic rays. That's why the experiment is being built 4,850 feet (1,478 meters) underground in the Sanford Underground Research Laboratory (SURF) in Lead, S.D.

"Cosmic rays transmute germanium atoms into long-lived radioactive atoms, at the rate of about two atoms per day per kilogram of germanium," Radford said. "Even those two atoms a day will add to the background in our experiment. So we use underground storage to reduce the exposure to cosmic rays by a factor of 100."

So far, Radford and his Oak Ridge colleagues have delivered nine of the enriched detectors to the South Dakota facility. The full suite of 30 detectors is expected to be complete by 2015.

"The research effort is the first major step towards building a one-ton detector — a potentially Nobel-Prize-worthy project," Radford said.

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Clara Moskowitz
Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She has written for both Space.com and Live Science.