What radioactive decay ever seen before could tell us about neutrinos

Bill Fairbank is not looking for anything.

The physics professor at Colorado State University is studying the fundamental particles of matter known as neutrinos, as well as an extremely rare case of radioactive decay in which neutrinos – present in such decays – can not be found. .

This process theorized but never observed before, called "double decay without beta without neutrinol", would rock the world of particle physics. If it was discovered, it would solve long-standing mysteries about the fundamental properties of neutrinos, which are among the most abundant but least understood particles in the universe.

Since 2005, the Fairbank laboratory has been part of the international scientific collaboration EXO-200 (Enriched Xenon Observatory), which aims to detect neutronol-free double beta decay using a particle detector filled with very cold liquid xenon.

In a new advance published on April 29 in the newspaper NatureThe Fairbank team laid the groundwork for a single-atom lighting strategy called barium marking. Their realization is the first known imaging of single atoms in a solid noble gas.

Barium labeling may prove to be a key technology for detecting beta-beta double decay without neutrin in a future enhanced experiment called nEXO. Crucially, barium staining would give scientists the power to clearly identify the byproducts of an atom of double beta decay by separating actual events from impulse signals. background.

The EXO-200 particle detector is located in Carlsbad, New Mexico, 800 meters underground. It contains 370 pounds (about 170 kilograms) of xenon atoms enriched isotopically in liquid form. Sometimes unstable xenon isotopes undergo radioactive decay, releasing two electrons and two neutrinos, transforming the xenon atoms into barium atoms.

If the decay produces only two electrons and a barium atom, it indicates that a double beta decay without neutrinol could be produced. And this can only happen if the neutrino is its own opposite antiparticle, an exceptional question that scientists would like to answer through these experiments.

Confirmation of such decay without neutrinol would be historic, requiring updates to the standard model of particle physics. In addition, the measured half-life of disintegration would help scientists indirectly measure the absolute masses of neutrinos – a feat never achieved before. Finally, if there is a double beta decay without neutrinol, scientists could use this information to understand why the universe contains so much matter, but so little antimatter. Until now, the EXO-200 detector has produced decay events of the correct energy, but no definite excess over what was expected from the bottom of the measured detector.

"In the EXO-200, we had about 40 decomposition events in two years," Fairbank said. "But we can not say exactly how many of them are real."

As for sifting stacks of balls of identical appearance, the distinction between actual decomposition and background events of similar appearance has been a central problem for researchers. This is where Fairbank barium labeling comes into play. If barium labeling is successfully implemented in a subsequent upgrade of the nEXO detector being designed, the detector sensitivity to double beta decay without neutronol could be multiplied by four. upgrade for nEXO experience of several million dollars. If a positive signal is observed, scientists can use barium marking to know for sure that they have seen the decay that they are looking for.

The barium marking work was supported by the INSPIRE program of the National Science Foundation.

"It's amazing to think about the sensitivity of these experiments," said John Gillaspy, a physicist at the National Science Foundation. "In experiments 30 years ago, I found it difficult to search for" one in a million "exotic atoms." This new study looked for ten times rarer atoms. "Physics and chemistry have come a long way. "I'm excited to think about what Fairbank and his colleagues might possibly find using this new technique because it has the potential to really change what we know about the fundamental nature of reality."

In their Nature The Fairbank team described the use of a cryogenic probe to freeze the "daughter" barium atom, produced by the radioactive decay of the xenon-136 isotope, in solid xenon at the end of the probe. Then they use laser fluorescence to illuminate individual barium atoms in the now solid xenon.

"Our group was very excited when we had images of simple barium atoms," said Fairbank, who has been leading the experiment for several years. The Fairbank atomization technique could also be generalized for other applications, with implications for areas such as nuclear physics, optical physics and chemistry.