Everything in our universe is made of matter, but a century of physics has revealed that at the beginning of time, an exactly equal amount of antimatter existed. Then, two seconds after the Big Bang, something changed and suddenly there was more matter than antimatter. What we don't know is how matter won and opened the door to existence as we know it.

Now, in a former salt mine next door to a nuclear weapons waste repository, Stanford physicists are completing the installation of a new particle detector, the Enriched Xenon Observatory 200, that they hope will provide the answer to that question.

"We're in the land where the theorists can't really tell us what to expect," said Jesse Wodin, a researcher at the Stanford Linear Accelerator Lab who is working on the EXO-200. "No one has done this at this scale."

When it's fully installed next year, the EXO-200 will be one of, if not the most, sensitive radiation detectors in the world. Located inside the Department of Energy's Waste Isolation Pilot Plant near Carlsbad, New Mexico, the new underground lab will be tracking the behavior of neutrinos, mysterious particles that hardly interact with anything. Though fundamental to our understanding of the universe, we know next to nothing about them.

The new detector will try to fill in the picture, determining basic features of the particles, like their mass and whether or not they, unlike almost all other particles, are their own antiparticles.

That quirk is why some scientists believe neutrinos could be the mechanism for the creation of our matter-filled universe. Almost all other particles have an antiparticle twin that, if it comes into contact with the particle, immediately annihilates it. But if neutrinos are their own antiparticles they could conceivably be knocked onto matter's

"team," thereby causing the cascading win for matter over antimatter that we know occurred. As the Indian theoretical physcist G. Rajasekaran put it in a speech earlier this year, neutrinos that are their own antiparticles would explain "how, after

[the] annihilation of most of the particles with antiparticles, a finite but small residue of particles was left to make up the present Universe."



But to test that theory, they'll need to catch one of the rarest events predicted by particle physics, a particularly strange kind of radioactive decay of purified xenon.

Driven by faster computers, physicists have been able to build increasingly sophisticated detectors and analysis equipment. They bury them underground to avoid interference from cosmic rays and other less exotic sources. For decades, scientists have been building larger and larger particle detectors with greater and greater sensitivities. But for exceedingly rare events, they run into some hard physical limits.

"If you had a single atom of Xenon 136, you'd have to wait 1025

years for it to have a reasonable probability of decaying," Wodin said.

"That's orders of magnitude longer than the age of the universe.... The only way to increase your odds of seeing it is to get a lot of atoms."

Getting more atoms requires building bigger detectors. The new experimental rig is composed of 200 kilograms of liquid xenon, made up of an almost unthinkable number of atoms of the noble gas. It is the largest detector purpose-built for catching what scientists call double beta decay.

"You can imagine a big vat of liquid xenon," said Wodin. "This one atom goes and becomes a barium. Basically, you want to have detected that process."



There are two ways that the double beta decay can occur. In the standard version, two electrons and two antineutrinos are sent hurtling out from an atom's nucleus. Scientists were only able to experimentally confirm that type of decay in 1986, after decades of trying.

Now, the search has shifted to the more improbable, neutrinoless radioactive decay, wherein only the electrons are emitted. That would only be possible if the two neutrinos that are normally emitted actually annihilate each other, which would confirm that neutrinos are their own antiparticles.

And with a neutrinoless beta decay observation in hand, the physicists could also determine the mass of the neutrino.

"You combine the results of regular beta decay and neutrinoless beta decay and basically do a kind of subtraction, and you can figure out what the mass is," Wodin said.

Back in 2006, some members of a team running an experiment called the Moscow-Heidelberg made a much-disputed announcement that they'd witnessed the special type of decay, but their claim is not generally accepted within the particle physics community.

At its current size, the EXO-200 team expects to have a shot at observing a few decay events per year. To further increase their odds of catching neutrinoless decay, they are planning an even larger 1-ton detector. The main limiting factor had been finding a place that could purify xenon enough to avoid any bad data sneaking into their experiment.

To enrich their xenon to the experimental levels they wanted, the team had to reach out to our former Cold War opponents.

"We used money available from the U.S. government to keep former

Soviet weapons scientists busy," Wodin said. "This was a fantastic partnership. We gave them our Xenon and they purified it to a single isotope. That increases our sensitivity massively."

That relationship, Wodin said, is a key advantage the team of Stanford researchers and their international collaborators has over their competitors in the search to figure out the neutrino's fundamental properties like the fantastically named Cryogenic Underground Observatory for Rare Events in Italy, and the Germanium Detector Array and the Cadmium-Zinc-Telluride 0-Neutrino Double Beta Research Apparatus in Germany.

And that's important because with a prize as large as understanding how matter came to persist in the universe, the competition is intense.

"It's basically at the top of the list in terms of rare decays,"

Wodin said. "It's certainly competition, but it's also friendly and there's a lot of information exchanged."

Images: Courtesy of Jesse Wodin and the Enriched Xenon Observatory, except #2.

1. The underground cavern at WIPP where the detector will be located. 2. An illustration of the situation in the early universe, two seconds after the Big Bang. Courtesy Berkeley Lab. 3. The cryostat for the detector. 4. A piece of the detector 5. Construction underway at WIPP.

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