NSF funds Pocar's study of neutrinoless double beta decay

Andrea Pocar, assistant professor of Physics, recently received a $510,000 National Science Foundation grant through the federal economic stimulus funding program to study neutrinoless double beta decay.

If it exists and can be observed, neutrinoless double beta decay might shed new light, in this simplest of elementary particles, on what mass is and on fundamental processes of how it is generated. “It might also tell us something about the asymmetry between matter and antimatter that we observe in the universe,” he says.

As Pocar explains, neutrinos are fundamental particles (like electrons and quarks) with no electric charge (neutral). They interact with other particles only via the weak nuclear force. An electron and a neutrino are simultaneously emitted from an atom’s nucleus during nuclear beta decay under the influence of this force. For many decades, scientists thought neutrinos were completely without mass, but they now understand that neutrinos have very tiny masses, at least 500,000 times smaller than that of the electron.

In this two-part project, Pocar will work with his Physics department colleague Krishna Kumar and others at the Enriched Xenon Observatory (EXO) located deep underground in a New Mexico salt mine, as well as in a smaller laboratory detector on campus. At the EXO, a five-foot-diameter copper barrel housed inside a clean room half a mile underground serves as a cryostat or refrigerator. The researchers place a bucket-sized detector inside, seal it and fill it with about 440 pounds (200 kg) of enriched xenon (with more of the isotope xenon136 than usual). They then cool the EXO-200 detector to minus 100 degrees Celsius, at which temperature xenon turns into a liquid that’s three times more dense than water.

The smaller lab to be built on campus will support this work by offering Pocar and colleagues a handy place to test individual detector components and observations made underground at EXO in a setup that’s easier to handle. The EXO-200 detector is built to very strict cleanliness standards to achieve the lowest possible intrinsic radioactivity, the physicist notes. Radioactive decays within and around the detector might otherwise be mistaken for double beta decay events. Others in the group are postdoctoral researcher Tim Daniels, graduate students Peter Morgan and Jessica Cook, and undergraduates Chris Sterpka, Kyle Schmoll and Andy Dowd.

If neutrinoless double beta decay of this isotope takes place, the detector will “see” the nucleus emitting two electrons simultaneously during decay, with no neutrinos. The mine’s salt and rock shield the detector from most cosmic rays found on the surface, which would make the search for such a rare event impossible, Pocar says.

Standard double beta decay (where two electrons each accompanied by a neutrino are emitted by a nucleus), though rare, can be fully explained by current theory and has been observed in a few elements, Pocar says. In their new experiments, he and colleagues are checking the possibility that such decay can occur without any neutrino emissions, which would require new theoretical arguments. “A number of factors make this seem possible,” he says.

For example, as the only fundamental particle without an electric charge, a neutrino is the only candidate particle that may prove to be its own anti-particle, with finite mass. If neutrinos are their own anti-particles then neutrinoless double beta decay becomes possible. “It’s a deep question in physics and the answer could shed some light on understanding matter and antimatter in the universe, one of the fundamental questions in cosmology. Why is the universe we know made of matter and not anti-matter? Finding a particle that is its own anti-particle is a little piece of information that could address this,” Pocar says.

EXO-200 is due to start collecting data in 2010, Pocar notes. It is designed to be sensitive to neutrinoless double beta decay lifetimes in xenon136 longer than 10²⁵ years, or about 10¹⁵ times the age of the universe. He adds that about 60 collaborating scientists from institutions in the United States, Canada, Switzerland and Russia, must train as miners in order to take shifts on the experiment.

Even if EXO-200 doesn’t observe it, the search for neutrinoless double beta decay might not be over, because a larger detector has been planned for EXO to hold one to 10 tons of xenon, with sensitivity to the lifetime of neutrinoless double beta decay of xenon-136 of 10²⁷ or even 10²⁸ years.

October 20, 2009.

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