One half of EXO's detector being constructed in a clean room.

Physicists are used to dealing with rare events and very small quantities, but rarely do they tackle a challenge of the kind facing the Enriched Xenon Observatory, or EXO. To find what they're looking for, not only will they try to find a rare event, but to be sure they will need to find a single barium atom in the 10 ton bath of liquid xenon--1028 atoms.

The EXO collaboration, involving SLAC National Accelerator Laboratory, Stanford University, and many other partners, is looking for a never-before-observed process called neutrinoless double beta decay. In their case, this means watching for an isotope of xenon decaying into barium, giving off two electrons (the double beta decay), but without giving out any neutrinos. A beta decay process gives off one neutrino, so how could this even be possible? It only works if the neutrino is its own antiparticle, so that the two beta decays each have a neutrino which essentially cancel each other out, like matter and antimatter annihilating. And the possibility that process exists is the reason for the experiment.

If neutrinoless double beta decay is observed, it means the neutrino must be its own antiparticle, a key unknown in the study of neutrinos. If the neutrino is indeed its own antiparticle, it has all kinds of implications for the structure of the Standard Model and the relationships between the fundamental particles.

The EXO experiment

EXO-200, the first version of the experiment, involves 200 kg of enriched xenon in an ultra-clean tank with light and electrical sensors that would detect the decay of xenon into barium. Steven Herrin of SLAC referred during the American Physical Society meeting in Denver, Colorado to the xenon as "the largest non-weapons based isotopically enriched stockpile in the world of any element."

Herrin said that the whole experiment is to be transported from Stanford University in northern California to the Department of Energy's Waste Isolation Pilot Plant in Carlsbad, New Mexico in June or July. The experiment is being installed 2000 feet underground in clean rooms at WIPP, a salt bed well suited to radioactive shielding. A technical run will begin in the late summer or autumn this year, with scientific data taking starting before the end of 2009.

A later version of EXO would include perhaps 10 times as much xenon and would be correspondingly more sensitive to the very rare neutrinoless double beta decay, if it exists at all.

Avoiding false alarms

Detecting a rare process like neutrinoless double beta decay is particularly challenging because it involves looking for a signal that occurs rarely but happens in an environment prone to generating false signals, called background. The very low-level but inherent radioactive decay of the experiment's equipment, combined with the chance of random cosmic rays hitting the apparatus all could give rise to signals that could be confused for the signal physicists are seeking.

The experiment is being placed underground to help avoid the effects of cosmic rays, and the materials from which the apparatus is made are all specially prepared to avoid the low-level radiation that essentially all materials naturally emit.

But even cutting down the background to as low a level as possible isn't ideal and experience with other related experiments trying to directly detect dark matter have shown that the major issue is dealing with these backgrounds.

The EXO team has an ambitious plan to try to make the backgrounds completely irrelevant, with no chance of them causing a false alarm. It requires finding the barium ion that is created when the xenon decays.

As graduate student Brian Mong from Colorado State University understated, "Detecting a single barium ion in 1028 xenon atoms is a unique challenge."

If the barium ion can be found and its creation matched to the time of the xenon emitting its two electrons, physicists can be sure that double beta decay really happened. But where do they begin?

Finding the needle

Mong described his graduate work on a process for detecting the barium ion. His is one of three different approaches being developed to try to find the barium. None of these will be ready for the EXO-200 experiment, but collaborators hope one of them will work well enough to implement in a larger-scale version of EXO.

Mong's approach is to feed an optical fiber into the liquid xenon and, using a small cryogenic system at the end of the fiber, freezing a sample of the xenon, which might include the barium ion. The barium ion would be trapped at the end of the fiber, where its existence could be confirmed.

The team is still unsure how they will trap the barium ion. The barium ion could be pushed toward the fiber using electric fields, or perhaps the fiber could be inserted on a probe into the middle of the xenon at the place identified by the flashes of light that will accompany the xenon decay. That is a problem for later and Mong is currently concentrating on getting the identification to work.

The basic principle for identification is that laser light of a specific frequency tuned to the properties of barium will be shone into the optical fiber. The light would hit the barium ion, if present, and the barium would absorb the light and then fluoresce, or re-emit some of it back toward the fiber. The light going back into the fiber could be detected and the existence of the barium confirmed.

Mong has a prototype of the equipment built and to test it, he is using a fluorescent dye called rhodamine 6G. He diluted some rhodamine many times over until there should have been about 100 molecules in the test tank. The fiber and laser system was able to very clearly identify the presence of the molecules showing the technique works in principle.

In what might be the closest physics has ever come to homeopathy, the next step was to dilute the sample even further until he expected to have only one molecule in the test tank. In a series of tests, he was able to extract a signal corresponding to the presence of a molecule, while tests with samples containing no rhodamine came up empty as expected.

Although the technique is not foolproof yet--there were a couple or results that didn't work which Mong fears might have come from two samples being confused--it shows promise for actually finding that elusive barium ion when xenon decays. The technique will be re-tested over the summer with many more samples.

Finding a single barium ion in the pool of xenon would be one of the most difficult detections of a single atom yet performed, but the payoff is enormous. It would bypass the problem of backgrounds and the kind of controversy that they have caused in related dark matter experiments.

Are the neutrino and antineutrino the same?

Even with detection of barium operating well, the physicists will still need to see what nature has in store for them. There is no doubt that barium will be created, as the xenon isotope being used will definitely decay.

But whether it decays only emitting neutrinos, or whether it also decays with no neutrinos must be disentangled. The trouble is that the neutrinos are invisible to the detector.

All that physicists have to work with is the energies of the electrons that are emitted during the decay of xenon to barium. Instruments will be able to determine the energies of the electrons with about one percent precision.

There is a maximum amount of energy that the electrons could carry away from the decay. But some of the energy would go into neutrinos if they are created. That means the energy spectrum of the electrons should be spread out over a wide range depending on just how much energy the neutrinos take with them.

If there is neutrinoless double beta decay, all of the energy goes into the electrons and that should form a spike of detections at the maximum energy. Physicists are confident that their equipment will be sensitive enough to resolve whether that spike exists.

If it does, physicists will know definitively that the neutrino is its own antiparticle. If the experiment has been running long enough and no such spike appears, physicists can be quite confident that the neutrino and the antineutrino are different particles. The longer the experiment runs, the more confident they will be.

Either way, with the system working and a little time, it should provide a key piece of the puzzle surrounding neutrinos, perhaps the least understood particles physicists have ever detected.