Double or nothing: physicists bet the neutrino's its own eerie twin.

Deep underground, in a cavern beside the Gran Sasso Tunnel in the Apennines Mountains near Rome, physicists are stacking blocks made of small, transparent crystals containing the isotope tellurium-130. It's one of only a handful of isotopes expected to undergo a proposed sort of nuclear disintegration. Within months, Ettore Fiorini of the University of Milan-Bieocca in Milan, Italy, and his colleagues expect their stack of crystals to begin serving as a detector of the long-sought disintegration, known as neutrinoless double-beta decay. Other researchers, conducting different types of experiments using other isotopes, are hunting for the same trophy. One group claims to already have it, but other scientists are skeptical of the finding.

Finding this disintegration could deeply affect the way physicists describe the universe.

One incentive for these difficult and increasingly expensive experiments is the hope of filling one of physics' most important knowledge gaps--the mass of the neutrino (SN: 1/30/99, p. 76). What's more, the information would help physicists answer the tough question, For neutrinos, are matter and anti-matter one and the same?

Measuring the neutrino's mass would also shed light upon a recently discovered oversight in the prevailing theory, or standard model, of particle physics and help researchers better understand how neutrinos have influenced the evolution of the universe.

"Looking for neutrinoless double-beta decay is really shining a light on the unknown," says Giorgio Gratta of Stanford University and spokesman for an experiment being developed there. Finding that disintegration would provide evidence for "physics that is not in our current description of the world," he says.

MASS DELUSION Neutrinos were first inferred to exist in 1930 to account for missing energy in a nuclear disintegration process known as beta decay. For decades thereafter, they were described as massless, uncharged particles. They're so abundant that 10 trillion of them pass every second through an area the size of your hand.

In 1998, researchers working at the SuperKamiokande detector in Japan demonstrated that neutrinos, which come in three varieties--electron neutrino, muon neutrino, and tau neutrino--can change into one another (SN: 6/13/98, p. 374). Subsequent findings from the Sudbury (Ontario) Neutrino Observatory in the past year (SN: 5/11/02, p. 301) confirmed the result. That flip-flopping of identity, known as neutrino oscillation, implies that the particles have mass, physicists say. Because the standard model assumes that neutrinos are massless, the oscillation findings provide the first crack in the theory that has been the bedrock of particle physics for decades.

"Now that we know the neutrino has a mass, it's critical we know what [that mass] is," says Steven R. Elliott of the University of Washington in Seattle.

From the results of the neutrino-oscillation experiments, scientists have calculated that the neutrino mass is at least 0.05 electron volts (eV), or about a 10-millionth the mass of the electron. Other measurements indicate that the neutrino mass is less than 2.2 eV. However, for an exact figure, scientists are giving new emphasis to a search that has previously only interested a few physicists.

"This orphan child has suddenly become like Tiger Woods," exclaims physicist Frank T. Avignone III of the University of South Carolina in Columbia.

NEUTRONS TO GO The new crop of experiments is seeking neutrinoless double-beta decay of neutrons. Neutrons are surprisingly unstable particles, considering that they make up roughly half the mass of all ordinary matter. When confined within stable nuclei, neutrons can last essentially forever. Outside of a nucleus, however, free neutrons last only about 10 minutes before disintegrating by means of beta decay.

When a neutron decays in an unstable nucleus, the particle transforms into a proton, while an electron and an antineutrino flee the scene. The upshot of each beta decay is an atom with a nucleus that contains one more proton than it did before. This is legitimate alchemy--you end up with a different element. For example, the radioactive form of hydrogen called tritium changes into helium.

Ordinarily, a single beta decay permits a neutron to assume a less energetic state. However, "in some isotopes, the regular, single-beta decay is forbidden. It would violate energy conservation [rules]," Gratta notes. "That opens the possibility of a different way of decay that doesn't violate energy conservation--and that's double-beta."

The standard model predicts a type of double-beta decay in which two neutrons simultaneously decay, while two electrons and two antineutrinos are emitted. In a major discovery in 1987, Elliott and other physicists led by Michael K. Moe of the University of California, Irvine found an example of this double-beta decay (SN: 9/5/87, p. 148). There's a 50 percent chance that any nucleus in a given sample will undergo such a decay in [10.sup.20] years--the decay's half-life. This double-beta decay is the rarest form of nuclear decay so far observed, says John F. Wilkerson of the University of Washington in Seattle.

In the late 1930s, Italian physicist Ettore Majorana postulated a strange characteristic of neutrinos that implies there is a second type of double-beta decay. A reclusive colleague of Enrico Fermi, Majorana died at an early age under mysterious circumstances.

While most elementary particles have a corresponding antimatter particle, the young Majorana proposed that neutrinos are their own antiparticles. This proposal opens the possibility that two neutrons may decay so that the antineutrino emitted by one is promptly absorbed by the other. The two neutrons would simultaneously disintegrate without the nucleus emitting any antineutrinos--hence the "neutrinoless" part of the decay's name.

Like a magician's rabbit, "[the antineutrino] appears and then disappears into the hat," says Xavier Sarazin of the University of Paris-South in Orsay, France. Sarazin and his colleagues are searching for signs of the decay in an experiment under the French Alps.

Even in Majorana's day, scientists recognized that this magic trick would depend on the neutrino having some mass. As long as the standard model was unchallenged in its assertion that neutrinos were massless, the search for neutrinoless double-beta decay was considered an extreme long shot. Now, in the wake of the oscillation experiments and their implication of neutrino mass, many scientists find the possibility of such a decay more plausible.

If neutrinoless double-beta decay exists, it's much rarer than the other type of double-beta decay. Researchers have set the most stringent limits so far with two experiments that Elliott calls the "crown jewels of the field." One of these studies, completed in a tunnel in Spain, observed 6 kilograms of the isotope germanium-76, while the other, known as the Heidelberg-Moscow collaboration, focused on about 11 kg of that same isotope.

Those experiments have shown that the half-life associated with this decay must exceed [10.sup.24] years. That's a period so long that if a second were equivalent to the age of the universe, a clock ticking off those eternal seconds would run for more than 3 million years.

WHERE'S NEUTRINO? If a neutrinoless double-beta decay occurs within the knee-high crystal tower that Fiorini and his colleagues are building, pairs of electrons should intermittently shoot out of a tellurium-130 nucleus into the rest of the crystal and deposit their energy there. As a result, the crystal, which is kept ultracold, would heat up slightly but measurably.

In the only neutrinoless double-beta decay experiment currently operating--the trial under the French Alps--electrons from a decay would zip through a helium-filled chamber laced with charged metal wires and then plow into sheets of plastic that emit light when struck. Helium atoms ionized by the electrons and attracted to the wires would delineate the electrons' paths, while the light produced would be proportional to their energy. Other detectors depend on the electrons from a decay to ionize other atoms and produce an electric current proportional to the decay energy.

Because the number of decays is certain to be tiny, any neutrinoless double-beta signal would have to be extremely faint. That means researchers must be able to tease that signal from stronger ones that result from natural radioactivity, including traces of radioactive contaminants in the detectors' components. To avoid those pitfalls, researchers build their detectors out of purified low-radioactivity materials, blanket the equipment in tons of shielding, and conduct their experiments deep underground to avoid cosmic rays.

"Double-beta experiments are the lowest background experiments that mankind has produced," Gratta says. An ordinary chair "is at least a billion times too radioactive to be close to our detector," he notes.

To have a hope of seeing the uncommon breakdowns, some teams also chill their set-ups to cryogenic temperatures and use highly enriched isotopes that are fabulously expensive. In May, Gratta's experiment received 100 kg of xenon-136 costing about $500,000. Gratta says these experiments are creating a demand for enriched isotopes--supplied mostly from Russian facilities--second only to the demand for enriched uranium for nuclear weapons and nuclear power.

Scaling up the experiments, which requires using greater masses of isotopes, may be the most important means of picking up the faint signal of neutrinoless double-beta decays. Physicists have determined that the lighter the neutrino, the rarer those disintegrations. Given the extremely small neutrino mass now indicated by oscillation experiments, researchers suspect that neutrinoless double-beta decay tests to date have contained too little of their respective isotopes to generate detectable signals. After all, the more isotope in an experiment, the more nuclei there are to break down.

Researchers are planning two new experiments designed to observe a whopping half-ton and 1-ton, respectively, of the enriched isotope germanium-76. The follow-up to Fiorini's current tellurium experiment is also expected to be in that range. Gratta says that his experiment eventually will incorporate up to 10 tons of enriched xenon-136.

Yet the big leap in sensitivity expected from massive experiments may not even be needed to confirm Majorana's hypothesis, says Hans V. Klapdor-Kleingrothaus of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and the leader of the Heidelberg-Moscow collaboration and one of the large proposed germanium experiments.

From an analysis of data acquired during the 10-year run of the Heidelberg-Moscow experiment, he and several colleagues claim to have the first evidence of neutrinoless double-beta decay. They reported the analysis in the Dec. 7, 2001 Modern Physics Letters A.

If verified, the observation of the decay "would be an extremely important result. It would put the neutrino physics community into a frenzy," Elliott says.

However, many hunters of neutrinoless double-beta decay have objected to the recent analysis. Random variations in background-radiation sources can explain the apparent signal, they argue. Moreover, they note, the statistical significance of the finding falls below the level generally considered noteworthy in particle physics. The conclusions that Klapdor-Kleingrothaus and his colleagues have made are "very dicey," says Wilkerson.

Dismissing such criticisms, Klapdor-Kleingrothaus says he expects to "confirm this signal with very high probability in a short time" using as little as 100 kg of germanium.

Even if he succeeds, there's plenty of reason to continue with a variety of experiments, scientists say. "No one would believe it if [neutrinoless double-beta decay] was found in just one [type of] nucleus," says Fiorini. Moreover, even results demonstrating neutrinoless double-beta decay would leave some uncertainty in the neutrino mass. Researchers could use the results from multiple experiments on different isotopes to reduce that uncertainty. "You need 4 to 5 isotopes to nail down the effective mass of the neutrino," Avignone says.

AMASSING EVIDENCE Physicists expect data from neutrinoless double-beta decays, if they're ever found, to inform a variety of questions beyond the exact mass of the neutrino. They want to know why the neutrinos seem to have masses so much smaller than those of other members of their class of fundamental particles, which includes electrons and their heavy cousins, muon and tau particles.

"Why neutrinos are so much lighter begs for an explanation," says theorist Petr Vogel of the California Institute of Technology in Pasadena.

In their experiments on neutrinoless double-beta decay, physicists may also find clues as to why the universe today is almost totally made up of matter, although it presumably started with an equal mix of both matter and antimatter, adds Wilkerson.

Physicists are curious whether neutrinos played a guiding role in the formation of the large-scale structure of the universe. The minimum mass calculated from the oscillation experiments suggests not, but the mass that Klapdor-Kleingrothaus reports--0.39eV--is large enough for neutrinos to have had a cosmos-shaping effect.

Investigations of neutrinoless double-beta decay may strongly affect theories about deep interconnections between the fundamental forces of nature. Having already uncovered a profound link between the electromagnetic force that governs electricity and magnetism and the weak force that acts in such processes as beta decay, physicists have been constructing so-called grand unified theories that go even further.

Those theories unite the electromagnetic and weak forces with the strong force that holds atomic nuclei together. They join these forces into a greater superforce that may have existed briefly at the birth of the universe. A linchpin of many of those theories is that the neutrino is a so-called Majorana particle--one that is its own antiparticle, Vogel says.

Although finding neutrinoless double-decay would answer many of the questions, not finding it could also be instructive, scientists say.

If the next round of neutrinoless double-beta decay experiments are done properly but come up empty-handed, that could lead to one of two outcomes. On the one hand, by combining those null results with data from neutrino-oscillation experiments and other efforts to measure neutrino mass, the experiments may even succeed in finally ruling out neutrinoless double-beta decay, Elliott says.

Should that happen, scientists would be forced to conclude that the neutrino is just like the rest of the fundamental particles, with separate and distinct matter and antimatter versions. "That would knock many grand unified theories right off the table," Aviguone says. It would also bring to a close more than 60 years of searching for a long-anticipated nuclear process.

On the other hand, the failure to find the elusive decay could mean simply that this nuclear event is even rarer than expected--a prospect that could keep physicists laboring in their underground labs for many decades to come.