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May 30, 2006
A New Way to Look at Neutrinos

Neutrinos are cosmology's version of the remark Winston Churchill made about the Soviet Union: "a riddle wrapped in a mystery inside an enigma." Experiments at neutrino telescopes such as the Sudbury Neutrino Observatory (SNO) and KamLAND have substantially increased our scientific knowledge of neutrinos, yet much about these tiny, phantomlike intergalactic travelers remains mysterious.

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Studies at observatories like SNO reveal that neutrinos have mass and can oscillate, or change flavor, during their 93 million mile journey from the sun to the earth. (Courtesy NOAA)

No other known particle in nature is quite like a neutrino. According to the Standard Model, long the bedrock theory of particle physics, neutrinos were supposed to have no mass and come in three unchanging varieties, or "flavors." Observations over the past few years at major neutrino experiments, however, have revealed that neutrinos do have mass, albeit incredibly small, and that neutrinos can oscillate, or change flavor, as they travel.

"It just goes to show that we really don't know nothin' about neutrinos," Nobel laureate physicist Leon Lederman famously remarked, upon learning of the first evidence for neutrino mass and oscillation. One of the biggest questions is whether neutrinos are their own antiparticle. A Berkeley Lab scientist has a new idea on how to learn the answer.

"Borrowing theoretical and conceptual tools from astronomy, optics, and particle physics, it can be shown that interfering two neutrinos with one another could be used to determine if the neutrino is its own antiparticle," says Thomas Gutierrez, a theoretical nuclear physicist doing postdoctoral research with Berkeley Lab's Nuclear Science Division.

Dirac versus Majorana

While experiments like those at SNO and KamLAND are setting ever-improving upper limits on the neutrino's mass (by some estimates, one sixty-thousandth that of an electron), they say nothing with respect to the quantum nature of the neutrino, namely whether the elusive particle is a "Dirac" fermion or a "Majorana" fermion.

Fermions are particles of matter. Dirac fermions are named for theorist Paul A.M. Dirac, who predicted that every particle of matter should have an antimatter counterpart of equal mass but opposite electrical charge.

But since neutrinos have no electrical charge, their matter and antimatter versions could be identical; fermions that serve as their own antiparticles are named for Ettore Majorana, the theorist who first proposed their existence.

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Theoretical physicist Thomas Gutierrez has proposed a new way of answering the question of whether neutrinos are their own antiparticles. (Photo Roy Kaltschmidt, CSO)

If the Standard Model is to be modified to allow for neutrinos with mass, scientists must first determine into which of these two fermion classifications a neutrino falls. Gutierrez's idea for making the choice was presented in a paper entitled "Distinguishing between Dirac and Majorana neutrinos with two-particle interferometry," in a recent issue of Physical Review Letters.

"The American Physical Society has identified determining the absolute mass of the neutrino and discovering if the neutrino is a Dirac or Majorana fermion as top priorities in the study of neutrinos," says Gutierrez. "Two-particle intensity interferometry can, theoretically, provide answers to both questions. Two-particle intensity interferometry has been used extensively in many areas of physics and has served to cross-pollinate ideas in different subfields for over forty years, so it seems only natural to think this technology might play an important role in neutrino physics."

Would You Like Some Neutrinos With That?

"The question about the quantum nature of the neutrino is currently being addressed through a growing industry of neutrinoless double-beta-decay experiments," says Gutierrez. Double beta decays are events in which two electrons, otherwise known as beta particles, are emitted.

Gutierrez explains that evidence of neutrinoless double beta decay would indicate that neutrinos are Majorana fermions. "At Berkeley Lab, we are involved with two proposed neutrinoless double-beta-decay experiments, one called Majorana and another called CUORE. These experiments will also be sensitive to the absolute mass of the neutrino."

In a standard double beta decay, two neutrons are converted to two protons, releasing a pair of electrons and a pair of neutrinos. In a neutrinoless double beta decay, an antineutrino emitted in one of the beta decays can be absorbed as a neutrino in the other.

In standard double beta decay (left), two neutrons become protons by emitting two electrons (beta particles) and two neutrinos. If neutrinos are their own antiparticles, however, an antineutrino emitted in one decay can be absorbed as a neutrino in the other, resulting in a neutrinoless double beta decay (right).

The net result is that no neutrinos are released, but the two electrons carry as much energy as the four particles emitted in a standard double beta decay. While evidence of a neutrinoless double beta decay would identify neutrinos as Majorana fermions, the phenomenon is a rare event; finding it has been compared to looking for a needle in a haystack. Thus Gutierrez's interferometry proposal.

Two-particle intensity interferometry, in which two detectors measure the interference intensities of a beam of particles to reveal a variety of correlation and anticorrelation effects, is a well established research tool for both particle physics and astronomy. Gutierrez's proposed application of this tool to neutrino research is unique.

"The same principles that allow two photons to interfere with one another can be applied to neutrinos to yield information on both the quantum nature and the absolute mass of neutrinos," says Gutierrez. "While two-particle interferometry with neutrinos is not yet practical with today's technology, this approach may spark fresh ideas about future neutrino experiments and stimulate new ways of thinking of old problems."

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