Few physicists have had the privilege of bringing a new elementary particle into the world. When Wolfgang Pauli hit on the idea of the neutrino in 1930, however, internal misgivings tempered his response. “I have done a terrible thing,” Pauli later told his colleagues. “I have postulated a particle that cannot be detected.” The neutrino is indeed elusive—its ghostly nature allows it to slip through almost all physical barriers, including the materials that physicists use in their particle detectors. In fact, most neutrinos pass cleanly through the earth without so much as brushing against another particle. Yet Pauli's fears turned out to be slightly overblown: the neutrino can be detected—although doing so requires great effort and experimental ingenuity.

Neutrinos are the oddest of the fundamental particles on other grounds as well. They do not make up atoms, nor do they have anything to do with chemistry. They are the only electrically neutral matter particles. They are extremely light—less than a millionth the mass of the next-to-lightest matter constituent, the electron. And neutrinos, more than other particles, metamorphose; they shift among three varieties, or “flavors.”

These tiny particles have kept physicists in continuous astonishment for more than 80 years. Even today fundamental questions about the neutrino remain unanswered: Are there only three flavors of neutrino, or do more exist? Why are all neutrinos so lightweight? Are neutrinos their own antimatter counterparts? Why do neutrinos shift character with such amazing verve?

Around the world—at particle colliders, at nuclear reactors, in abandoned mine shafts—new experiments that can address these questions are coming online. The answers they deliver should provide essential clues to the inner workings of nature.

The neutrino's oddities make it a lodestar guiding particle physicists on the daunting voyage toward a so-called grand unified theory describing all particles and forces, except gravity, in a consistent mathematical framework. The Standard Model of particle physics, the best theory of particles and forces to date, cannot accommodate all the complexities of the neutrino. It must be extended.

Lightweight but Pressing

The most popular way to build on the neutrino segment of the Standard Model is to introduce new entities called right-handed neutrinos. Handedness is a variant of electrical charge that determines whether a particle feels the weak interaction, the force responsible for radioactive decay; a particle must be left-handed to feel the weak force. These hypothetical right-handed particles would thus be even slipperier than their left-handed fellows, the experimentally proved neutrinos of the Standard Model. All neutrinos are classified as leptons—the extended family of particles that also includes the electrons—meaning that they do not feel the strong force holding together the protons and neutrons in the atomic nucleus. Lacking electrical charge, neutrinos do not directly feel electromagnetic forces, either. That leaves only the force of gravity and the weak interaction for the three known neutrino flavors, but a right-handed neutrino would be impervious even to the weak force.

If a right-handed neutrino exists, it would provide a very reasonable explanation for another neutrino puzzle: the reason the three left-handed varieties—the electron, muon and tau neutrinos—all have such tiny masses.

Most elementary particles gain their mass by interacting with the ubiquitous Higgs field. (Higgs made headlines in March when physicists at the Large Hadron Collider, or LHC, at CERN near Geneva announced results strongly indicating that a new particle identified in 2012 was indeed the long-sought Higgs boson. That boson is the particle counterpart to the Higgs field, just as the photon is the counterpart to the electromagnetic field.) In the process, the Higgs carries away the particles' weak force version of electrical charge. Because right-handed neutrinos lack this charge, their mass does not rely on the Higgs field. Instead it may emerge from a different mechanism altogether at the extremely high energies of grand unification, which would make the right-handed neutrino enormously heavy.

Quantum effects could link right-handed neutrinos to their left-handed siblings in a way that would cause the enormous mass of one to “infect” the other. The contagiousness would be very weak, though—if the right-handed neutrino came down with pneumonia, the left-handed one would catch only a minor cough—which means the left-handed mass would be very tiny. This relation is known as the seesaw mechanism because a large mass raises, or lifts up, a smaller mass.

An alternative explanation for neutrino masses arises from supersymmetry, a leading candidate for new physics beyond the Standard Model. In the supersymmetry hypothesis, every particle within the Standard Model has an as yet undiscovered partner. The so-called superpartner particles, which must be exceedingly massive to have thus far escaped detection, would instantly at least double the number of elementary particles. If supersymmetric particles exist, the LHC may be able to produce them and measure their properties.

One of the most appealing traits of supersymmetry is that a superparticle known as the neutralino makes a fine candidate for dark matter—the mass in galaxies and galaxy clusters that exerts a gravitational pull but does not emit light nor reveal itself in other obvious ways. The neutralino would fit the bill for dark matter only if it were stable over long periods, rather than decaying away rapidly to some other particle.

A short-lived neutralino would thus send dark matter researchers back to the drawing board but could prove a boon to neutrino physicists. The neutralino's stability depends on a hypothetical property called R-parity, which prevents the superpartners from decaying into any of the ordinary Standard Model particles. If R-parity does not hold, however, the neutralino becomes unstable—and its decay depends in part on the mass of the neutrino.

Two of us (Hirsch and Porod), in collaboration with José Valle of the University of Valencia in Spain and Jorge C. Romão of the Technical University of Lisbon in Portugal, have shown that the link between neutrinos and the neutralino could be testable at the LHC. If the stability of the neutralino indeed depends on neutrinos, the neutralino's lifetime would be predictable from known neutrino properties. And it just so happens that the superparticle should exist long enough for physicists to track its entire lifetime—from production to decay—inside the detectors of the LHC.

What's the Antimatter?

All plausible explanations for the neutrino's meager masses point to unexplored realms of physics. Yet one of those explanations, the seesaw mechanism, may also bear on the mystery of how matter came to reign over antimatter—a triumph that enabled the formation of cosmic structure and, ultimately, the development of life.

Every particle in the Standard Model has an antimatter counterpart, a sort of Bizarro World version with an opposing charge. The electron, for instance, has an electrical charge of −1, and the antielectron, or positron, has a charge of +1. When an electron and positron collide, their charges cancel out, and the particles annihilate in a burst of radiation. The complete chargelessness of the right-handed neutrino may have an important consequence: it could mean that, for neutrinos, matter and antimatter are one and the same. In the terminology of physics, the electron and positron are known as Dirac particles. A particle that is its own antimatter counterpart, on the other hand, is a Majorana particle.

If the seesaw theory accurately reflects the workings of the particle world, then the left-handed neutrinos are infected not just with mass but also with the Majorana-ness of the right-handed neutrinos. In other words, if some neutrinos are their own antiparticles, then all neutrinos are.

Neutrinos and their antiparticles being one and the same would have a variety of fascinating implications. For instance, neutrinos could trigger transitions among particles and antiparticles. In most particle reactions, the so-called lepton number, or the number of leptons minus the number of antileptons, is conserved—it does not change. Neutrinos, however, might violate this rule, creating an imbalance of matter and antimatter. For us humans, the imbalance is a very good thing because if matter and antimatter were equally paired in the aftermath of the big bang, they would have completely annihilated each other and left nothing behind to build galaxies, planets and life-forms. The explanation for matter's dominance over antimatter has long eluded physicists and cosmologists.

Disappearing Act

The connection between neutrinos and their antiparticles does not have to languish in the realm of tantalizing but ultimately unsettled theory. Many experiments, past and present, have sought to answer definitively whether neutrinos are in fact their own antiparticles by searching for a type of radioactive event known as nuclear double beta decay.

Neutrinos and antineutrinos were first observed in nuclear beta decay, by which an atom emits an electron, along with an antineutrino. In several nuclear isotopes, two beta decays can occur simultaneously, which, under normal circumstances, emits two electrons and two antineutrinos. Yet if the neutrino is a Majorana particle, then the same antineutrino emitted in the first decay can be absorbed in the second. The result is a double beta decay that does not emit any neutrinos or antineutrinos [see box on opposite page]. In an instant, where there had previously been no leptons, two leptons (the electrons) emerge without their usual, counterbalancing antileptons (the antineutrinos). In other words, this so-called neutrinoless double beta decay violates the conservation of the lepton number.

At present, the search for neutrinoless double beta decay is the best test we have for Majorana neutrinos in particular and for lepton number violation in general. In principle, a neutrinoless double beta decay experiment is simple: collect a nuclear isotope such as germanium 76, in which simultaneous beta decays can occur, and wait for the emergence of two electrons unaccompanied by neutrinos. In practice, however, the experiments are very difficult. Double beta decay of any kind is exceedingly rare, so experimenters must gather large quantities of germanium, or other source materials, to have a hope of documenting the neutrinoless variety. To make matters worse, the constant stream of subatomic particles raining down on the earth from cosmic rays tends to drown out the minuscule signal from double beta decays. So experimentalists must bury their detectors deep underground or in former mines or other subterranean labs, where the overlying rock screens out nearly all cosmic rays.

Unfortunately, the only report to date of neutrinoless double beta decay, from the Heidelberg-Moscow Double Beta Decay Experiment in Italy, has been vigorously contested by other physicists. Next-generation detectors just starting to take data or currently under construction will conduct a more thorough search. An experiment in New Mexico called EXO-200 and one in Japan named KamLAND-Zen recently published the first data from their searches for neutrinoless double beta decay, which caused friction with the earlier claim but did not unambiguously rule it out.

The GERDA experiment in Italy, which came online in 2011, uses the same isotope as the Heidelberg-Moscow setup in an improved design that aims to directly confront its predecessor's controversial finding. Both the EXO-200 and KamLAND-Zen experiments are continuing their operations, and an apparatus known as CUORE is scheduled to start taking data in Italy in 2014. The number of advanced experiments now under way provides a very reasonable hope that neutrinoless double beta decay may be confirmed before the end of this decade.

Light Switchers

Finding an as yet undiscovered neutrino or proving that neutrinos and antineutrinos are one and the same would add an entirely new layer of intrigue to these already puzzling particles. But even as we physicists hunt for new facets of these particles, we continue to wrestle with the mechanism underlying a well-documented but poorly understood attribute of neutrinos—their strong propensity to metamorphose. In the literature, we say that the amount of lepton flavor violation, or neutrino mixing, is large in comparison with the mixing among flavors of quarks, the elementary particles that make up protons and neutrons.

Many research groups worldwide are investigating how newly conceived symmetries of nature—key commonalities between apparently distinct forces and particles—could explain such behavior. One example would be the symmetries inherent in the ways that the known particles transform from one to another. Gautam Bhattacharyya of the Saha Institute of Nuclear Physics in Calcutta, Philipp Leser of the Technical University of Dortmund in Germany and one of us (Päs) recently discovered that such symmetries would conspicuously affect the Higgs field. The interaction of flavor-swapping quarks and neutrinos with the Higgs field would manifest itself in exotic decay products of Higgs bosons that ought to be observable at the LHC. Such a signal could point to the underlying mechanism for neutrinos' hyperactive transmutations, which would certainly be one of the most spectacular discoveries at the LHC.

In the meantime, a different family of experiments is nailing down just how often the particles switch identities. Long-baseline experiments such as T2K in Japan, MINOS in Minnesota and OPERA in Italy detect beams of neutrinos that originate at particle accelerators hundreds of kilometers away, to measure changes in flavor as neutrinos traverse long distances through the earth [see box on page 23]. The scales of these experiments are so large that the neutrinos may cross state lines or even international borders on their journeys. (In 2011 OPERA made news when physicists from the collaboration announced that neutrinos in their experiment appeared to travel from CERN to an underground Italian lab faster than the speed of light—a measurement that soon proved to be flawed.) In complement to these long-distance neutrino experiments, the Double Chooz project in France, the Daya Bay Reactor Neutrino Experiment in China and RENO in South Korea all measure the short-range oscillation of neutrinos coming from nuclear reactors.

Only in 2012 did these experiments finally determine the last and smallest of the so-called mixing angles—the parameters governing the transitions among neutrino flavors. The final mixing angle to be pinned down, known as the reactor angle, describes the probability of an electron neutrino or antineutrino's conversion over a short baseline. The measurements of the reactor angle opened the possibility that future neutrino experiments might be able to compare the properties of neutrinos and antineutrinos. An asymmetry between particles and their antimatter counterparts would be known as CP violation and, along with studies of neutrinoless double beta decay, could bear on the mystery of why there is more matter than antimatter in our universe.

Of the ongoing searches, T2K probably has the first decent chance to see hints of CP violation. But the race is on among this new generation of experiments to answer key neutrino questions—and it promises to be exciting. The long-baseline NOvA experiment, now under construction in the U.S., also has the potential to uncover CP violation in neutrinos. NOvA will fire a neutrino beam through the earth from Fermi National Accelerator Laboratory in Batavia, Ill., clear across the state of Wisconsin and the tip of Lake Superior, to a detector in Ash River, Minn., 810 kilometers away. The neutrinos will make the trip in less than three milliseconds.

Among its research goals, NOvA also aims to clarify the neutrino mass hierarchy—determining which of the neutrinos is the lightest and which is the heaviest. At present, physicists know only that at least two neutrino species have nonzero masses, but, as with so many aspects of these ghostly particles, the details elude us.

Lingering Mysteries

With so many neutrino experiments under way—featuring different aims, different designs and different particle sources—the varied data emerging from around the globe have sometimes yielded conflicting interpretations. One of the most tantalizing—as well as controversial—experimental hints suggests the existence of a new particle called the sterile neutrino.

Echoing Pauli's fears in 1930, the sterile neutrino would be only indirectly detectable, just like the much heavier right-handed neutrino of the seesaw mechanism. (From a theoretical point of view, however, the two proposed particles are nearly mutually exclusive.) Nevertheless, two experiments may have caught a whiff of the sterile neutrino. LSND, which ran at Los Alamos National Laboratory in the 1990s, found early but controversial evidence for an elusive type of neutrino flavor conversion—muon antineutrinos morphing into electron antineutrinos. Fermilab's MiniBooNE, which began producing scientific results in 2007, also hinted at such conversions. Yet the LSND and MiniBooNE oscillations did not fit neatly into the standard three-neutrino picture.

Quantum mechanics permits neutrinos to oscillate between flavors only if they have mass—and only if each flavor has a different mass. The various neutrino masses could trigger neutrino conversion to explain the LSND and MiniBooNE anomalies but only if another mass difference exists in addition to the ones already known—in other words, only if four neutrino types exist instead of three. An additional neutrino coupling to the weak force would make the Z boson—a carrier of the weak force—decay too fast, so this particle would not interact with the weak force at all. Hence the “sterile” designation: this hypothetical neutrino would be almost entirely decoupled from the rest of the particle zoo.

Detectors of a different kind altogether, which capture neutrinos from nearby nuclear reactors, have also registered surprising results that could point to a sterile neutrino. The data from several reactor experiments indicate an anomalous disappearance of electron antineutrinos over very short distances, which, if interpreted in terms of neutrino oscillations, would imply the existence of sterile neutrinos. The anomaly has been around for some time, but recent recalculations of the neutrino output from the various reactors have strengthened the case for a new particle.

The evidence for sterile neutrinos, such as it is, remains sketchy, indirect and conflicted—all of which is to be expected in pursuit of a notoriously elusive, and possibly nonexistent, particle. Yet MiniBooNE and a companion experiment called MicroBooNE, which is now under construction at Fermilab, may soon have something firmer to say on the matter. And a new crop of proposed experiments, which would study the reactor anomaly, is also under discussion.

It is remarkable that the mighty LHC and the comparatively low-energy experiments on the humble neutrino provide such complementary routes to explore the inner workings of nature. More than 80 years after Wolfgang Pauli conceived of his “particle that cannot be detected,” neutrinos continue to guard their secrets closely. Still, the potential payoff in unraveling those secrets justifies the decades-long effort to pry ever further into the neutrino's private life.