Neutrinos are everywhere in the universe, but we cannot see them or feel them and can almost never stop them. They stream through our bodies by the trillions every second, flitting through the spaces between our atoms with nary a collision. These ghostly particles were created in abundance during the big bang, and stars like the sun pump out more all the time. Yet for all their plentitude, neutrinos may be the most mysterious particles in the cosmos.



For decades physicists thought neutrinos weighed nothing, and they were shocked in 1998 to discover that the particles do have very small, but nonzero, masses. Exactly how much mass they have is still unknown. The larger question, however, is why they have mass at all.



The unexpected mass of neutrinos represents a deviation from the reigning laws of particle physics, called the Standard Model, so teasing out the reason for this mass could lead the way to a deeper and fuller explanation of the particles that make up our world. “It’s really the first crack in the Standard Model in quite some time,” says Indiana University physicist Mark Messier, “and there’s a lot of interest to try to pull that crack open and see what’s going on there.” Furthermore, neutrinos seem to be at the heart of a larger mystery: the question of why we live in a universe made of matter and not antimatter. The two types of stuff should have been created in roughly equal measures at the beginning of time, but somehow matter won out. Scientists suspect that solving the neutrino mass problem may help reveal why that is.



A new experiment that beams neutrinos underground from Illinois to Minnesota is taking aim at the puzzle. The NuMI Off-Axis Electron Neutrino Appearance, or NOvA, experiment, creates neutrinos by accelerating protons and slamming them into carbon nuclei inside a facility at the Fermi National Accelerator Laboratory near Chicago. These crashes produce a slew of new particles, including some that decay into neutrinos. The neutrinos then travel near light speed 800 kilometers straight through the earth to the Ash River Laboratory in northern Minnesota. Along they way, the particles do something incredible—they change identities.



Neutrino flavors



Neutrinos come in three "flavors," called electron, muon and tau. They are not bound to one, however—they can change flavor on the fly. The experiment at Fermilab was designed to generate only muon-flavored neutrinos. By the time they reach Minnesota, however, some portion of them will have changed into electron and tau neutrinos. The particles pass through two detectors—one at the beginning of their journey and one at the end—to show scientists how many have flipped flavor during the trip. These transformations are affected by the masses of the neutrinos, and NOvA’s data may point the way toward understanding how these masses arise.



[Slide Show: Giant Experiments Seek Out Tiny Neutrinos]



The Ash River detector in Minnesota is designed to identify electron neutrinos. It contains a lattice of thin plastic tubes filled with a liquid material that lights up when excited. The 14,000-ton construction is the largest freestanding plastic structure in the world. “At one point we were going to get in touch with the Guinness Book of Records to get them to certify it, but I don’t know if that actually happened,” says Fermilab physicist Steve Brice. Most neutrinos will fly right though the liquid without incident, just as they usually pass through matter without interacting. Very rarely, however, one will interact with an atom in the liquid, causing the material to release a particle according to the neutrino’s flavor. If an electron is produced, it creates a flash in the liquid that registers as the detection of a single electron neutrino. Despite the multitudes of neutrinos passing through in every moment, such interactions happen only a few times a day. Over time, though, the researchers can build up a reliable estimate of how often the muon neutrinos traveling from Fermilab transform, or oscillate, into electron neutrinos.



Knowing that rate could provide new clues about the neutrinos' mass. Not only do scientists not know their absolute masses , they also do not know how far apart the masses are spaced or which flavor is associated with the heaviest. The fact that neutrinos can oscillate is what tells researchers that they have mass—and that the flavors do not have the same masses. The differing masses cause the various neutrinos, which according to quantum mechanics can be thought of as both particles and waves, to travel with slightly different frequencies. These waves gradually fall in and out of phase with each other, and researchers see the different combinations as changes in the neutrino’s flavor.



Because neutrino oscillations and masses are so intertwined, measurements of these transformations in past experiments have already revealed that there are limited choices for the arrangement of neutrino masses. Essentially, the electron neutrino must be either heavier or lighter than the other two flavors. The latter case is called the normal mass hierarchy, because the neutrino flavors would then parallel the particles they are named for: electron neutrinos would be lighter than muon neutrinos, and muon neutrinos lighter than tau neutrinos, just as electrons themselves are lighter than muons and muons lighter than taus. The other option, called the inverted hierarchy, is that electron neutrinos are the heaviest flavor. According to theoretical predictions, “generally the neutrino oscillation process we’re looking at happens more often when the hierarchy is normal,” says Messier, a co-spokesperson for NOvA, “and when the hierarchy is inverted that process gets suppressed.”



The origin of neutrino mass



Each hierarchy option is tied to different theories explaining why neutrinos have mass. For example, the inverted mass hierarchy would mean the two heavier neutrinos have almost exactly the same mass. “Having particles with exactly the same mass is a big sign that there’s some kind of substructure to them,” says theorist André de Gouvêa at Northwestern University. “One logical possibility is that neutrinos might be made of more fundamental particles. It might be an indication that deep down all the neutrinos are different manifestations of the same object, which may look like different particles at low energies or long distances.”



Neutrinos do not seem to get their masses the way other particles do—through the Higgs boson. The now famous particle, discovered in 2012 at the Large Hadron Collider, is associated with a “Higgs field” that pervades the universe. As particles move through this field, they pick up mass through their interactions with it. The particularities of neutrinos—specifically the fact that they are “left-handed” particles, a designation having to do with the direction of their spin—mean that neutrinos probably cannot interact with the Higgs field. Furthermore, particles that get their masses from the Higgs tend to be heavier. As far as scientists can tell, neutrinos have masses about a million times lighter than other particles of their class, such as electrons. “There must be some new mechanism that’s giving mass to neutrinos,” Messier says. “This factor of a million sort of begs for an explanation.”



The neutrino mass picture is complicated by the strange fact that neutrino flavors do not have absolute, fixed masses. Rather, each flavor state is what’s called a superposition, or overlapping, of mass states—in effect a probability of having certain masses. This juxtaposition of mass states inside neutrinos is actually what gives the particles the ability to change flavors.



At a deeper level,learning the ordering of the neutrino mass states, as well as why they are so small, could reveal something not just about neutrinos, but about the nature of physics. “There are some people out there who feel these are somewhat random numbers,” says Fermilab theorist Stephen Parke. “I don’t believe that. I believe there is some mechanism that we will eventually discover that explains why the mass states have particular mixtures of the electron, muon and tau. That’s the big question, the trip-to-Stockholm-in-December question.”







Antimatter mysteries



Once scientists know the neutrino mass hierarchy, they will be better able to clear up another pressing mystery about the particles: are neutrinos their own antimatter counterparts? All regular matter particles are thought to have antimatter partners with equal mass and opposite charge. Physicists have suggested that some particles—so-called Majorana particles—are both matter and antimatter. If that is true of neutrinos, then when two of them collide they should annihilate each other, as matter and antimatter do on contact.



Determining whether neutrinos are Majorana particles relies on a difficult experiment looking for a possible process called neutrinoless double beta decay, in which two neutrinos that are produced as normal by-products of radioactive decays cancel each other out, resulting in missing neutrinos. If the neutrino mass hierarchy is inverted, the decay process happens faster. So far no one has observed neutrinoless double beta decay, but scientists are still looking. “The sensitivities of these experiments depend on the effective mass of the electron neutrino,” says NOvA co-spokesperson Gary Feldman, a physicist at Harvard University. “In the normal hierarchy their sensitivity is going to be very low. The measurement could be in the range of those experiments if it’s the inverted ordering, though.”



Knowing whether neutrinos are Majorana particles could in turn help scientists solve the mass puzzle. “If the neutrino is Majorana, that would be a really big hint about where its mass comes from,” de Gouvêa says. “In one path the neutrino is Majorana, and in another it’s not. From the theoretical point of view, understanding neutrino masses from these two perspectives is very different. If we could find out which possibility is correct, we would exclude a whole bunch of different theories.”



Ultimately, physicists think neutrinos could resolve an even larger mystery about antimatter—why isn’t the universe made of it rather than matter? To understand why this is puzzling, consider what physicists currently believe happened just after the big bang: copious amounts of the matter and antimatter that had just been created came together and destroyed each other. The small amount of matter left over after all these annihilations is what makes up the galaxies, stars and planets we have today.



To find out how matter got the upper hand, scientists are searching for asymmetries in the ways matter and antimatter behave. They have ruled out the possibility of such asymmetries for most types of particles. “There are only a few places where you can hide that matter-antimatter asymmetry in the Standard Model,” Messier says. “There’s a lot of evidence pointing to neutrinos as the origin of this very big question.” If neutrinos are not Majorana particles, then perhaps neutrinos oscillate to other flavors at different rates than antineutrinos do. These different rates, then, could have caused more neutrinos to survive the era of matter-antimatter annihilation in the early universe.



NOvA has the ability to measure both neutrinos and antineutrinos, so it will look for differences in their oscillation rates. On its own, the experiment is not likely to be sensitive enough to discover an asymmetry. The combined data from NOvA and other neutrino experiments, however, could point in the right direction. NOvA’s main competitor is the Japanese Tokai to Kamioka (T2K) experiment, which has a similar setup. Neutrinos travel just 300 kilometers (compared to NOvA’s 800 kilometers) at T2K, but its main detector is larger and more sensitive. “You can begin to scratch the surface of the [antimatter asymmetry] question when you combine the NOvA data with the data from other experiments around the world,” says Fermilab director Nigel Lockyer.



To really answer the question, he says, an even bigger experiment is probably necessary. Fermilab scientists are already putting together plans for a next-generation project called the Long Baseline Neutrino Experiment (LBNE), set to replace NOvA when its run is over. This facility would beam neutrinos from Fermilab to South Dakota, through 1,290 kilometers of earth. Its detectors would also be placed underground for further shielding from contaminating radiation. Much of the U.S. high-energy physics community has rallied around the plan, and Europe and India say they want to join in. “It requires a grassroots coming together of the world community to say we want to do this experiment,” Lockyer says. “It is a very difficult time in terms of funding, but I’m an optimist. I think we’ve positioned ourselves very well.”

