What have neutrinos done for you lately?

Don't call neutrinos elusive. Francis Halzen, who makes it his job to detect them, said that the particles' unusual properties, like their minuscule masses and tendency not to interact with matter, tend to make "elusive" one of the most common adjectives used to describe them. But in reality, they're all around us: each cubic centimeter of the Universe has hundreds of them, left over from the Big Bang. Every second, trillions of them flow through our bodies. In fact, there's probably as much matter in the form of neutrinos as there is visible matter. If we could just get a bit better at detecting them, they could tell us a lot about the Universe.

The World Science Festival hosted a discussion led by three people who have focused their careers on detecting neutrinos. Halzen helps run the new Ice Cube detector at the South Pole, while MIT's Janet Conrad works on the MiniBooNE detector, which picks up neutrinos made by the Fermilab accelerator chain. Conrad was joined by her fellow MITer, Joe Formaggio, who works on the Sudbury Neutrino Observatory, buried in a mine in Canada.

The panel was rounded out by theoretician Lawrence Krauss, and moderated by noted science writer John Rennie.

So why have neutrinos picked up the reputation of being hard to get a handle on, even though they're just about everywhere? Part of it is historical; the particles were predicted decades before anyone figured out how to confirm they actually existed. They are very hard to detect because their primary means of interaction with other matter is through the weak force, which only governs nuclear reactions like fusion and radioactive decay. Of course, that "only" is a rather big deal, since Halzen noted without the reactions that involve neutrinos, we wouldn't produce neutrons, and the Universe wouldn't have much in the way of chemistry.

The association with nuclear reactions and tendency not to interact with matter make them the best way of observing what goes on in the interior of the Sun. Krauss said that it takes photons about a million years to escape from the core of the Sun because there's so much matter to interact with. When we first tried to use neutrinos to probe the Sun, however, there didn't appear to be enough of them being emitted. On its own, the apparent shortage wasn't that surprising—as Krauss said, the average solar neutrino would go through about 10,000 light years of lead without interacting with anything, and we're just lucky that the Sun produces a lot of neutrinos.

But the deficit kept showing up in additional experiments. Eventually, we found out why: neutrinos, through a process called flavor oscillations, sometimes change their identity. The missing neutrinos were there—they just didn't look like we expected them to. And that was a bit of a surprise. "The interference between flavors only works cleanly if there are mass differences," Formaggio said, "which means some of them must have mass." As Krauss described it, "if they didn't have mass but did change flavors, then there would be no way to distinguish them."

A neutrino with mass is the clearest indication that the Standard Model has to be either wrong or incomplete. In fact, a physicist in the audience pointed out that, had we known neutrinos had mass early enough, we might never have accepted the Standard Model in the first place.

Halzen called the identification of neutrinos' mass the biggest discovery in particle physics—while noting that it didn't come from a particle accelerator. In fact, it came from an instrument made to look at something else entirely, the possibility that protons decay. (That study came up empty.)

But we still don't know what that mass is, as the flavor oscillations only tell us what the relative masses of the three types of neutrinos are. Formaggio compared that to having a car dealership tell you, "'I can get you $500 off the car'—you kind of still want to know what the price is." What we do know, in part from information in the radiation left over from the Big Bang, is that it's tiny. While the lightest particle with a known mass, the electron, is a half Mega-electron-volt, the upper limit on the mass of the neutrinos is less than one electron-volt.

Formaggio said that there are a couple of ways we might be able to get a measurement of the neutrino's mass: analysis of their effects on galaxy clusters or an identification of mass that's carried away from particle decays without registering in a detector. "Hopefully, they'll get the same number," he said, at which point both Krauss and Conrad argued that life is more fun when the numbers don't agree.

But mass isn't the only challenge facing neutrino physicists. There have been some hints that neutrinos might be the only fundamental particle that is its own antiparticle (making them Majorana fermions, for the technically inclined). This, Conrad said, would explain the fact that all the neutrinos we see have their spin oriented in one direction, while the antineutrinos have the opposite spin. She suggested that a rare form of radioactive decay that should produce two neutrinos might help us sort this out.

If neutrinos were their own antiparticles, these two should annihilate at an appreciable rate, leaving the decay neutrino-less. But, just days after the session, SLAC's EXO-200 experiment announced that it hadn't seen neutrino-less decays, making this prospect much less likely. (Also this week, Fermilab announced that new data had eliminated the possibility that antineutrino masses were different from those of regular neutrinos.)

What's next for neutrino work? Some cosmological data hints there might be a fourth type of neutrino, one that may not interact via the weak force. The Fermi announcement mentioned above indicated that its Minos detector would now focus on testing whether we can see an indication of these so-called "sterile" neutrinos on Earth.

Another big goal is the detection of the neutrinos that remain from the Big Bang, which were formed as the first atoms in the Universe condensed. They should, much like the cosmic microwave background, preserve information about the state of the early Universe. But the "neutrino background" is very low energy, which makes the neutrinos themselves much harder to detect. Formaggio said that, if large atoms are kept cold enough, it might be possible to detect a neutrino interacting with an entire nucleus, rather than just a proton or neutron. So far, however, no hardware capable of this has been built.

Meanwhile, Conrad has been inspired by people who ask her about the practical impact of her work. "What have neutrinos done for you lately?" she asked, rhetorically. "I get that all the time." Her answer is that they could do useful things because of their association with nuclear decays. So she's working on getting neutrino detectors down in size from something like MiniBooNE (which is 40 feet tall) to something that's portable enough that it could be moved to a site like Fukushima in order to tell us something about what's going on there.