OPERA Finds a Tau Neutrino

Big news emerged last week from the OPERA experiment at Gran Sasso National Laboratory: researchers there made the first direct observation of one of the rarest events in high-energy physics: a specific kind of neutrino oscillation, in which one type of neutrino changes into another.

It's actually a big deal, building on experimental evidence culled over the last 15-20 years, so a bit of context might be helpful in understanding its significance.

Neutrinos are tiny subatomic particles that travel very near the speed of light. The two most defining features of neutrinos are that they have no charge and, until quite recently, physicists believed they had no mass.

They're extremely difficult to detect, because they very rarely interact with any type of matter, even though they're the most abundant type of particle in the known universe. Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth. Isaac Asimov dubbed them “ghost particles.”

The current Standard Model of particle physics posits three different kinds of neutrinos (electron, muon and tau). The most common are the solar neutrinos that come from our own sun -- specifically, the nuclear processes taking place at its core. When a neutron inside an atom decays, it produces a proton, an electron and a neutrino.

This occurs hundreds of billions of times every second in the core of stars like ours, as hydrogen is converted into heavier elements like helium, releasing huge amounts of energy in the process (i.e., sunshine). Trillions of neutrinos are produced by the sun every day.

But for decades, experiment after experiment showed far fewer solar neutrinos than predicted by theory, and it wasn't until a few years ago that physicists realized those neutrinos weren't really "missing," but were merely in disguise.

Solar neutrinos are sneaky little particles: they can actually change into another kind of neutrino as they shoot through space on their way to Earth -- a phenomenon called “neutrino oscillation.” It's a bit like piano strings, which are tuned to specific notes: let’s say G, E and C (notes that comprise the C Major chord). Scientists previously assumed that if a neutrino was born as a G, it would always be a G. But neutrinos can “de-tune” over time, just like the strings on a piano. So a G can gradually become a E, or a C.

Here's why neutrino oscillations mean that neutrinos must have a tiny bit of mass. Like most elementary particles, neutrinos also have a wavelike nature, and waves oscillate back and forth. Add two waves together and you get a new composite wave.

And when two very similar notes are played together, there's an interference effect that causes the sound to wobble between loud and soft, producing "beats."

Similarly, oscillating neutrinos are comprised of three different waves that combine in different ways as they travel through space. The "beats" are caused by small physical differences in mass that lead to those telltale interference effects.

Particle physicists have been studying this phenomenon for the last 15 years or so, but while several experiments clearly shown evidence for neutrino oscillations as they travel long distances through space, all of those were looking at only certain types of neutrinos. Could, say, a tau neutrino emerge from a beam of muon neutrinos through the oscillation process? It took four years but we now have an answer: yes!

The folks at OPERA got their neutrinos from CERN. Basically, a bunch of protons smash into a fixed target, thereby creating a horde of scattered mesons, which last for fractions of a second before decaying into a bunch of neutrinos (of the muon variety).

OPERA's detector, about 732 meters away, picks up the telltale underground "signatures" of these decay patterns and records them. The subsequent analysis is all-important, because it's easy to confuse the signatures of the events of interest with background noise or an entirely different kind of event altogether.

And lo and behold, after much patient and careful sifting through the data, the telltale signature of a tau neutrino popped up, out of the billions upon billions of muon neutrinos generated at CERN. It's not much, granted, but it's so rare that it's highly significant.

As more of such events are detected, it'll give us that much better understanding of neutrino oscillations, and bolster the evidence that neutrinos really do have mass.