The possibility that neutrinos might travel a smidgen faster than light has been widely publicized. It seems to have already been disproven, but there’s something else that neutrinos do that’s no longer in doubt – and almost as interesting.

Neutrinos come in three “flavors“: electron, muon, and tau (denoted by ν_{e}, ν_{μ}, ν_{τ}.) These three types are parallel with and correspond to the three flavors of better-known leptons: electron, muon, and tau. “Flavor” is a facetious term used by particle physicists to refer the fact that leptons (as well as neutrinos and quarks) come in three distinct types with different masses, but no other distinguishing characteristics. And nobody really knows why different flavors exist, or why there are (or seem to be) exactly three “generations” of them.

But what neutrinos do that is no longer disputable, and which neither other leptons nor quarks do, is to spontaneously change their flavor – undergo “oscillation” – on the fly. More precisely, neutrino flavor is a quantum mechanical property, which has no definite value until it’s measured. And the probability of observing one definite flavor or another varies over time. Not only can neutrinos have flavors different from what they had when created, but even the probability of what will be measured fluctuates.

Neutrino oscillation is a very interesting phenomenon, but rather technical. See this article for a more detailed presentation.

Neutrinos have mass (but very little), so they are subject to gravitational force. They are also subject to the weak nuclear force. But they have no electrical charge, so they don’t feel the electromagnetic force, and they don’t feel the strong force either.

Because neutrinos are subject to the weak force, they are created or destroyed in events governed by that force. For example, when a free neutron spontaneously decays, by the process of beta decay, n → p + e^{–} + ̅ν_{e}, the result is a proton, an electron, and an electron antineutrino. Nuclear reactions governed by the weak force are how energy is produced in nuclear reactors, so large quantities of neutrinos – of the electron flavor – are produced in such reactors. The Sun’s energy is also generated in this kind of reaction, so it’s also a strong source of electron neutrinos.

In fact, the phenomenon of neutrino oscillation was first observed in attempts to measure the solar neutrino flux. Such measurements always found a significantly smaller flux than expected. This was known as the solar neutrino problem. It’s now understood that the cause of the problem is the fact that some electron neutrinos undergo oscillation so that they are no longer detectable by equipment that’s sensitive only to electron neutrinos. Oscillation has been observed in neutrinos and antineutrinos from nuclear reactors as well as the Sun, and has been a well-established fact for over 10 years.

There’s a reasonably simple (approximate) formula for the probability that an electron neutrino or antineutrino will have oscillated to a different flavor:

P ≈ sin

^{2}(2θ_{13}) sin^{2}(1.267Δm^{2}_{31}L/E)

Here θ_{13} is one of three parameters that describe neutrino oscillation, Δm^{2}_{31} is the difference in squared mass between an electron and a tau neutrino, L is the distance (in meters) the neutrino has traveled from the source, and E is the neutrino energy (in MeV). From the formula, it’s apparent that for a nonzero probability of oscillation (as there clearly is), θ_{13} must be nonzero, and neutrinos must also have mass. Note that the probability varies periodically in time as neutrinos move at near the speed of light, due to the dependence on distance (L).

There are two other parameters like θ_{13} that govern oscillation – namely θ_{12} and θ_{23}. The exact meaning of these parameters is rather technical – they determine elements of a matrix that describes neutrino oscillation. They are called “mixing angles” because they occur as values of trigonometric functions.

Up until now, there have been three other experiments that measured θ_{13}, but their results varied somewhat and were not at a high confidence level. The values obtained (for sin^{2}(2θ_{13}) rather than the angle itself) were 0.11, 0.04, and 0.086. θ_{12} and θ_{23} have been determined much more precisely. The latest research has determined sin^{2}(2θ_{13}) with much more significance – it’s 0.092±0.017 with a statistical significance of 5.2σ (meaning less than 1 chance in several million that 0 is the correct value). So θ_{13} is about 8.8° – and almost certainly nonzero. Earlier measurements indicated this, but a value of zero wasn’t ruled out with high significance. If θ_{13} actually turned out to be 0, the whole theory of neutrino oscillation would need a major overhaul.

The indicated value of sin^{2}(2θ_{13}) means that there’s at most about a 9% chance of having an oscillation. This is reflected in the finding that the observed flux of antineutrinos at the more distant detectors was about 6% less than the flux at the detectors closer to the reactors.

Having precise values for all three mixing angles is important, because it will make possible further neutrino experiments. For example, it is now possible to conduct experiments that can detect differences between oscillation rates of neutrinos and antineutrinos, or whether neutrino and antineutrinos can change into each other. Although such behavior would be a violation of CP symmetry, if a violation can be confirmed and measured, it will help determine the degree of asymmetry between amounts of matter and antimatter created in the big bang. Such an asymmetry is necessary in order for the universe to be left with only ordinary matter after annihilation between corresponding particles of matter and antimatter.

The experiment that measured θ_{13} was conceptually simple, though technically demanding. There are a total of six nuclear reactors at Daya Bay in the Guangdong province of China – 4 in one group and 2 in another. Sophisticated antineutrino detectors were installed close (~500m) to both reactor groups, and also at another location about 1.7 kilometers from the reactors. This last set of detectors was located close to where the probability of oscillation was expected to be near a maximum.

Electron antinneutrinos (̅ν_{e}) are produced in the reactors by ordinary beta decay, and they are detected by an inverse beta decay reaction (the opposite of neutron decay) with protons in the detectors: ̅ν_{e} + p → e^{+} + n. Since neutrinos hardly interact with other matter at all, the probability of this reaction is very low. However, the antineutrino flux from the reactors is large. Comparison with the rate of interactions in the detectors close to the reactors with interactions in the more distant detectors makes it possible to estimate sin^{2}(2θ_{13}), since the other quantities that appear in the probability for oscillation of electron neutrinos and antineutrinos are fairly well established. The lower rate of interactions in the more distant detectors tells the whole story, since antineutrinos with muon and tau flavor will not be detected at all.

One additional parameter known as “phase” needs to be measured in order to understand the possibility of CP symmetry violation better. The Daya Bay experiments may be able to give information on this eventually. Several other experiments that measure neutrino oscillations over much longer distances can also be done now, given the value of θ_{13}. And very different kinds of experiments that measure CP symmetry violation in strong force interactions are also being done. All of these should eventually make it possible to explain the universe’s puzzling matter-antimatter asymmetry.

**Further reading:**

Observation of Electron Antineutrino Disappearance at Daya Bay

Neutrino oscillations measured with record precision

Physicists in China Nail a Key Neutrino Measurement

Neutrino Measurement May Help Solve Mystery of Matter’s Domination over Antimatter

Daya Bay experiment makes key measurement, paves way for future discoveries

Daya Bay nails neutrino oscillation

Elusive neutrino oscillations pinned down

Neutrinos could help explain missing antimatter

Daya Bay observes theta13 at 5 sigma!

The third mixing angle measured! Or, maybe we learn a reason matter exists..

Daya Bay Reactor Neutrino Experiment

Observation of electron-antineutrino disappearance at Daya Bay

**Press releases:**

Announcing the First Results from Daya Bay: Discovery of a New Kind of Neutrino Transformation

Missing: Electron antineutrinos; Reward: Understanding of matter-antimatter imbalance

Announcing the First Results from Daya Bay: Discovery of a New Kind of Neutrino Transformation

Experiment Observes Elusive Neutrino Transformation

New discovery is key to understanding neutrino transformations

A big discovery in the study of neutrinos, tiny particles that have a big role in the universe

Key neutrino behavior observed at Daya Bay