Neutrino 'Costume Change' Mystery Solved?


Another piece of the neutrino puzzle has fallen into place, thanks to new results announced last week by the Daya Bay collaboration in China.

The experiment has only been up and running for a couple of months, but the international collaboration’s latest measurement might explain how neutrinos change “flavors” — akin to a costume change — as they move through space.

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Neutrinos are subatomic particles that travel very near the speed of light. 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. Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth.

There are three different kinds of neutrinos (electron, muon and tau). In recent years, physicists have discovered that neutrinos have another bizarre feature: they can change into another kind of neutrino as they travel. This is called “neutrino oscillation.” Why does this happen?

It has to do with the wavelike nature of neutrinos. Waves oscillate back and forth. Add two waves together and you get a new composite wave. For instance, when two very similar musical notes are played together, there’s an interference effect that causes the sound to wobble between loud and soft, producing “beats.”

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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 tell-tale interference effects.

So each flavor of neutrino is a mix of three different masses that constantly fluctuate over time. There’s no way of knowing which flavor it is (or has become) until it’s remeasured at its final destination — in this case, the detectors at Daya Bay.

The Daya Bay experiment gets its (electron) antineutrinos from nuclear power reactors owned by the China Guangdong Nuclear Power Group. The neutrinos then travel 1.7 kilometers to six massive underground detectors filled with liquid laced with gadolinium, hitting another group of detectors midway at about the 500-meter mark.

When electron antineutrinos collide with the atoms in the liquid, the result is a pretty blue glow. This glow is then picked up by the photomultiplier tubes lining the detectors and recorded for posterity.

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Okay, but how can they tell if any of the neutrinos changed flavors en route to the detectors? The number of antineutrinos measured at Daya Bay should be slightly less than the number produced by the nuclear power plants, picked up by the midrange detectors.

And that’s what the Daya Bay collaboration found: some of the original electron antineutrinos seem to disappear en route to the final detector site. From that data, physicists were able to deduce the value of something called the “mixing angle.” It’s the last of three critical numbers needed to plug into the equations used to describe neutrino oscillations.

The next step is to bring the remaining two detectors online in order to collect more data and further refine these preliminary results. That’s right, they got these significant results even before the experiment was fully online.

Those refined results will likely help physicists design future neutrino experiments, thereby gleaning even more insights into what Isaac Asimov dubbed “ghost particles.”

We still don’t know the specific masses of the neutrinos — all are likely to have masses significantly less than one-millionth the mass of an electron — or even if there’s any significant difference between neutrino and antineutrino oscillations.

Mostly, however, physicists are hoping that learning more about neutrinos will shed light on one of the current grand challenges in particle physics and cosmology: why we live in a universe dominated by matter. There should have been equal amounts of each in the early universe, colliding and annihilating with reckless abandon, releasing their energy into the nascent cosmos.

But if that were the case, there would no matter, only energy. And that means no planets, galaxies, or other celestial structures. Since we see these things, we know there was a slight asymmetry between matter and antimatter in the early universe — matter had an edge in the cosmic war of attrition.

Neutrinos may be tiny ghostly particles,but they could hold the secret to one the universe’s enduring mysteries.

Images: Roy Kaltschmidt, Lawrence Berkeley National Laboratory.

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