T2K neutrino detector

Inside the Super-Kamiokande Detector, receiving end of the T2K experiment in Japan. Physicists with T2K announced that they had caught neutrinos switching flavors, a finding that could help them pin down why the universe is made of matter. (T2K Experiment (http://t2k-experiment.org) / July 23, 2013)

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Using an underground neutrino detector in Japan, physicists may have found the key to answering a fundamental question: Why does our universe, as we know it, exist at all?

Scientists have caught tiny, ghostlike particles called neutrinos in the act of turning from one type into another using the Tokai-to-Kamioka experiment. The ground-breaking findings may help physicists understand a strange asymmetry in the early cosmos that allowed matter to beat out antimatter shortly after the big bang.

"Now I see the path, I see the journey to get to this Holy Grail," said Chang Kee Jung, an experimental particle physicist at the State University of New York at Stony Brook and one of the lead scientists for the international collaboration.

The T2K experiment, which shoots high-energy pulses of particles from an accelerator on Japan's east coast to a detector roughly 185 miles away on the west coast, reported at the European Physical Society meeting in Stockholm that it had caught neutrinos changing from one flavor to another mid-flight.

This shape-shifting ability is a weird quirk of neutrinos. There are three flavors, or types — electron neutrino, muon neutrino and tau neutrino — and somehow, they seem to switch from one flavor to another, even flipping their masses. Imagine if your ice cream spontaneously changed flavors, or added an extra scoop.

"So you have vanilla ice cream, you throw it into space — some time later, it turns into chocolate ice cream, or strawberry ice cream," Jung said. "This is a very, very weird phenomenon."

This feat, known as oscillation, is rooted in the mind-boggling realm of quantum physics, which works on exceedingly tiny scales. (Ice cream is too big to actually work, but the neutrino fits the bill: It's millions of times lighter than an electron, which is itself less than a thousandth the mass of the proton.)

Scientists have been trying to catch neutrinos in the act of flipping flavors for years, but it’s tricky business. Usually they can really only say for sure that, whatever flavor of neutrino they produce, some of them vanish en route. To extend the ice cream metaphor:

"You throw ten vanilla ice creams and five of them disappear," Jung said. "You can deduce that five of them [became] chocolate or strawberry, but you have no ways of detecting them."

How it works

To catch these chameleon particles in the act of shape-shifting, an experiment would have to detect what happens to neutrinos at a particular energy level. But that can be extremely difficult. Particle accelerators work by shooting 100 trillion protons at a graphite target, which results in a shower of pions, which decay into muons and muon neutrinos. A second graphite layer blocks the muons, allowing only the muon neutrinos to filter through to the detector. These pulses of protons, produced about once every 2.5 seconds during testing, generate "kazillions" of muon neutrinos, Jung said.

Why make so many? That’s because neutrinos hardly interact with matter at all. Roughly 50 billion neutrinos pass through your finger every second; imagine how many whoosh right through the Earth. The more neutrinos hurled at the detector, the higher the chances that a precious few will stick.

But the neutrinos caught by the detector span a wide range of energies, even though the T2K scientists only needed neutrinos at a lower energy band. And all that these extra, energetic neutrinos do is muddle up the readings, like white noise covering up a low note.

The researchers got around this with an ingenious solution: Rather than hit the target dead-on, they offset it by 2.5 degrees. This meant that, rather than capture all the meddlesome high-energy neutrinos at the center of the beam, they caught the lower-energy ones off to the side. These neutrinos are the ones that are most likely to flip — to go from vanilla to chocolate or strawberry.

Lo and behold, out of about 400 to 500 neutrino hits, the researchers picked up 28 electron neutrinos — even though they'd only made muon neutrinos. The researchers had expected just 4.6 electron neutrinos to show up from background contamination. This meant the extra 23 or so neutrinos had flipped from the "muon" flavor to the "electron" flavor en route. Here were the tell-tale shape-shifters.

Why it matters

In the beginning, just after the big bang, the universe was filled with both matter and anti-matter. For every proton, there was an anti-proton. For every muon neutrino, there was an anti-muon neutrino. For every quark, an anti-quark. And when a particle meets its nemesis, they annihilate.

Presumably, if the big bang generated equal amounts of matter and anti-matter and they kept colliding, then it should have generally canceled out by now.

And yet, matter clearly won the day. We're here, and we're made of the stuff. And antimatter is relatively hard to come by in the cosmos, unless you’re making it in a high-energy physics lab.

So researchers believe there must have been some sort of asymmetry at play, one that clearly favored matter. If neutrinos behaved differently from their antimatter counterparts — particularly the jumbo-sized neutrinos thought to have existed in the universe's early days — then perhaps they could have tipped the scales in the matter/anti-matter battle.

But to figure out if neutrinos were the key to matter's triumph, scientists need to know how neutrinos and anti-neutrinos shape-shift, and then compare the two patterns. T2K's new measurement of roughly two dozen flavor-flipping neutrinos gives them a baseline — a key — to work with.

The next step is to shoot a beam of anti-neutrinos and find out how many of these antiparticles flip flavors, too. If they don’t mirror the neutrinos' behavior, then the physicists may have nailed their culprit — a source of the asymmetry that allowed matter to dominate the universe. 

Finding this asymmetry — known as a charge-parity violation — would be a scientific Mt. Everest, said Jung, who used to be a mountaineer. This current discovery of flavor-flipping neutrinos may not be Mt. Everest yet, he said — but it’s definitely a K2 (the second-highest peak on Earth).