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Japan's 279 Kilometer T2K Experiment Offers Three Antineutrino Detections

Hunting the ghostly particles that may explain why the universe even exists.

by Michael Byrne
Jul 26 2015, 7:34pm

Image: T2K

Physicists at Japan's T2K neutrino experiment announced this week a series of three antineutrino detections registered at the detection end of its 279 kilometer island-spanning neutrino beam. The observations bring researchers that much closer to the "holy grail of neutrino physics," in the words of Chang Kee Jung, the experiment's spokesperson. And that holy grail also happens to promise an answer to why the universe hasn't been completely annihilated by its own matter-antimatter interactions, e.g. why we're even here to ask such questions.

The most recent phase of the T2K effort began in May of 2014 with the switching on of a long-range muon antineutrino beam spanning from the Super-Kamiokande (Super-K) detector on Japan's west coast to the J-PARC facility on its east coast. (Muons are part of a family of elementary particles including neutrinos, electrons, and taus.) Last month, the beam was switched off, leaving researchers with 10 percent of the project's anticipated total data set. The first phase, which ran from 2010 to 2013, consisted of the beam operating in neutrino (not anti-) mode.

Image: T2K

Neutrino physics is pretty weird. First of all, the ultralight particles come in three different "flavors," which are neutrinos (and antineutrinos) that have different masses and are created in the same reactions that create electrons, taus, and muons (electron neutrinos, tau neutrinos, and muon neutrinos). While they may start off in one of these flavors, neutrinos have the curious ability to oscillate into other varieties of neutrinos, and this is why the particles are conveyed along a 279 kilometer beam—the oscillations occur as the particles travel different distances. So, the idea behind T2K is to observe these oscillations (collect statistics on them) as the neutrinos travel through the Earth to a distant detector. Because neutrinos only interact with the universe via the weak force, the force responsible for radioactive decay, they remain pretty much oblivious to their surroundings, whether they consist of solid rock or a complete vacuum.

What does this have to do with the very existence of the universe? Neutrinos may offer an answer to the lingering mystery of the universe's matter-antimatter imbalance. That is, physics tells us that matter is only created side by side with antimatter. So, if I were to whip up an electron right now, I'd wind up with a positron as well. That's just part of the deal. But if matter and antimatter are produced equally, then all of the matter created in the early universe should have been cancelled out by the equal amounts of antimatter, nuking existence itself instantly.

Put matter and antimatter together and, poof, they destroy each other. And yet, somehow that didn't happen. There is some asymmetry between the two opposing matters—know properly as CP violation—and by observing and comparing the oscillations of neutrinos and antineutrinos as they cruise along deep beneath the surface of Japan, it's hoped that some difference in these oscillations will become apparent. If, say, antineutrinos oscillated with more or less frequency than neutrinos, we would have some indication of a fundamental difference.

The first phase of the T2K experiment showed pretty conclusively that oscillations were in fact occurring. At the Super-K detector, 28 different events were registered—light-emitting interactions involving electrons and neutrinos—while only five interactions should have occurred in the same time-frame if the neutrinos were not oscillating as expected.

"Many of the interactions of muon neutrinos produce muons, while interactions of electron neutrinos often produce electrons," the T2K project page explains. "Muons and electrons are charged particles, and they displace electrons in the water as they pass. As the water electrons return to their equilibrium positions after the passage of the charged particle, they emit light. If the passing charged particle is travelling faster than the velocity of light in water (which is three-quarters of its velocity in a vacuum), this light is emitted as a cone known as Cerenkov radiation. The walls of Super K are lined with more than 10,000 sensitive photo-multipliers, which detect the cone of Cerenkov light as a ring. Super K can distinguish muons (which produce a sharp ring) from electrons (which produce a more diffuse ring)."

The three detections announced this week are really more just a taste of what's to come. The beam will fire back up this fall and the T2K physicists will keep on gathering data. That said, we'll likely have to wait until the construction of the more powerful 800 mile long DUNE project for more concrete answers, but a proposed upgrade to the Super-K detector may also go (literally) a long way.