Do Neutrinos Change Flavor at Night?

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A fascinating observation has been tentatively announced by scientists using the Japanese SuperKamiokande neutrino detector. After analyzing 18 years of data it appears that neutrinos generated by fusion in the sun’s core ‘flip’ flavors when detected on the night-side of Earth.

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Neutrinos are the chargeless ‘ghosts’ of the quantum world. They have very little mass and travel near the speed of light. They are so weakly interacting with normal matter that they can blast through our entire planet, from one side to the other, without hitting a thing. The only force they interact with is the weak force.

Although they may seem impossible to detect, physicists have devised a means to snare stealthy neutrinos should they score a direct hit with terrestrial matter.

In the case of the SuperKamiokande detector, a vast cavern under a mountain 300 kilometers (190 miles) from Tokyo has been filled with 50,000 tons of ultra-pure water and thousands of detectors cover the cavern’s walls. Occasionally, should a direct collision between a neutrino and water molecule occur, high-energy electrons or muons are generated. These collision particles generate Cerenkov radiation, causing a brief flash that can be measured. If you have a big enough vat of water, it’s statistically likely that enough neutrino collisions can be spotted to create a kind of “neutrino telescope” (though, technically, it’s more of a particle detector than a telescope).

Although the Universe is swarming with a flood of these neutral particles, in our cosmic neighborhood the sun is the main neutrino generator.

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Neutrinos can come in three different flavors — “electron,” “tau” and “muon” — and, through a quantum quirk, oscillate between these flavors. The nature of this oscillation has been the focus of physics studies for decades.

The fascinating thing about neutrino flavor is that only electron neutrinos are detected by SuperKamiokande. A longstanding mystery has been why far fewer than expected neutrinos are detected from the sun — it turns out that electron neutrinos (that can be detected) oscillate into muon and tau neutrinos (that can’t be detected) on their journey through interplanetary space, explaining the discrepancy.

As noted by Physicsworld.com, about half of the electron neutrinos with energies of less than 2MeV change flavor before reaching Earth. At higher energies, this oscillation rate is greater. There’s a trend; the higher the energy, the less likely the neutrino detection. This strange behavior is known as the Mikheyev–Smirnov–Wolfenstein (MSW) effect, named after the Soviet physicists Stanislav Mikheyev and Alexei Smirnov who, in 1986, built on 1978 work by US theorist Lincoln Wolfenstein. The MSW effect is also theorized to reverse this oscillation.

As muon and tau neutrinos travel through our planet, they can interact with the electrons contained within the dense terrestrial matter. This can cause the neutrinos to flip back to electron neutrinos. And it appears SuperKamiokande has detected observational evidence of this effect in action.

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After analyzing 18 years of data, SuperKamiokande physicists noticed a 3.2 percent increase in neutrino detections at night than during the day. As the detector is facing away from the sun (at night), the neutrinos have to travel through the Earth before hitting the detector. During the day, the solar neutrinos hit the detector after traveling through space (and a few miles of atmosphere). This is a strong hint that after passing through our planet, muon and tau neutrinos are being influenced by the MSW effect, flipping them into electron neutrinos SuperKamiokande can detect.

However, the researchers urge caution. The statistical significance of this finding is not a “discovery” or definitive proof that the MSW effect is fiddling with neutrinos. This result has a statistical significance of 2.7σ, which is interesting, but cannot be considered a discovery. Only when the statistical significance reaches 5 σ can a discovery be announced. It seems we’re going to need a bigger detector for that to happen.

Fortunately, plans are afoot to build a HyperKamiokande that could possibly even use this strange neutrino flavor change to measure the different densities of rock within our planet.

“HyperKamiokande will be 25 times the size of SuperKamiokande, so we will get a much larger data set,” neutrino expert David Wark of the University of Oxford told Physicsworld.com (who was not involved in this study). “Whether it would be big enough to make measurements of the Earth’s density with interesting sensitivity, I am not sure, but we will certainly be looking at that as we further develop HyperKamiokande.”

Source: Physicstoday.com

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