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What Are the Different Flavors of Neutrinos?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

The Discovery

The Standard Model theory predicts that neutrinos are massless, and yet scientists know that three different types of neutrinos exist: the electron, the muon, and the tau neutrinos. Therefore, because of the changing nature of these particles, we know it cannot be massless, and therefore must travel slower than the speed of light. But I am getting a head of myself.

The muon neutrino was discovered in 1961 during the Two Neutrino Experiment at the Alternating Gradient Synchrotron in Brooklyn, New York. Jack Steinberger, Melvin Schwartz, and Leon Lederman (all Columbia University professors) wanted to look at the weak nuclear force, which happens to be the only one impacting neutrinos. The goal was to see if neutrino production was possible, for up to then, you detected them via natural processes like nuclear fusion from the sun.

To accomplish their goal, protons at 156 GeV were fired into beryllium metal. This mostly created pions, which can then decay into muons and neutrinos, all at high energies because of the collision. All the daughters move in the same direction as the impacting proton, making their detection easy. To get just the neutrinos, a 40-foot collects all the non-neutrinos and allows our ghosts to pass through. A spark chamber then records the neutrinos that happen to hit. To get a feel for how little this happens, the experiment went for 8 months and a total of 56 hits were recorded.

The expectation was that as radioactive decay occurs, neutrinos and electrons are made, and neutrinos should therefore help to make electrons. But with this experiment, the results were neutrinos and muons, so shouldn’t the same logic apply? And if so, are they the same type of neutrino? Couldn’t be, because no electrons were seen. Hence, the new type was uncovered (Lederman 97-8, Louis 49).

Detecting neutrinos.

Detecting neutrinos.

Changing Neutrinos

The variety of flavors alone was puzzling, but what was even stranger was when scientists found out that the neutrinos could change from one to the other. This was discovered in 1998 at Japan’s Super-Kamiokande detector, as it observed neutrinos from the sun and the number of each type fluctuating. This change would require an exchange of energy which implies a change of mass, something that runs counter to the Standard Model. But wait, it gets weirder.

Because of quantum mechanics, no neutrino is actually any one of those states at once, but a mix of all three with one being dominant over the other. Scientists are not currently sure as to the mass of each the states, but it's either two small and one large or two large and one small (large and small being relative to each other, of course). Each of the three states is different in its mass value and, depending on the distance travelled, the wave probabilities for each state fluctuate. Depending on when and where the neutrino is detected, those states will be in different ratios and, depending on that combination, you get one of the flavors we know of. But don’t blink because it can change in a heartbeat or on a quantum breeze.

Moments like this make scientists cringe and smile all at once. They love mysteries, but they don’t like contradictions, so they began to investigate the process under which this occurs. And ironically, antineutrinos (which may or may not essentially be neutrinos, pending on the aforementioned work with germanium-76) are helping scientists learn more about this mysterious process (Boyle, Moskowitz “Neutrino,” Louis 49).

At the China Guangdong Nuclear Power Group, they put out a big number of electron antineutrinos. How big? Try one followed by 18 zeros. Yeah, it’s a big number. Like normal neutrinos, the antineutrinos are hard to detect. But by making such a large amount, it helps scientists increase the odds in their favor of getting good measurements. The Daya Bay Reactor Neutrino Experiment, a total of six sensors distributed at different distances from Guangdong, will count the antineutrinos that pass by them. If one of them has disappeared, then it is likely a result of a flavor change. With more and more data, the probability of the particular flavor it is becoming can be determined, known as the mixing angle.

Another interesting measurement being done is how far apart the masses of each of the flavors are from one another. Why interesting? We still do not know the masses of the objects themselves, so having a spread on them will help scientists narrow down the possible values of the masses by knowing how reasonable their answers are. Are two significantly lighter than the other, or just one? (Moskowitz “Neutrino,” Moskowitz 35).

Do neutrinos change consistently between the flavors regardless of charge? Charge-parity (CP) says yes they should, because physics shouldn't favor one charge over another. But evidence is mounting that this may not be the case.

At J-PARC, the T2K experiment streams neutrinos along 295 kilometers to the Super-K and found that in 2017 their neutrino data showed more electron neutrinos than there should have been and less anti-electron neutrinos than expected, something that further hints at a possible model for the aforementioned neutrinoless double beta decay being a reality (Moskvitch, Wolchover "Neutrinos").

Deep Underground Neutrino Experiment (DUNE)

One experiment that will help with these flavor mysteries is the Deep Underground Neutrino Experiment (DUNE), a huge feat starting at Fermilab in Batavia, Illinois and ending at the Sanford Underground Research Facility in South Dakota for a total of 1,300 kilometers.

That is important, because the largest experiment before this was only 800 kilometers. That extra distance should give scientists more data on the oscillations of the flavors by allowing comparisons of the different flavors and seeing how they are similar or different to the other detectors. That extra distance through Earth should encourage more particle hits, and the 17,000 metric tons of liquid oxygen at Sanford will record the Chernokov radiation from any hits (Moskowitz 34-7).

Works Cited

  • Boyle, Rebecca. “Forget the Higgs, Neutrinos May Be the Key to Breaking the Standard Model” ars technician. Conde Nast., 30 Apr. 2014. Web. 08 Dec. 2014.
  • Lederman, Leon M. and David N. Schramm. From Quarks to the Cosmos. W.H. Freeman and Company, New York. 1989. Print. 97-8.
  • Louis, William Charles and Richard G. Van de Water. “The Darkest Particles.” Scientific American. Jul. 2020. Print. 49-50.
  • Moskovitch, Katia. “Neutrino Experiment in China Shows Strange Particles Changing Flavors.” HuffingtonPost. Huffington Post, 24 Jun. 2013. Web. 08 Dec. 2014.
  • ---. "The Neutrino Puzzle." Scientific American Oct. 2017. Print. 34-9.
  • Moskvitch, Katia. "Neutrinos Suggest Solution to Mystery of Universe's Existence." Quantuamagazine.org. Quanta 12 Dec. 2017. Web. 14 Mar. 2018.
  • Wolchover, Natalie. "Neutrinos Hint of Matter-Antimatter Rift." quantamagazine.com. Quanta, 28 Jul. 2016. Web. 27 Sept. 2018.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley

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