What Is a Neutrino and All Its Flavors?
Punch the wall.
Yeah, I started this article with that recommendation. Go ahead (gingerly, of course)! When your fist hits the surface, it stops unless you have enough force to penetrate it. Now imagine you punching the wall, and your fist goes right through it without breaking the surface. Weird, right? Well, it would be even weirder if you fired a bullet into a stone wall and it too went through it without actually piercing the surface. Surely this all sounds like science fiction, but tiny nearly massless particles called neutrino’s do just that with everyday matter. In fact, if you had a light-year of solid lead (a very dense or particle-heavy material), a neutrino could go through it unscathed, not touching a single particle. So, if they are so hard to interact with, how can we do any science with them? How do we even know they exist?
First, it is important to establish that neutrinos are easier to detect than it would seem. In fact, neutrinos are one of the most common particles in existence, only outnumbered by photons. Over a million pass through the nail of your pinky every second! Because of their high volume, all it takes is the right set-up, and you can start collecting data. But what can they teach us?
One rig, the IceCube Observatory, located near the South Pole, is going to try to help scientists such as Francis Halzen uncover what causes high-energy neutrinos. It uses over 5000 light sensors several kilometers below the surface to (hopefully) record high energy neutrinos colliding with normal matter, which would then emit light. Such a reading was spotted in 2012 when Bert (@1.07 PeV or 1012electron volts) and Ernie (@1.24PeV) were found when they generated 100,000 photons. Most of the other, normal energy neutrinos range ones come from cosmic rays hitting the atmosphere or from the sun’s fusion process. Because those are the only known local sources of neutrinos, anything that is above the energy output of that range of neutrinos may not be a neutrino from around here, such as Bert and Ernie (Matson, Halzen 60-1). Yeah, it could be from some unknown source in the sky. But don’t count on it being a by-product of a Klingon’s cloaking device.
In all likelihood, it would be from what is creating cosmic rays, which are difficult to trace back to their source because they interact with magnetic fields. This causes their paths to be altered beyond hopes of restoring their original flight path. But neutrinos, no matter what of the three types you look at, are not affected by such fields and thus if you can record the entry vector one makes in the detector all you have to do is follow that line back, and it should reveal what created it. Yet when this was done, no smoking gun was found (Matson).
As time went on, more and more of these high energy neutrinos were detected with many in the 30-1,141 TeV range. A bigger data set means more conclusions can be reached, and after over 30 such neutrino detections (all originating from the southern hemisphere's sky) scientists were able to determine that at minimum 17 did not come from our galactic plane. Thus, they were created in some far-off location outside the galaxy. Some possible candidates for what is then creating them include quasars, colliding galaxies, supernovas, and neutron star collisions (Moskowitz “IceCube,” Kruesi "Scientists").
Some evidence in favor of this was found on December 4, 2012, when Big Bird, a neutrino that was over two quadrillion eV. Using the Fermi Telescope and the IceCube, scientists were able to find that blazar PKS B1424-418 was the source of it and UHECRs, based on a 95% confidence study (NASA).
Further evidence for black hole involvement came from Chandra, Swift, and NuSTAR when they correlated with IceCube on a high energy neutrino. They backtracked the path and saw an outburst from A*, the supermassive black hole residing in our galaxy. Days later, some more neutrino detections were made after more activity from A*. However, the angular range was too large to definitely say it was our black hole (Chandra "X-ray").
That all changed when 170922A was found by IceCube on September 22, 2017. At 24 TeV, it was a big event (over 300 millions times that of its solar counterparts) and after backtracking the path found that blazar TXS 0506+056 was in it. On top of that it too had recent activity that would correlate to a neutrino, 13 prior neutrinos had come from that direction from 2014 to 2015 and the result was found to be within 3 standard deviations. And this blazar is a bright object, in the top 50 known, showing that its active and likely to be producing much more than we see. Radio waves also showed high activity for the blazar (Timmer "Supermassive," Hampson).
And as a brief sidebar, IceCube is looking for Greisen-Zatsepin-Kuznin (GZK) neutrinos. These special particles arise from cosmic rays that interact with photons from the cosmic microwave background. They are very special because they are at the EeV (or 1018 electron volt) range, way higher than the PeV neutrinos seen. But so far, none have been found, but neutrinos from the Big Bang have been recorded by the Planck spacecraft. They were found after scientists from the University of California observed minute temperature changes in the cosmic microwave background that could have only come from neutrino interactions. And the real kicker is that it proves how neutrinos cannot interact with each other, for the Big Bang theory accurately predicted the deviation scientists saw with the neutrinos (Halzan 63, Hal).
Neutrinoless Double Beta Decay
Besides high energy neutrinos, other science is being done on standard variations of neutrinos. Specifically, scientists were hoping to witness a key feature of the Standard Model of Particle Physics in which neutrinos were their own antimatter counterpart. Nothing prevents it because they would both still have the same electrical charge. If so, then if they were to interact they would destroy each other. This idea of neutrino behavior was found in 1937 by Ettore Majorana. In his work, he was able to show that a neutrinoless double beta decay, which is an incredibly rare event, would happen if the theory was true. In this situation, two neutrons would decay into two protons and two electrons, with the two neutrinos that would normally be created would instead destroy each other because of that matter/antimatter relation. Scientists would notice that a higher level of energy would be present and that neutrinos would be missing. If neutrinoless double beta decay is real, it potentially shows that the Higgs boson may not be the source of all mass and can even explain the matter/antimatter imbalance of the Universe, hence opening the doors to new physics (Ghose, Cofield, Hirsch 45, Wolchover "Neutrino").
How is that possible? Well, it all stems from the theory of leptogenesis or the idea that heavy versions of neutrinos from the early universe didn't break down symmetrically like we would have expected them to. Leptons (electrons, muons, and tau particles) and antileptons would have been produced, with the latter more prominent than the former. But by a quirk in the Standard Model, antileptons lead to another decay - where baryons (protons and neutrons) would be 1 billion times more common than antibaryons. And thus, the imbalance is resolved so long as these heavy neutrinos existed, which could only be true if neutrinos and antineutrinos are one in the same (Wolchover "Neutrino").
So how would one even start to show such a rare event as neutrinoless double beta decay is even possible? We need isotopes of standard elements because they usually undergo decay as time progresses. And what would be the isotope of choice? Manfred Linder, the director of the Max Planck Institute for Nuclear Physics in Germany and his team, decided on germanium-76 which barely decays (into selenium-76) and thus requires a large amount of it to increase the chances of even potentially witnessing a rare event (Boyle, Ghose, Wolchover "Neutrino").
Because of this low rate, scientists would need the ability to remove background cosmic rays and other random particles from producing a false reading. To do this, scientists put the 21 kilograms of the germanium almost a mile below the ground in Italy as a part of the Germanium Detector Array (GERDA) and surrounded it with liquid argon in a water tank. Most sources of radiation cannot go this deep because the dense material of the Earth absorbs most of it by that depth. Random noise from the cosmos would result in about three hits a year, so scientists are looking for something like 8+ a year to have a finding. Scientists kept it down there, and after a year no signs of the rare decay had been found. Of course, it is so unlikely an event that several more years will be needed before anything definitive can be said about it. How many years? Well, maybe at least 30 trillion trillion years if it is even a real phenomenon, but who is in a rush? So stay tuned viewers (Ghose, Cofield, Wolchover "Neutrino," Dooley).
The Three Flavors
Of course, it would be too easy if this was the only challenge neutrinos presented to the Standard Model. That 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. That alone was puzzling but 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. 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 germainium-76) are helping scientists learn more about this mysterious process (Boyle, Moskowitz “Neutrino”).
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").
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).
Left-Handed vs. Right Handed
Another component of neutrinos that may bring light to their behavior is how they relate to electrical charge. If some neutrinos happen to be right handed (responding to gravity but not to the other three forces) otherwise known as sterile, then the oscillations between flavors as well as the matter-antimatter imbalance would be resolved as they interact with matter. This means that sterile neutrinos only interact via gravity, much like dark matter. Unfortunately, all evidence points to neutrinos being left-handed based on their reactions to the weak nuclear force. This arises from their small masses interacting with the Higgs field, but before we knew that neutrinos had mass, it was possible for their massless sterile counterparts to exist and thus resolve those aforementioned physics difficulties. The best theories to resolve this included the Grand Unified Theory, SUSY, or quantum mechanics, all of which would show that a mass transference is possible between the handed states. But evidence from 2 years of observations from IceCube published in the August 8, 2016 edition of Physical Review Letters showed that no sterile neutrinos had been found. Scientist are 99% confident in their findings, implying that sterile neutrinos may be fictitious. But other evidence keeps the hope alive. Readings from Chandra and XMM-Newton of 73 galaxy clusters showed X-ray emission readings that would be consistent with the decay of sterile neutrinos, but uncertainties related to the sensitivity of the telescopes makes the results uncertain (Hirsch 43-4, Wenz, Rzetelny, Chandra "Mysterious," Smith).
Weird Before, Crazy Now
So remember when I mentioned that neutrinos don’t interact very well with matter? While true, it does not mean that they don’t interact. In fact, depending on what the neutrino is passing through it can have an impact on the flavor it is at a moment. In March of 2014, Japanese researchers found that muon and tau neutrinos, which are the result of electron neutrinos from the Sun changing flavors, could become electron neutrinos once they have passed through the Earth. According to Mark Messier, a professor at Indiana University, this could be a result of an interaction with Earth’s electrons. The W boson, one of the many particles from the Standard Model, exchanges with the electron, causing the neutrino to revert to an electron flavor. This could have implications for the debate of the antineutrino and its relation to the neutrino. Scientists wonder if similar mechanism will work on antineutrinos. Either way, it is another way to help resolve the dilemma they currently pose (Boyle).
Then in August of 2017 evidence for a neutrino colliding with an atom and exchanging some momentum was announced. In this instance, 14.6 kilograms of cesium iodide was placed in a mercury tank and had photodetectors places around it, waiting for that precious hit. And sure enough, the expected signal was found nine months later. The light emitted was a result of a Z boson being traded to one of the quarks in the nucleus of the atom, causing an energy drop and therefore a photon to be released. Evidence for a hit was now backed by data (Timmer "After").
Further insight into neutrino-matter interactions was found by looking at IceCube data. Neutrinos can take many paths to get to the detector, such as a direct pole-to-pole journey or via a secant line through the Earth. By comparing the trajectories of neutrinos and their energy levels, scientists can gather clues about how the neutrinos interacted with the material inside the Earth. They found that higher energy neutrinos interact more with matter than lower ones do, a result that is in line with the Standard Model. The interaction-energy relation is almost linear, but a slight curve does appear at high energies. Why? Those W and Z bosons in the Earth act on the neutrinos and cause a slight change to the pattern. Maybe this can be used as a tool to map the interior of the Earth! (Timmer "IceCube")
Those high energy neutrinos may also carry a surprising fact: they may be traveling faster than the speed of light. Certain alterative models that could replace relativity predict neutrinos that could exceed this speed limit. Scientists looked for evidence of this via the neutrino energy spectrum that hits Earth. By looking at the spread of neutrinos that have arrived here and taking into account all known mechanisms that would cause neutrinos to lose energy, an expected dip in the higher levels than anticipated would be a sign of the fast neutrinos. They found that if such neutrinos exist, they only exceed the speed of light by only "5 parts in a billion trillion" at most (Goddard).
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