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Do Neutrinos Break the Laws of Physics?

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

Neutrinoless Double Beta Decay

Besides high energy neutrinos, other science is being done on standard variations of neutrinos that often yields surprising results. 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 one 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").

Normal double beta decay on the left and neutrinoless double beta decay on the right.

Normal double beta decay on the left and neutrinoless double beta decay on the right.

Germanium Detector Array (GERDA)

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).

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. Scientists 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).

A Fourth Flavor of Neutrinos?

But that isn't the end of the sterile neutrino story (of course not!). Experiments done in the 1990s and 2000s by LSND and MiniBooNE found some discrepancies in the conversion of muon neutrinos to electron neutrinos. The distance required for the conversion to take place was smaller than anticipated, something that a heavier sterile neutrino could account for. It would be possible for its potential state of existence to cause oscillations between the mass states to be enhanced.

Essentially, instead of the three flavors there would be four, with the sterile causing quick fluctuations making its detection hard to spot. It would lead to the observed behavior of muon neutrinos disappearing faster than anticipated and more electron neutrinos being present at the end of the rig. Further results from IceCube and such may point to this as a legitimate possibility if the findings can be backed up (Louis 50).

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 alternative 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).

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.
  • Chandra. "Mysterious X-ray signal intrigues astronomers." Astronomy.com. Kalmbach Publishing Co., 25 Jun. 2014. Web. 06 Sept. 2018.
  • Cofield, Calla. "Waiting for a Neutrino No-Show." Scientific American Dec. 2013: 22. Print.
  • Ghose, Tia. “Neutrino Study Fails to Show Interaction of Weird Subatomic Particles.” HuffingtonPost. Huffington Post, 18 Jul. 2013. Web. 07 Dec. 2014.
  • Goddard. "Scientist gives 'outlaw' particles less room to hide." Astronomy.com. Kalmbach Publishing Co., 21 Oct. 2015. Web. 04 Sept. 2018.
  • Hirsch, Martin and Heinrich Pas, Werner Parod. "Ghostly Beacons of New Physics." Scientific American Apr. 2013: 43-4. Print.
  • Rzetelny, Xaq. "Neutrinos Traveling Through the Earth's Core Show No Sign of Sterility." arstechnica.com. Conte Nast., 08 Aug. 2016. Web. 26 Oct. 2017.
  • Smith, Belinda. "Search for fourth type of neutrino turns up none." cosmosmagazine.com. Cosmos. Web. 28 Nov. 2018.
  • Timmer, John. "After 43 Years, Gentle Touch of a Neutrino is Finally Observed." arstechnica.com. Conte Nast., 03 Aug. 2017. Web. 28 Nov. 2017.
  • ---. "IceCube Turns The Planet into a Giant Neutrino Detector." arstechnica.com. Kalmbach Publishing Co., 24 Nov. 2017. Web. 19 Dec. 2017.
  • Wenz, John. "Sterile Neutrinos Search Comes Back Lifeless." Astronomy Dec. 2016: 18. Print.
  • Wolchover, Natalie. "Neutrino Experiment Intensifies Effort to Explain Matter-Antimatter Asymmetry." quantamagazine.com. Simons Foundation, 15 Oct. 2013. Web. 23 Jul. 2016.

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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