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- This Review is about he mysterious Neutrinos. They are in the same family as electrons except they carry no charge. They are neutral particles. Like the electron they have 3 levels of energy, or mass: lightest, middle, and heaviest. In the case of the electron the heavier, negative charged particles are the muon and the tau.
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- The other family of particles carrying a positive charge are the Quarks, but, that is a different Review.
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- When the Big Bang first occurred nearly 14 billion years ago, a universe appeared in an unthinkably high-energy blast. Particles started to materialize out of that energy, as did their antiparticles, which were a mirror image with the opposite electric charge.
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- Each and every particle had an antiparticle, and they would annihilate each other in a pop of energy. Most particles met their end at the hands of their antiparticle in those earliest days.
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- But not all particles. A small amount of matter persisted over the antimatter, and it condensed into galaxies, stars, planets, and eventually people. What could possibly have been different between the matter and antimatter such that the matter dominated? It is still a mystery.
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- Why are we here?
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- The difficult-to-detect neutrino seems to undergo a strange identity-flipping process, and if this reaction occurs differently between neutrinos and antineutrinos, then this process, called neutrino oscillation, could help physicists explain why matter dominates over antimatter. Neutrinos are mysteriously flipping between the lightest, middle and heaviest particles.
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- How does the universe really operate in a fundamental way to produce what we see today? Why and how do neutrinos change energy levels?
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- Neutrinos pass directly through most matter without so much as a bump, no collisions, so they are invisible to most experiments. Neutrino-hunting detectors all fill the biggest container you can imagine with a detecting medium, like water or liquid argon, and wait for the rare neutrino interactions to happen. Any collisions are exceedingly rare events.
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- In “water detectors“, neutrinos interact with some of the water molecules, producing particles that in turn generate small flashes of detectable light as they travel faster than the speed light travels through water.
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- In “liquid argon detectors“, some of the neutrinos will interact with the argon nuclei medium, producing particles that in turn knock electrons off of the atoms. The electrons drift in the direction of an electrically charged surface containing a particle-detecting element.
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-The data and timing information gathered by these detectors tell researchers where the neutrino came from and about its energy and identity. The detector contains 68,000 tons of liquid argon built 4,850 feet underground.
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- Physicist Wolfgang Pauli first theorized neutrinos in 1930 as a way to explain missing energy during a radioactive process called “beta decay“. When an atomic nucleus emits an electron, the atom must also spit out energy in the form of a chargeless particle. He called them neutrons.
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- Physicist James Chadwick discovered a much heavier neutral particle in the atomic nucleus two years later and called his particle the neutron as well. It wasn’t until 1956 that physicists discovered neutrinos in a nuclear reactor, where they observed neutrino’s antiparticle partners, the antineutrinos, interacting with a proton to produce a neutron and the antiparticle partner of the electron, the “positron“.
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- Experiments continued to expand our neutrino knowledge. Physicists at Brookhaven National Lab discovered a neutrino that would interact with muons, the heavier particle related to the electron, which they called the “muon neutrino“. Others theorized the presence of an even heavier neutrino flavor, the “tau neutrino“, which was discovered in the year 2000 at Fermilab in Illinois.
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- Hints that there was something especially weird about neutrinos began to crop up in the 1970s. An experiment used a 100,000-gallon tank filled with the dry-cleaning chemical perchloroethylene, built 4,850 feet beneath the surface at the Homestake Mine in order to shield it from particles from space.
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- The experiment detected neutrinos coming from the Sun. But, only around a third of how many they expected to find. Follow-up searches continued until 1998, when the Super-Kamiokande (Super-K) experiment in Japan discovered that neutrinos from the atmosphere could oscillate between flavors.
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- In 2001, the Sudbury Neutrino Observatory in Canada discovered neutrino oscillations in solar neutrinos. In order for neutrino oscillations to make sense, neutrinos would need to have mass which was contrary to the predictions of the Standard Model of particle physics.
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- The neutrino oscillation phenomenon, implying that the neutrino has mass, is the only phenomenon beyond the Standard Model seen in the laboratory venue. Particle physicists are always looking for holes in the Standard Model to explain the unexplained pieces of our universe, such as “dark matter“. Neutrinos having mass when the Standard Model predicts that they wouldn’t could therefore be an inroad toward solving some of these mysteries.
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- By 2007, physicists began presenting studies containing various designs for the Long Baseline Neutrino Experiment, an upgraded particle accelerator that would send a beam of neutrinos through around 800 miles of Earth before striking a detector deep underground. At that distance, physicists hoped that they would be able to spot neutrinos as they swapped flavors between leaving the accelerator and arriving at the far detector.
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- This experimental setup would also provide a way to look for charge/parity (CP) symmetry violation, places where matter acts differently from antimatter. If a muon neutrino produced in the particle accelerator arrives as an electron neutrino at the far detector at a different rate than the same process in antineutrinos, then neutrino physicists would be able to confirm that neutrinos differ from their antiparticle.
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- By 2012, physicists had built up some robust theory surrounding neutrino oscillations, devising a host of parameters that together described the oscillating behavior. But of all of these numbers, the least well-known was called “θ13,” or theta 1-3. The future of neutrino physics stood with this number.
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- Beginning in late 2011, six detectors placed both near and far from the Daya Bay nuclear power plant in Shenzhen, China watched in wait for neutrinos to, well, disappear. These detectors were designed only to measure electron neutrinos.
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- That means that, if neutrinos oscillated, then the close detector would detect more neutrinos than the far detector as the neutrinos changed identities. This disappearance would allow the physicists to calculate the value of the theta 1-3 parameter.
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- In April 2012, the Daya Bay team released results better than they could have hoped for. Not only did the electron neutrinos disappear, but the calculated value of theta 1-3 was surprisingly high. That meant that physicists would be able to see neutrinos oscillate over the 800-mile distance between a neutrino beam from Fermilab and a detector at the Homestake Mine and that such an experiment would be able to see whether or not neutrinos violated CP symmetry.
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- In 2020 not only will scientists be able to compare oscillations between neutrinos and antineutrinos, but will attempt to solve another mystery relating to the neutrino’s mass. Not only are there three neutrino flavors, but there are three masses as well, called m1, m2, and m3, and the three masses, (lightest, middle, and heaviest), don’t cleanly line up with the three flavors.
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- Scientists hope to understand whether m3 is heavier or lighter than m1 and m2, which has implications for understanding how particles behaved in the early universe.
- Physicists have also theorized a new kind of particle, called the “right handed neutrino“, that would offer a mechanism to give neutrinos mass and perhaps be part of the story for why there’s more matter than antimatter. There are already hints of a fourth kind of neutrino, called a “sterile neutrino“, showing up in existing experiments.
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- For all we know, actually answering the question “why are we here?” might take decades or longer. And maybe it’s impossible to ever know. Maybe the universe never wanted to create the same amount of matter and antimatter right from the start, for no reason at all.
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- Like all major accelerator undertakings, new non-physics technology will inevitably come as a side effect. These advances can lead to better particle accelerators used for cancer treatment, and the radio frequency cavities used to accelerate particles may one day be useful for quantum computing.
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- But with neutrinos, it isn’t obvious that an answer will be found if only we can build a big-enough experiment.
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- No matter what physicists find it is going to be new and could edge them ever closer to solving some of these outstanding cases that the Standard Model has failed to explain. Neutrino science sits at the precipice of the unknown.
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- Stay tuned, there is still more to learn. I hope this helps.
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- June 15, 2020 2762
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