- 3793 - NEUTRINOS - still a most mysterious particle? Neutrinos are some of the most mysterious particles in nature, capable of passing through the Earth like it wasn't there. Neutrinos are tiny subatomic particles, often called 'ghost particles' because they barely interact with anything else.
--------------------- 3793 - NEUTRINOS - still a most mysterious particle?
- Neutrinos are some of the most mysterious particles in nature, capable of passing through the Earth like it wasn't there. Neutrinos are tiny subatomic particles, often called 'ghost particles' because they barely interact with anything else.
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- Neutrinos are also the most common particle in the universe. Believe it or not, approximately 100 trillion neutrinos pass completely harmlessly through your body every second.
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- Their tendency not to interact very often with other particles makes detecting neutrinos very difficult, but it does not mean that they never interact. The probability that any given neutrino will interact with another particle is just very small.
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- Despite these small odds, the fact that there are so many neutrinos means that statistically, some will be seen to interact. For example, there is a 1 in 4 chance that a neutrino will interact with an atom in your body at some point in your life. Given that throughout your life an estimated 2.5 x 10^21 neutrinos will sweep through you, the probability of any given neutrino interacting with you is about 1 in a trillion trillion. (1 in 10^24).
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- Neutrinos play crucial roles in the standard model of particle physics, in stellar physics and black holes, and even in cosmology and the nature of the Big Bang. On the family tree of particles, called the Standard Model, neutrinos belong to the family of particles known as “leptons“. There are three main leptons, namely electrons, muons and tau particles, and each one has an associated neutrino and anti-neutrino.
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- Neutrinos have no charge; they are neutral. And while the neutrino mass has yet to be precisely measured, we know it must be very small. At KATRIN, the “Karlsruhe Tritium Neutrino Experiment” in Germany, scientists were able to measure the upper limit of the neutrino mass to be 0.8 electronvolts, or eV.
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- An electronvolt is the amount of kinetic energy acquired by an electron when it is accelerated through a potential difference of one volt. While it might at first seem strange to be measuring mass using units of energy, Albert Einstein showed us how mass and energy are two sides of the same coin (E = mc^2), and extremely small particle masses are often given in eV because the kilogram conversion is so tiny (0.8eV is about 1.4 x 10^–36 kg.) Neutrinos are about ten-thousand times less massive than electrons.
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- Neutrinos don't interact at all with the strong nuclear force that binds atomic nuclei together, but they do interact with the weak force that controls radioactive decay. This is how neutrinos are produced; the KATRIN experiment measured the mass of neutrinos that resulted from the decay of tritium isotopes.
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- The conservation of both energy and angular momentum are two fundamental tenets of physics. You can't produce energy out of nothing, and angular momentum can't just vanish. Back in 1930, the famous quantum physicist Wolfgang Pauli realized that in order to maintain the conservation of energy and angular momentum in beta decay (in which an electron or its anti-particle, a positron, are emitted from a radioactive atom) it required the presence of a new type of particle with no charge, none or very little mass, and a quantum spin of 1/2. This new, theoretical particle was the neutrino.
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- It remained purely theoretical until 1955 when neutrinos were discovered coming from beta decay inside a nuclear reactor at the Savannah River Site in South Carolina. Their neutrino detector consisted of scintillating fluid and photomultiplier tubes and didn't detect the neutrino directly.
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- Instead, the detector watched for neutrinos interacting with protons in the fluid, the interactions producing positrons and neutrons. The positrons annihilated when they encountered electrons, which are their antimatter equivalent, in the fluid. This annihilation converted all their mass into pure energy in the form of two gamma rays, while the neutrons also produced extra gamma rays when they were subsequently captured by another atom. The photomultiplier tubes were able to detect these gamma rays.
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- These neutrinos were being artificially produced by the nuclear reactor. The first 'natural' neutrino to be detected was found in 1965 at an experiment deep underground at the East Rand goldmine in South Africa.
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- The Homestake Mine, in South Dakota, was the largest gold mine in the United States. An experiment deep in the mine was built to detect neutrinos coming from the core of the sun, where nuclear fusion reactions turn hydrogen into helium.
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- They filled a tank in the mine with 100,000 gallons of a chlorine-rich dry-cleaning fluid, perchloroethylene. The methodology was simple, on the occasions that a neutrino interacted with an atom of chlorine-37, it turned into a radioactive isotope of argon-37, and by counting how many atoms of argon-37 had appeared every few weeks.
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- Physicists could calculate how many neutrinos from the sun had passed through the tank. Because it was 4,850 feet underground, the Homestake experiment was shielded from cosmic rays that could interfere with these results.
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- The Homestake results were a big problem as the numbers of neutrinos were well down on expectations, only a third as many neutrinos as predicted were measured to be coming from the sun. Other subsequent neutrino detectors, such as Super Kamiokanda in Japan, confirmed these results.
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- Either there was a big problem with our understanding of neutrinos, or there was an even bigger problem with our understanding of the sun. It became known as the “Solar Neutrino Problem” that perplexed scientists for three decades before they arrived at a solution. It wasn't lost on scientists that there are three kinds of neutrinos, and two-thirds of the expected neutrinos coming from the sun were missing.
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- Nuclear reactions inside the sun should emit only electron neutrinos, and that's what the experiments were set up to detect. Suppose, though, that in the 93,205,678 miles between us and the sun, two-thirds of the electron neutrinos were somehow transforming in muon and tau neutrinos.
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- Quantum physics says that this is possible, because the quantum states of all three types of neutrino can be superimposed on top of one another. These states can evolve over time, so a neutrino can start off with one state dominant, then it changes to another state, and so on. This is called “neutrino oscillation“, but it only works if neutrinos have mass, and until then they were thought to be mass-less.
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- In 2001, observations by the Sudbury Neutrino Observatory, based deep in a copper mine in Ontario, Canada, proved that neutrinos were oscillating between these different 'flavors'. They must have mass, but still trying to pin down exactly how much mass they have.
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- Neutrinos are produced inside nuclear reactors on Earth and fusion reactions inside the sun. However, they are also produced much further afield. In February 1987 a star exploded as a supernova in the Large Magellanic Cloud, which is a small, nearby galaxy. The supernova, known as SN 1987A, was visible to the unaided eye. However, two to three hours before the visible light of the supernova reached us, a burst of neutrinos was detected coming from the dying star.
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- Only a handful of neutrinos were detected at each detector around the world, but given how weakly neutrinos interact, the two-dozen detections was well above the background level and indicated a huge burst of neutrinos that had been produced as the core of the star collapsed. It was the first time that neutrinos had been detected coming from a supernova, and confirmed various theories about how massive stars end their lives.
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- Since then, neutrinos have also been detected coming from violent events around active supermassive black holes, such as those found in quasars and blazars. Neutrinos are also relevant to cosmology since primordial neutrinos that formed in the first second after the Big Bang are also prevalent in the universe.
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- One estimate suggests there are about 300 Big Bang neutrinos in every cubic centimeter. These neutrinos from the Big Bang have been detected, as well as how they affect the size of baryonic acoustic oscillations in the cosmic microwave background (CMB) radiation. Therefore, understanding Big Bang neutrinos will help us to understand the CMB and the Big Bang itself better.
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- Dark matter is the mysterious substance that many scientists believe can explain the extra gravity observed holding together galaxies and galaxy clusters. Dark matter cannot be seen, and only interacts with ordinary matter via gravity. If it interacts with ordinary matter in any other ways, then it does so only very weakly.
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- Neutrinos appear to fit the bill, but there's a problem: they're not massive enough. Even with countless neutrinos filling the universe, at a maximum of 0.8eV, the three known flavors of neutrino — electron, muon and tau — are still not enough to account for all the dark matter.
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- However, what if there is another type of neutrino that has previously gone undetected? An experiment at the “Liquid Scintillator Neutrino Detector” at Los Alamos National Laboratory found that more muon antineutrinos were oscillating into electron antineutrinos than theory predicted. The MiniBooNE experiment (BooNE stands for Booster Neutrino Experiment) at FermiLab also found a stronger oscillation signal than expected.
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- Astronomers have postulated that a fourth kind of neutrino, known as a “sterile neutrino“, could exist as a way of explaining these strange oscillation patterns. The sterile neutrino would have very specific properties. It would only interact via gravity, and would not interact with the other forces of nature at all, unlike the other three flavors of neutrino that interact with the weak force.
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- The sterile neutrino would also have no electric charge. Moreover, its mass could be anywhere between 1eV and an enormous 15GeV (about 15 times more massive than a proton). There could even be several types of sterile neutrinos. If sterile neutrinos are at the upper end of the estimated mass range, they could explain at least some of the mysterious dark matter. However, subsequent searches for sterile neutrinos have been inconclusive, and their existence remains firmly hypothetical.
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- The leading present-day neutrino detector is the “IceCube Observatory“. Why fill tanks with tens of thousands of gallons of cleaning fluid when you can use a cubic kilometer of naturally formed ice buried at the South Pole? Implanted within the ice are 5,160 digital optical modules, arranged in strings hanging down 86 frozen boreholes.
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- When an incoming high-energy neutrino interacts with a molecule of ice, it smashes the ice apart, creating a cascade of particles including muons, which move slower than the speed of light in a vacuum. Because the speed of light in the medium of ice is slower than the speed of light in a vacuum, the muons are effectively traveling faster than light in the ice. Therefore they release a flash of light, the optical equivalent of a sonic boom, called “Cherenkov radiation“. The digital optical modules then detect the flash of Cherenkov radiation, recording the presence of a neutrino interaction.
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- The latest observing run of the Large Hadron Collider is also set up to detect neutrinos. Previously the LHC has not had the capability to detect neutrinos created in its particle collisions, but for its latest observing run two new neutrino-detecting instruments — the Forward Search Experiment (FASER) and the Scattering and Neutrino Detector, have been introduced, and among other things they will be searching for evidence of sterile neutrinos.
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- Looking further into the future, scientists are hoping to build the Pacific Ocean Neutrino Experiment (P-ONE), which would be a giant neutrino detector at least two miles deep, with strands of photodetectors kept afloat across several square miles, and which would detect Cherenkov light like IceCube.
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- Neutrinos are one of the universe's best-kept secrets, and we're only now beginning to unlock some of their mysteries
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December 22, 2022 NEUTRINOS - still a most mysterious particle? 3793
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