Neutrinos are tiny subatomic particles, often called 'ghost particles' because they barely interact with anything else.
Neutrinos are, however, the most common particle in the universe. Believe it or not, approximately 100 trillion neutrinos pass completely harmlessly through your body every second!
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.
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).
Related: 10 cosmic mysteries the Large Hadron Collider could unravel
Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.
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.
What exactly are these 'ghost particles'?
Neutrinos have no charge; they are neutral, as their name implies. 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. (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 (as described by his famous equation, 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). To put that into context, neutrinos are about ten-thousand times less massive than electrons.
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. Hence this is how neutrinos are produced; the KATRIN experiment, for instance, measured the mass of neutrinos that resulted from the decay of tritium isotopes.
How were neutrinos discovered?
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, of course, the neutrino.
It remained purely theoretical until 1955, when physicists Clyde Cowan and Frederick Reines of the Los Alamos National Laboratory led a team to detect neutrinos for the first time, 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. 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.
These neutrinos were being artificially produced, however, 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, but it wasn't until the famous Homestake Mine detector was built that neutrino physics really came of age.
Homestake Mine, in South Dakota, was once upon a time the largest gold mine in the United States. Physicists John Bahcall and Ray Davis, Jr built an experiment deep in the mine to detect neutrinos coming from the core of the sun, where nuclear fusion reactions turn hydrogen into helium. To do so, Bahcall and Davis filled a tank in the mine with 100,000 gallons (454,600 liters) of a chlorine-rich dry-cleaning fluid — perchloroethylene to be precise. 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, Davis and Bahcall could calculate how many neutrinos from the sun had passed through the thank. Because it was 4,850 feet (1,478 meters) underground, the Homestake experiment was shielded from cosmic rays that could interfere with the results.
Neutrino oscillations
The Homestake results were a big problem, however, 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 Kamiokande in Japan, confirmed these results.
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, which Bahcall describes for us in his own words at the Nobel Prize website (Bahcall and Davis won the 2002 Nobel Prize for Physics for their discovery), and it 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. Coincidence? Scientists didn't think so.
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 (150 million kilometers) between us and the sun, two-thirds of the electron neutrinos were somehow transforming in muon and tau neutrinos. 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 and so forth. This is called neutrino oscillation, but it only works if neutrinos have mass, and until recently they were thought to be mass-less.
In 2001, observations by the Sudbury Neutrino Observatory, based deep in a copper mine in Ontario, Canada, proved that neutrinos were oscillating between different 'flavors'. Ergo they must have mass, and we have seen earlier in the article, scientists are still trying to pin down exactly how much mass they have.
Where do neutrinos come from?
As we have seen, 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. 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.
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 — 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.
Are neutrinos dark matter?
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.
Neutrinos appear to fit the bill, but there's a problem: they're not massive enough. Even with countless neutrinos filling every nook and cranny of 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.
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.
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. 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 neutrino. 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 at this point.
The future of neutrino astronomy
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. 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 just a smidgen 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.
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.
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.
Neutrinos are one of the universe's best-kept secrets, and we're only now beginning to unlock some of their mysteries. With plans in the National Academies' recent Astro2020 Decadal Survey for an upgraded IceCube Observatory for the 2030s and beyond, we will learn more and more about neutrinos, their bizarre behavior, and what they mean for the universe at large.
Additional resources
Learn more about neutrinos with these resources from the Department of Energy. Find out if neutrinos are the reason matter exists with the informative website all things neutrino. Investigate electron neutrinos and antineutrinos in more detail with HyperPhysics from Georgia State University.
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