What are neutrinos?
In the most basic sense, neutrinos are elementary subatomic particles that have almost no mass and can travel vast distances without interacting with normal matter. “They can go through light-years of lead without stopping,” says Nathan Whitehorn, an IceCube physicist at the University of Wisconsin-Madison. In fact, trillions of neutrinos are passing right through you each second.
Neutrinos owe their ghost-like powers to the fact that they have no electric charge (are unperturbed by electromagnetic forces) and are only affected by the weak nuclear force, which works at very short ranges, and gravity, which is quite weak at subatomic scales.
Famed physicist Wolfgang Pauli initially proposed the concept of the neutrino back in 1930 to explain what happens when tritium (an isotope of hydrogen) undergoes beta decay. You see, when tritium decays into helium-3, an electron is also released, carrying away some of the energy. But scientists noticed that the equation was unbalanced - tritium had more energy than its resulting decay product, suggesting that an invisible particle must be carrying away additional energy.
Scientists didn’t detect neutrinos until over thirty years later. A team led by Clyde Cowan and Fred Reines detected neutrinos coming from a nuclear reactor and published their results in the journal Science.
There are three known types (flavors) of neutrinos: electron neutrino νe, muon neutrino νμ and tau neutrino ντ, named after their partner leptons in the Standard Model. The current best measurement of the number of neutrino types comes from observing the decay of the Z boson. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types is 3. The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists’ intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
Where do neutrinos come from?
There are numerous sources of neutrinos in the universe, both in space and here on Earth. Wherever there is nuclear physics, there are neutrinos; they are the liaisons that make nuclear physics possible.
Neutrinos are produced in the sun and other stars. They are the byproduct of nuclear fusion, which involves the merging of two protons (hydrogen atoms) to form a deuteron, releasing a positron (antielectron) and an electron neutrino at the same time.
High-mass stars end their lives in supernova explosions, which also produce neutrinos. Before the explosion, the star collapses in on itself, forcing protons to combine with electrons, forming neutrons and electron neutrinos. Eventually a neutron core will form in the center of the dying star - as it cools, it releases neutrino-antineutrino pairs of all flavors.
Scientists also believe that tons of neutrinos were created during the Big Bang, so there should currently be a “cosmic neutrino background” similar to the cosmic microwave background radiation. These particles, however, are thought to have energies too low to detect with current neutrino detectors.
Ultimately, the scientists hope the neutrinos will help them better understand the origins of high-energy cosmic rays, which are a mystery to scientists. Essentially, anything that could produce cosmic rays would also produce these high-energy neutrinos. So they are now trying to trace the neutrinos back to their source(s) - a task that should be easier than tracing back cosmic rays because neutrinos travel in a straight line, unperturbed by strong magnetic fields.
"These neutrinos could be clues to the origins of cosmic rays," Whitehorn says. "We could be opening a new window into the universe."
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