Neutrino

A neutrino is an elementary particle that exists at the most fundamental level of our universe. These particles are electrically neutral, meaning they carry no electric charge. They also have an incredibly tiny rest mass. For a long time, scientists believed their mass was exactly zero. Because they lack charge and have such little mass, they interact with the world in very specific ways. They only interact via gravity and the weak interaction. The weak interaction is a force that has a very short range. Because they do not participate in electromagnetic or strong interactions, neutrinos usually pass through normal matter without hitting anything.

To understand how neutrinos behave, we must look at their three distinct flavors. These flavors are the electron neutrino, the muon neutrino, and the tau neutrino. Each flavor is associated with a corresponding charged lepton. For example, the electron neutrino is linked to the electron. One of the most surprising things about neutrinos is a process called oscillation. This is when a neutrino changes its flavor while it is in flight. A neutrino created as an electron neutrino during beta decay might interact in a distant detector as a muon or tau neutrino. This happens because a neutrino with a specific flavor is actually a quantum superposition. It is a mixture of three different mass states.

The history of the neutrino began with a mystery in nuclear physics. In 1930, Wolfgang Pauli proposed the particle to explain beta decay. During beta decay, particles are emitted from an atomic nucleus. Pauli wanted to show how energy, momentum, and angular momentum, also called spin, were conserved. He originally called his idea a "neutron." However, James Chadwick discovered a much heavier neutral particle and named it a neutron in 1932. To avoid confusion, the name "neutrino" was used. This name was jokingly coined by Edoardo Amaldi. In Italian, neutrino means "little neutral one." Enrico Fermi later used this name during scientific conferences in 1932 and 1933.

For many years, neutrinos remained theoretical because they were so hard to detect. In 1956, Clyde Cowan and Frederick Reines finally confirmed their existence. They conducted the Cowan–Reines experiment using a nuclear reactor. They used antineutrinos, which are the antiparticles of neutrinos. These antineutrinos reacted with protons to produce neutrons and positrons. The positron quickly finds an electron and they annihilate each other. This annihilation produces two detectable gamma rays. The coincidence of these gamma rays and the neutron capture provided a unique signature. This massive achievement earned them the Nobel Prize in 1995.

Neutrinos are produced by many different natural and artificial processes. They are created during the beta decay of atomic nuclei or hadrons. They are also produced in natural nuclear reactions inside the core of a star. Artificial reactions in nuclear reactors, nuclear bombs, or particle accelerators also create them. Massive cosmic events like a supernova also release them. A supernova is the explosion of a star.

Even the spin-down of a neutron star can produce neutrinos. The majority of neutrinos detected on Earth come from the Sun. At the surface of the Earth, the flux is about 65 billion solar neutrinos per second per square centimeter.

One of the greatest mysteries in science was the solar neutrino problem. In the 1960s, the Homestake experiment measured electron neutrinos from the Sun. It found only one third to one half of the predicted number. This discrepancy lasted for thirty years. Scientists eventually realized that the neutrinos were not missing. Instead, they were oscillating into different flavors that the experiments could not detect. This discovery was confirmed by experiments at Super-Kamiokande and the Sudbury Neutrino Observatory. This work helped explain how neutrinos change flavors as they travel through matter. This is known as the Mikheyev–Smirnov–Wolfenstein or MSW effect.

Today, neutrinos connect many different fields of study. They can be used for tomography of the interior of the Earth. This means scientists can use them to map what is inside our planet. They also provide clues about the history of the entire universe. A general background of neutrinos pervades all of space. Some of these came from the Big Bang about one second after it began. This is called the cosmic neutrino background. Others come from a diffuse background created by supernovas. By studying these tiny particles, we learn about the largest structures in existence.