# What Is a Neutrino? Neutrino Facts

A neutrino is a subatomic particle and also an elementary or fundamental particle. In other words, it is smaller than an atom and does not consist of smaller subunits. It is a fermion, which is a particle with a spin of 1/2. The symbol for a neutrino is Greek letter nu (ν).

### Why It’s Called a Neutrino

The word “neutrino” means “little neutral one” and reflects two properties of this particle. First, it is electrically neutral (the “neutr-” part of the name). Second, it is extremely tiny (“-ino”, with a rest mass very nearly zero.

### Neutrino Facts

• A neutrino has a neutral electrical charge and very small mass. Its mass is estimated as at least six orders of magnitude smaller than that of the electron, which has a mass of 9.1×10-31 kilograms. The exact mass of a neutrino has yet to be measured.
• Neutrinos travel at speeds approaching the speed of light.
• A neutrino only reacts to gravity and the weak nuclear force (weak interaction). Because of this, it very rarely interacts with matter.
• For example, billions of neutrinos pass through your body every day. Despite this, scientists estimate only one solar neutrino (from our Sun) interacts with a person throughout their entire lifetime.
• At present, there are three known “flavors” of neutrinos: electron, muon, and tau. A neutrino oscillates between these three flavors. There are also antimatter particles: anti-electron (antineutrino), anti-muon, and anti-tau.
• There may be other neutrino flavors. For example, scientists predict the existence of the sterile neutrino. A sterile neutrino interacts only with gravity, not the weak nuclear force.
• Neutrinos are extremely common. They come from nuclear reactions. Sources include the Sun and other stars, supernovae, nuclear decay, fission, and fusion.
• Like neutrons, neutrinos induce nuclear fission of heavy nuclei. Only neutrino fission of deuterium has been observed in labs, but the process likely occurs within stars and influences the isotope abundance of elements.
• Scientists estimate between 2% to 3% of the Sun’s radiation takes the form of neutrinos. About 99% of a supernova’s energy gets released as neutrinos.
• Researcher see the Sun, day or night, using neutrinos. They pass through the Earth when it is night time. Based on neutrino images, astronomers know nuclear reaction only occur in the Sun’s core, which is its inner 20-25%.
• Neutrinos may be hot dark matter. That is, they neither emit nor absorb light, so they appear dark. Yet, they have energy, so they are hot.

### Discovery and History

Wolfgang Pauli proposed the existence of the neutrino in 1930 as a means of conservation of energy in beta decay. Both Pauli and Enrico Fermi referred to the hypothetical particle as a neutrino in scientific conferences in 1932 and 1933.

### Neutrino Detection

Because neutrinos so rarely interact with matter, detecting them is a difficult task. Basically, the particles are too small and unreactive for direct detection. Scientists look for particles or radiation that can be observed and measured.

Wang Ganchang proposed using beta capture for experimental neutrino detection in 1942. But, it was not until July 1956 that Clyde Cowan, Frederick Reines, Francis B. “Kiko” Harrison, Austin McGuire, and Herald Kruse announced discovery of the particle. The discovery of the neutrino led to a 1995 Nobel Prize. The Cowan-Reines neutrino experiment involved releasing neutrinos produced by beta decay in a nuclear reactor. These neutrinos (antineutrinos, actually) reacted with protons and formed neutrons and positrons. The highly reactive positrons quickly encountered electrons. The gamma radiation released from the positron-electron annihilation and the neutron formation gave evidence of neutrino existence.

The first neutrino found in nature was in 1965 at a chamber in the East Rand gold mine in South Africa, 3 kilometers underground. Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for discovering neutrino oscillations, proving that neutrinos have mass.

At present, the largest neutrino detector is Super Kamiokande-III in Japan.

### Practical Applications

The low mass and neutral charge of a neutrino make it perfect as a probe for exploring places other forms of radiation can’t penetrate. For example, neutrinos detect conditions inside the core of the Sun because most of them pass through the intensely dense material. Meanwhile photons (light) get blocked. Other targets for neutrino probes include the Earth’s core, the galactic core of the Milky Way, and supernovae.

In 2012, scientists sent the first message using neutrinos through 780 feet of rock. Theoretically, neutrinos allow for transmission of binary messages through the densest matter at nearly the speed of light.

Because neutrinos do not decay, detecting one and following its path lets scientists locate extremely distant objects in space. Otherwise, the study of neutrinos is vital for understanding dark matter and extending the Standard Model of particle physics.

### References

• Alberico, Wanda Maria; Bilenky, Samoil M. (2004). “Neutrino oscillations, masses, and mixing”. Physics of Particles and Nuclei. 35: 297–323.
• Barinov, V.V.; et al. (2022). “Results from the Baksan Experiment on Sterile Transitions (BEST)”. Phys. Rev. Lett. 128(23): 232501. doi:10.1103/PhysRevLett.128.232501
• Close, Frank (2010). Neutrinos (softcover ed.). Oxford University Press. ISBN 978-0-199-69599-7.
• Mertens, Susanne (2016). “Direct neutrino mass experiments”. Journal of Physics: Conference Series. 718 (2): 022013. doi:10.1088/1742-6596/718/2/022013
• Tipler, Paul Allen; Llewellyn, Ralph A. (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 978-0-7167-4345-3.