Neutrino
A neutrino (/njuːˈtriːnoʊ/ new-TREE-noh; denoted by the Greek letter ν) is a fermion (an elementary particle with spin of 1 /2) that interacts only via the weak interaction and gravity.[2][3] The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The rest mass of the neutrino is much smaller than that of the other known elementary particles (excluding massless particles).[1] The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the electromagnetic interaction or the strong interaction.[4] Thus, neutrinos typically pass through normal matter unimpeded and undetected.[2][3]
Not to be confused with neutron, neuron, or neutralino.Composition
Leptons, antileptons
First (
ν
e), second (
ν
μ), and third (
ν
τ)
ν
e ,
ν
μ ,
ν
τ ,
ν
e ,
ν
μ ,
ν
τ
spin: ±+ 1 /2ħ, chirality: Left, weak isospin: + 1 /2, lepton nr.: +1, "flavour" in { e, μ, τ }
spin: ±+ 1 /2ħ, chirality: Right, weak isospin: − 1 /2, lepton nr.: −1, "flavour" in { e, μ, τ }
ν
e, electron neutrino: Wolfgang Pauli (1930)
ν
μ, muon neutrino: late 1940s
ν
τ, tau neutrino: mid-1970s
ν
e: Clyde Cowan, Frederick Reines (1956)
ν
μ: Leon Lederman, Melvin Schwartz and Jack Steinberger (1962)
ν
τ: DONUT collaboration (2000)
3 types: electron neutrino (
ν
e), muon neutrino (
ν
μ), and tau neutrino (
ν
τ)
< 0.120 eV (< 2.14 × 10−37 kg), 95% confidence level, sum of 3 "flavours"[1]
0 e
1 /2ℏ
LH: + 1 /2, RH: 0
LH: −1, RH: 0
−1
−3
Weak interactions create neutrinos in one of three leptonic flavors:
Each flavor is associated with the correspondingly named charged lepton.[5] Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values (the smallest of which could even be zero[6]), but the three masses do not uniquely correspond to the three flavors: A neutrino created with a specific flavor is a specific mixture of all three mass states (a quantum superposition). Similar to some other neutral particles, neutrinos oscillate between different flavors in flight as a consequence. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino.[7][8] The three mass values are not yet known as of 2024, but laboratory experiments and cosmological observations have determined the differences of their squares,[9] an upper limit on their sum (< 2.14×10−37 kg),[1][10] and an upper limit on the mass of the electron neutrino.[11]
For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has spin of 1 /2 and no electric charge. Antineutrinos are distinguished from neutrinos by having opposite-signed lepton number and weak isospin, and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with positrons (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos.[12][13]
Neutrinos are created by various radioactive decays; the following list is not exhaustive, but includes some of those processes:
The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 65 billion (6.5×1010) solar neutrinos, per second per square centimeter.[14][15] Neutrinos can be used for tomography of the interior of the Earth.[16][17]
History[edit]
Pauli's proposal[edit]
The neutrino[a]
was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum, and angular momentum (spin). In contrast to Niels Bohr, who proposed a statistical version of the conservation laws to explain the observed continuous energy spectra in beta decay, Pauli hypothesized an undetected particle that he called a "neutron", using the same -on ending employed for naming both the proton and the electron. He considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay and had a mass similar to the electron.[18][b]
James Chadwick discovered a much more massive neutral nuclear particle in 1932 and named it a neutron also, leaving two kinds of particles with the same name. The word "neutrino" entered the scientific vocabulary through Enrico Fermi, who used it during a conference in Paris in July 1932 and at the Solvay Conference in October 1933, where Pauli also employed it. The name (the Italian equivalent of "little neutral one") was jokingly coined by Edoardo Amaldi during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron.[19]
In Fermi's theory of beta decay, Chadwick's large neutral particle could decay to a proton, electron, and the smaller neutral particle (now called an electron antineutrino):
Scientific interest[edit]
Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.
Using neutrinos as a probe was first proposed in the mid-20th century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions. Whereas photons emitted from the solar core may require 40,000 years to diffuse to the outer layers of the Sun, neutrinos generated in stellar fusion reactions at the core cross this distance practically unimpeded at nearly the speed of light.[109][110]
Neutrinos are also useful for probing astrophysical sources beyond the Solar System because they are the only known particles that are not significantly attenuated by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas, and background radiation. High-energy cosmic rays, in the form of swift protons and atomic nuclei, are unable to travel more than about 100 megaparsecs due to the Greisen–Zatsepin–Kuzmin limit (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.
The galactic core of the Milky Way is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core might be measurable by Earth-based neutrino telescopes.[22]
Another important use of the neutrino is in the observation of supernovae, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an extremely dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their radiant energy in a short (10-second) burst of neutrinos.[111] These neutrinos are a very useful probe for core collapse studies.
The rest mass of the neutrino is an important test of cosmological and astrophysical theories. The neutrino's significance in probing cosmological phenomena is as great as any other method, and is thus a major focus of study in astrophysical communities.[112]
The study of neutrinos is important in particle physics because neutrinos typically have the lowest rest mass among massive particles (i.e. the lowest non-zero rest mass, i.e. excluding the zero rest mass of photons and gluons), and hence are examples of the lowest-energy massive particles theorized in extensions of the Standard Model of particle physics.
In November 2012, American scientists used a particle accelerator to send a coherent neutrino message through 780 feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent immense distances through even the densest materials, such as the Earth's core.[113]
In July 2018, the IceCube Neutrino Observatory announced that they have traced an extremely-high-energy neutrino that hit their Antarctica-based research station in September 2017 back to its point of origin in the blazar TXS 0506+056 located 3.7 billion light-years away in the direction of the constellation Orion. This is the first time that a neutrino detector has been used to locate an object in space and that a source of cosmic rays has been identified.[114][115][116]
In November 2022, the IceCube Neutrino Observatory found evidence of high-energy neutrino emission from NGC 1068, also known as Messier 77, an active galaxy in the constellation Cetus and one of the most familiar and well-studied galaxies to date.[117]
In June 2023, astronomers reported using a new technique to detect, for the first time, the release of neutrinos from the galactic plane of the Milky Way galaxy.[118][119]