Neutrino in the context of "Spin-1"

The tau (τ), also called the tau lepton, tau particle or tauon, is an elementary particle similar to the electron, with negative electric charge and a spin of ⁠1/2⁠. Like the electron, the muon, and the three neutrinos, the tau is a lepton, and like all elementary particles with half-integer spin, the tau has a corresponding antiparticle of opposite charge but equal mass and spin. In the tau's case, this is the "antitau" (also called the positive tau). Tau particles are denoted by the symbol τ and the antitaus by τ.

Tau leptons have a lifetime of 2.9×10 s and a mass of 1776.9 MeV/c (compared to 105.66 MeV/c for muons and 0.511 MeV/c for electrons). Because their interactions are very similar to those of the electron, a tau can be thought of as a much heavier version of the electron. Due to their greater mass, tau particles do not emit as much bremsstrahlung (braking radiation) as electrons; consequently they are potentially much more highly penetrating than electrons.

In particle physics, a lepton is an elementary particle of half-integer spin (spin ⁠1/2⁠) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), including the electron, muon, and tauon, and neutral leptons, better known as neutrinos. Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.

There are six types of leptons, known as flavours, grouped in three generations. The first-generation leptons, also called electronic leptons, comprise the electron (e
) and the electron neutrino (ν
_e); the second are the muonic leptons, comprising the muon (μ
) and the muon neutrino (ν
_μ); and the third are the tauonic leptons, comprising the tau (τ
) and the tau neutrino (ν
_τ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high-energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

The cosmic neutrino background is a proposed background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos or sometimes abbreviated CNB or CνB, where the symbol ν is the Greek letter nu, standard particle physics symbol for a neutrino.

The CνB is a relic of the Big Bang; while the cosmic microwave background radiation (CMB) dates from when the universe was 379,000 years old, the CνB decoupled (separated) from matter when the universe was just one second old. It is estimated that today, the CνB has a temperature of roughly 1.95 K.

The tau neutrino or tauon neutrino is an elementary particle which has the symbol ν
_τ and zero electric charge. Together with the tau (τ), it forms the third generation of leptons, hence the name tau neutrino. Its existence was immediately implied after the tau particle was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC–LBL group. The discovery of the tau neutrino was announced in July 2000 by the DONUT collaboration (Direct Observation of the Nu Tau). In 2024, the IceCube Neutrino Observatory published findings of seven astrophysical tau neutrino candidates.

As of 2022 they have been called the "least studied particle in the standard model" because of their low cross section, difficulty of production, and difficulty to distinguish from other neutrino flavors. One review argues they are worth studying more in order to finally completely measure their properties, test our knowledge of neutrino mixing, probe possible anomalies, and make full use of experiments that are sensitive to tau neutrinos in any case.

Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino created with a specific lepton family number ("lepton flavor": electron, muon, or tau) can later be measured to have a different lepton family number. The probability of measuring a particular flavor for a neutrino varies between three known states as it propagates through space.

First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by a multitude of experiments in several different contexts. Most notably, the existence of neutrino oscillation resolved the long-standing solar neutrino problem.

In quantum mechanics, spin is an intrinsic property of all elementary particles. All known fermions, the particles that constitute ordinary matter, have a spin of ⁠1/2⁠. The spin number describes how many symmetrical facets a particle has in one full rotation; a spin of ⁠1/2⁠ means that the particle must be rotated by two full turns (through 720°) before it has the same configuration as when it started.

Particles with net spin ⁠1/2⁠ include the proton, neutron, electron, neutrino, and quarks. The dynamics of spin-⁠1/2⁠ objects cannot be accurately described using classical physics; they are among the simplest systems whose description requires quantum mechanics. As such, the study of the behavior of spin-⁠1/2⁠ systems forms a central part of quantum mechanics.

Wolfgang Ernst Pauli (/ˈpɔːli/ PAW-lee; German: [ˈpaʊ̯li] ; 25 April 1900 – 15 December 1958) was an Austrian–Swiss theoretical physicist and a pioneer of quantum mechanics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics "for the discovery of the Exclusion Principle, also called the Pauli Principle". The discovery involved spin theory, which is the basis of a theory of the structure of matter.

To preserve the conservation of energy in beta decay, Pauli proposed the existence of a small neutral particle, dubbed the neutrino by Enrico Fermi, in 1930. Neutrinos were first detected in 1956.

In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino; or, conversely a proton is converted into a neutron by the emission of a positron with a neutrino in what is called positron emission. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. For either electron or positron emission to be energetically possible, the energy release or Q value must be positive.

Beta decay is a consequence of the weak force, which is characterized by relatively long decay times. Nucleons are composed of up quarks and down quarks, and the weak force allows a quark to change its flavour by means of a virtual W boson leading to creation of an electron/antineutrino or positron/neutrino pair. For example, a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks.

Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist and the co-discoverer of the neutrino along with Frederick Reines. The discovery was made in 1956 in the neutrino experiment. Reines received the Nobel Prize in Physics in 1995.