Is a neutrino an elementary particle?

Yes. In the Standard Model, neutrinos are elementary leptons with no known substructure.

How many neutrinos pass through a person per second?

Roughly 65 billion solar neutrinos pass through every square centimetre of the Earth-facing side of your body every second, virtually all of them without interacting.

Why was the neutrino so hard to detect?

Its only known interaction is the weak nuclear force, whose cross-section at MeV energies is of order 10⁻⁴³ cm² — vanishingly small. Enormous fluxes and huge detector masses are needed to record even a handful of events.

What Is a Neutrino?

A neutrino is an elementary particle of the Standard Model of particle physics. It is an electrically neutral lepton with an extraordinarily small mass and interacts only through the weak nuclear force and — through its mass — gravity. Three neutrino flavors are known: the electron neutrino $ν_{e}$ , the muon neutrino $ν_{μ}$ , and the tau neutrino $ν_{τ}$ , each paired with its corresponding charged lepton.

A postulate born of desperation

The neutrino entered physics not as an observation but as a bookkeeping device. In 1930, Wolfgang Pauli wrote his now-famous open letter to a nuclear physics conference in Tübingen, addressing the attendees as “dear radioactive ladies and gentlemen.” Beta decay appeared to violate energy conservation: electrons emitted by radioactive nuclei came out with a continuous spectrum of energies instead of the discrete line one would expect from a two-body decay. Pauli proposed a “desperate remedy” — a new neutral, spin-½ particle, emitted alongside the electron, carrying off the missing energy and momentum.

Enrico Fermi built this into a quantitative theory of beta decay in 1934, naming the particle the neutrino (Italian for “little neutral one”) to distinguish it from the heavier neutron discovered two years earlier. For the next twenty-six years it remained theoretical. The particle was too weakly interacting to detect.

The 1956 detection

Frederick Reines and Clyde Cowan set out to detect the antineutrino flux emerging from a nuclear reactor. Beside the Savannah River Plant reactor in South Carolina they built a detector of cadmium-doped water sandwiched between tanks of liquid scintillator. An incoming electron antineutrino $\overset{ν}{ˉ}_{e}$ could undergo inverse beta decay on a proton in the water:

$\overset{ν}{ˉ}_{e} + p \to e^{+} + n$

The positron annihilates immediately, producing two 511 keV gamma rays. The neutron thermalizes over a few microseconds, then is captured on a cadmium nucleus, which releases a cascade of gamma rays. The delayed coincidence of these two signals — prompt positron annihilation followed by neutron capture a few microseconds later — became the unambiguous signature of an antineutrino event. In 1956 Reines and Cowan telegraphed Pauli to confirm the detection. Reines received the Nobel Prize for this work in 1995; Cowan had died in 1974.

What makes a neutrino a neutrino

Three properties define the particle.

It is electrically neutral. A neutrino carries no electric charge and therefore does not participate in electromagnetic interactions. It cannot ionize matter, bend in a magnetic field, or radiate photons.

It has spin ½. Neutrinos are fermions. Like electrons, they obey the Pauli exclusion principle and follow Fermi–Dirac statistics.

It interacts only through the weak force. At low energies, the weak interaction cross-section is roughly $σ \sim G_{F}^{2} E^{2} / π \approx 1 0^{- 44} cm^{2} \cdot (E / MeV)^{2}$ where $G_{F}$ is the Fermi coupling constant. A 1 MeV neutrino has a mean free path through lead of roughly a light-year. This is why neutrinos from the centre of the Sun stream out freely while photons diffuse for tens of thousands of years.

Flavors and eigenstates

Every charged lepton has a partner neutrino defined by its production and detection channels. A $W^{+}$ boson decaying to $e^{+}$ emits a $ν_{e}$ ; to $μ^{+}$ , a $ν_{μ}$ ; to $τ^{+}$ , a $ν_{τ}$ . These are flavor eigenstates of the weak interaction.

Because neutrinos have mass, however, the states that propagate freely through space — the mass eigenstates $ν_{1}, ν_{2}, ν_{3}$ — are not the same as the flavor eigenstates. The two bases are related by a unitary mixing matrix, the PMNS matrix, with three mixing angles and at least one CP-violating phase. This mismatch gives rise to neutrino oscillations: a neutrino produced as $ν_{μ}$ can later be detected as $ν_{e}$ after propagating a sufficient distance. The phenomenon was discovered in atmospheric neutrinos by Super-Kamiokande in 1998 and in solar neutrinos by SNO in 2001–2002, and earned the 2015 Nobel Prize in Physics.

Mass

The Standard Model, as originally formulated, has massless neutrinos. The discovery of oscillations made this position untenable: oscillation requires at least two of the three mass eigenstates to have different, non-zero masses. Oscillation experiments measure only the squared mass differences, currently $Δ m_{21}^{2} \approx 7.4 \times 1 0^{- 5} eV^{2}, ∣Δ m_{31}^{2} ∣ \approx 2.5 \times 1 0^{- 3} eV^{2}$ The absolute mass scale remains unknown. The direct kinematic measurement at KATRIN sets $m_{ν} < 0.45$ eV at 90% confidence. Cosmological bounds on the sum $\sum m_{ν}$ are tighter but model-dependent, currently of order 0.1 eV.

Why neutrinos matter

Neutrinos carry unique information. They escape from stellar cores where photons cannot. They are produced copiously in beta decays, in the early Universe, in supernovae, in cosmic ray showers, in particle accelerators, and in the Earth’s crust. The 1987A supernova neutrino burst arrived before the optical signal and provided the first direct observation of core collapse. Today the IceCube observatory uses neutrinos as astronomical messengers, tracing distant blazars and gamma-ray bursts. Reactor neutrino experiments are used to monitor nuclear activity. And applied research now explores the possibility of converting the flux of invisible radiation — neutrinos among other components — into usable electrical energy through engineered nanostructures, as described on the Master Equation and Neutrinovoltaics pages.

Frequently asked

Is a neutrino an elementary particle?: Yes. In the Standard Model, neutrinos are elementary leptons with no known substructure.
How many neutrinos pass through a person per second?: Roughly 65 billion solar neutrinos pass through every square centimetre of the Earth-facing side of your body every second, virtually all of them without interacting.
Why was the neutrino so hard to detect?: Its only known interaction is the weak nuclear force, whose cross-section at MeV energies is of order 10⁻⁴³ cm² — vanishingly small. Enormous fluxes and huge detector masses are needed to record even a handful of events.