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A neutrino ( new-TREE-noh; denoted by the Greek letter ν) is an elementary particle that interacts via the weak interaction and gravity.

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). 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. Weak interactions create neutrinos in one of three leptonic flavors: electron neutrino, νe; muon neutrino, νμ and tau neutrino, ντ. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different values (all tiny, the smallest of which could be zero), 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. The three mass values are not yet known as of 2026, but laboratory experiments and cosmological observations have determined the differences of their squares, an upper limit on their sum (< 0.120 eV/c2), and an upper limit on the mass of the electron neutrino. Neutrinos are fermions, which have spin of ⁠1/2⁠ħ. For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has spin of ⁠1/2⁠ħ and no electric charge. 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. The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun.

The first evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay leading to the discovery of the electron neutrino.

Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations.

Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavor state is a linear combination of the three distinct mass eigenstates.

Majorana neutrinos would have the property that the neutrino and antineutrino could be distinguished only by chirality; what experiments observe as a difference between the neutrino and antineutrino could simply be due to one particle with two possible chiralities. The cosmic neutrino background is also a probe of whether neutrinos are Majorana particles, since there should be a different number of cosmic neutrinos detected in either the Dirac or Majorana case.

There are three known types (flavors) of neutrinos: electron neutrino νe, muon neutrino νμ, and tau neutrino ντ, named after their partner leptons in the Standard Model (see table at right).

International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain the neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavors.

Other efforts search for evidence of a sterile neutrino – a fourth neutrino flavor that would not interact with matter like the three known neutrino flavors.

The experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavor states with neutrino mass states (analogously to CKM mixing), requires neutrinos to have nonzero masses.

Neutrino oscillation – Phenomenon in which a neutrino changes lepton flavor as it travels

Quick Facts

• Weak interactions create neutrinos in one of three leptonic flavors: electron neutrino, νe; muon neutrino, νμ and tau neutrino, ντ.
• For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino.
• Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations.
• 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.
• There are three known types (flavors) of neutrinos: electron neutrino νe, muon neutrino νμ, and tau neutrino ντ, named after their partner leptons in the Standard Model (see table at right).

Source material: Wikipedia - "Neutrino". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.