tauoneutriino

The tauoneutriino, commonly called the tau neutrino (ντ), is a lepton with zero electric charge and represents the third-generation neutrino. It interacts via the weak force, couples to the W± bosons, and is produced in processes involving the tau lepton and in decays of heavy-flavor hadrons. In the Standard Model, it is the neutrino partner of the tau lepton and participates in lepton flavor conservation within purely Standard Model interactions.

Mass and mixing: The exact mass of the tau neutrino is unknown. Experiments place an upper limit on its direct mass at the tens of MeV scale, while the three-flavor oscillation framework implies that observed flavor states are superpositions of three mass eigenstates with very small splittings, described by the neutrino mixing (PMNS) matrix.

Production and detection: ντ is produced when a tau lepton is created and subsequently decays, or in decays of heavy hadrons and in atmospheric or accelerator neutrino beams. Detection relies on charged-current interactions that produce a tau lepton, which then decays rapidly, making ντ events challenging to identify. Notable experiments include DONuT, which provided direct evidence for ντ, OPERA, which observed νμ→ντ appearance, and confirmations from Super-Kamiokande and IceCube in atmospheric neutrinos.

Flavor oscillations and significance: The existence of ντ confirms three-flavor neutrino mixing. It is used in tests of lepton flavor universality, investigations of neutrino mass ordering, and searches for CP violation in the lepton sector. Ongoing experiments aim to improve measurements of ντ properties and constrain its mass further.