Neutrino oscillation
Introduction
Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton flavor (electron, muon, or tau) can later be measured to have a different flavor. The discovery of this effect, which requires neutrinos to have mass, led to the modification of the Standard Model of particle physics. It also confirmed the discovery of the phenomenon of neutrino oscillation, which was awarded the Nobel Prize in Physics in 2015.
History
The phenomenon of neutrino oscillation was first proposed by Ziro Maki, Masami Nakagawa, and Shoichi Sakata in 1962, to explain the results of the experiments of Leon Lederman, Melvin Schwartz and Jack Steinberger. These experiments had shown that more than one type of neutrino exists by detecting interactions of the muon neutrino (already hypothesised with the name 'neutrino' at the time), which earned them the 1988 Nobel Prize.
Theory
Neutrino oscillation is of theoretical interest because it is sensitive to the square of the neutrino masses and to the elements of the Pontecorvo–Maki–Nakagawa–Sakata matrix. The probability of a neutrino's changing into another is governed by the differences of the squares of the masses of the neutrinos, the mixing angles of the PMNS matrix, and the distance the neutrino travels from the point of creation.
Experimental Observations
The first experimental observation of neutrino oscillation came from a large scientific collaboration at the Super-Kamiokande detector in Japan, which showed that atmospheric neutrinos produced in the atmosphere by cosmic ray interactions were disappearing in a manner consistent with neutrino oscillation.
Implications
The discovery of neutrino oscillation implies that the neutrino has a non-zero mass, since the phenomenon is dependent on the differences of the squares of the masses of the neutrinos. This is inconsistent with the Standard Model of particle physics, in which neutrinos are massless. As such, the observation of neutrino oscillation has led to modifications of the Standard Model.
Future Research
Ongoing research in neutrino oscillation focuses on several major areas. These include precise measurements of the mixing angles and mass differences, the determination of the mass hierarchy, the search for CP violation in the lepton sector, and the search for sterile neutrinos.