Neutrinos and Neutrino Oscillations
What is a Neutrino?
All matter in the universe is composed of particles referred to as quarks and leptons. For example, a hydrogen atom is composed of one proton comprising three quarks and one electron, which belongs to the lepton particle family. Neutrinos are neutral leptons. It is very difficult to detect neutrinos, due to the fact that they rarely interact with other matter. Therefore the characteristics of neutrinos were not well measured for a long time.
There are tremendously large numbers of neutrinos flying around all the time. Trillions of neutrinos are passing through your body every second. But you do not feel them, since they rarely interact with matter.
There exist three types of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. These classifications are referred to as a neutrino’s “flavor.”
Neutrinos are also classified in terms of “mass.” Neutrino1, neutrino2 and neutrino3 have a mass of: m1, m2, and m3, respectively. These classifications, in terms of flavor and mass, are mixed with each other (see the right figure). Flavor eigenstates and mass eigenstates cannot be determined at the same time.
To give an example, the electron neutrino is the mixing state of neutrino1, neutrino2 and neutrino3. It is called neutrino mixing.
Neutrinos exhibit the properties of a particle as well as a wave. Therefore, neutrino1, neutrino2 and neutrino3, each with different mass eigenstates, travel through space as waves that have a different frequency. The flavor of a neutrino is determined as a superposition of the mass eigenstates. The type of the flavor oscillates, because the phase of the wave changes (see the right figure).
This phenomenon is called neutrino oscillation. Neutrino oscillation occurs when neutrinos have mass and non-zero mixing.
Neutrino oscillation was discovered by the Super-Kamiokande experiment in 1998. The atmospheric muon neutrinos generated by the collision between cosmic rays and the atmosphere on the earth were observed, the number of the upward going neutrinos was only half of the number of the down going neutrinos. This is because the muon neutrinos passing through the earth turn into tau neutrinos.
The discovery of neutrino oscillation verified that neutrinos have a small but finite mass. As until that point, it was thought that the mass of neutrinos was zero, this discovery was an epoch-making observation and required the framework of elementary particle theory to pass beyond the Standard Model.
Neutrino oscillations were later observed using solar neutrinos and the artificial neutrino beam.
Neutrino mixing is expressed in terms of three mixing angles: θ12, θ23 and θ13, and the parameter of CP violation. Neutrino oscillation experiments have previously measured three mixing angles and their squared mass differences: m12-m22, m22-m32.
The unsolved problems that remain are those of the CP violation parameter, the neutrino mass hierarchy and mass value of each neutrino. The reason why neutrino mass is more than one million times smaller than the mass of electron or quarks is shrouded in mystery.
The Standard Model explaining the framework of elementary particles seems to be completed by the discovery of the Higgs particle. However, the neutrino mass or neutrino mixing parameters measured by neutrino experiments show, for some reason, a large difference to those of quarks. That appears to lead to a collapse of the Standard Model. That is, the Standard Model is not a complete model to explain all physical phenomena and the existence of unkonwn theories beyond the Standard Model is indicated. The unified theory is thought to be one of candidates for these thories. Neutrino oscillation experiments are expected to unlock the clues to resolving the unified theory or as yet unknown theories of elementary particles.