We would like to introduce “neutrino oscillation,” the phenomenon discovered in 1998, which led to Prof. Takaaki Kajita’s Nobel Prize.
Neutrinos have no electric charge and can pass through anything without interaction
Neutrinos, as the name “”neutro”” (neutral) + “”ino”” (small particle) implies, are subatomic particles that do not have an electric charge.
All matter in the universe is made up of quarks and leptons, which are elementary particles. For example, a hydrogen atom is formed by combining a proton and an electron. A proton is made from three quarks, and an electron is one of the lepton.
All matter in the universe consists of quarks and leptons.
Quarks and leptons each have their corresponding particles that differ by one electric charge. For example, an up quark has a charge of +2/3, and a down quark has a charge of -1/3.
Neutrinos correspond to electron families. The electron’s charge is -1, and the neutrino’s charge is “0,” which differs by one. Neutrinos have no electric charge.
Neutrinos are among the most abundant subatomic particles in the universe, flying around us at the speed of light and passing through our bodies at several hundred trillion per second.
However, neutrinos have no electric charge and hardly interact with other matter, so we do not feel them. They can easily penetrate even the earth. For this reason, neutrinos are extremely difficult to observe, and their properties have long been shrouded in mystery.
Neutrinos can pass through anything, but because of the large number of neutrinos raining down from space, they can interact with matter on rare occasions. Super-Kamiokande detects neutrino interactions in a large water tank.
One particular flavor neutrino is a mixture of three different mass states.
There are three “flavors” of neutrinos: electron neutrino, muon neutrino and tau neutrino. On the other hand, there are three “mass states” of neutrinos: neutrino1, neutrino2 and neutrino3 with mass m1, m2 and m3.
Neutrinos are classified according to “flavor” and “mass”.
This classification by flavor and mass are not independent. When we choose one flavor, it is a “mixture of neutrinos of different masses.”
So, for example, when we say “electron neutrino,” it is a mixture of neutrino 1, neutrino 2, and neutrino 3. This is called “neutrino mixing.”
Neutrinos based on “flavor” and “mass” mix with each other.
Discover of neutrino oscillations
The groundbreaking discovery, which required a modification
of the Standard Theory of elementary particles
Neutrinos have properties of both “particles” and “waves.” Therefore, neutrino 1, neutrino 2, and neutrino 3 have different masses and propagate through space as “waves” with different frequencies. The neutrino flavor is a superposition of waves at each mass state. When the neutrino travels through space, the phase of the waves changes, and the type of flavor changes.
This phenomenon is called “neutrino oscillation.” Neutrino oscillation occurs when neutrinos have mass and non-zero neutrino mixing.
Neutrino oscillations were discovered by the Super-Kamiokande experiment in 1998. Observations of muon neutrinos in “atmospheric neutrinos,” produced when cosmic rays flying from space collide with the atmosphere, revealed that the number of muon neutrinos coming from the other side of the earth was only half of those coming from above. This is because the muon neutrinos were transformed into tau neutrinos during their passage through the earth.
The discovery of neutrino oscillations proved that neutrinos have a small amount of mass. In the Standard Model, which explains the framework of elementary particles, neutrino mass was thought to be zero, but this epoch-making discovery forced a revision of that theory.
Waves of neutrinos with slightly different frequencies compose and produce the beat, and the neutrino flavors change as they travel through space.
Indicates the existence of an “unexplained theory” beyond the Standard Model.
Neutrino oscillations have been observed in solar neutrinos and artificial neutrino beams.
Neutrino mixing is described with three mixing angles θ12, θ23, θ13, and a CP phase parameter. From previous studies of neutrino oscillations, the squared differences m12-m22 and m22-m32 between the three mixing angles and the neutrino masses have been measured.
Remaining unanswered questions are the parameters of the CP phase, the order of the neutrino masses, and the respective values of the neutrino masses. The question of why neutrino masses are so small, less than one-millionth of the mass of an electron or quark, is also a mystery.
The Standard Model, which explains how elementary particles work, is believed to have been completed with the discovery of the Higgs boson. However, the masses and mixtures of neutrinos found so far are somehow very different from those of quarks. The Standard Model is not a complete theory that explains all physical phenomena, suggesting that there may be unexplained particle theories beyond the Standard Model. The Unified Theory, for example, is considered to be a candidate for such a theory.
“The discovery of atmospheric neutrino oscillations in Super-Kamiokande.
The number of muon neutrinos flying through the earth was only half of those from above the detector.”
Learn about Neutrinos