Associate Professor, Yoshinari Hayato
Yumiko Takenaga
(Kamioka Observatory, ICRR,
The University of Tokyo)



Is is thanks to neutrinos
we can exist?

When you hear, “maybe it could have been a universe where none of our life, stars, or matter existed.”, do you think “that’s ridiculous”?
How did the universe become as full of matter as it is today?
The key to solving this mystery may lie in neutrinos, which Hyper-Kamiokande will observe.

The present universe is made of particles, not antiparticles.

Is a “universe without matter” more natural?

There were the same number of “antiparticles” that annihilated with the particles.

Every particle has its counterpart, the antiparticle. The present universe is made up of particles, and antiparticles are almost non-existent. On rare occasions, antiparticles are born and fly in space, but they disappear as soon as they hit matter such as the earth. This is because particles and antiparticles annihilate each other when they meet.

However, it is thought that at the beginning of the universe, particles and antiparticles existed in equal numbers. They soon annihilated each other repeatedly, but somehow, only particles remained. It seems unnatural, as if “1 billion – 1 billion = 1”.One of the reasons for this is thought to be related to a slight “difference” between particles and antiparticles, called “CP violation.”

The key is the “difference” between neutrinos and antineutrinos

The “C” in “CP symmetry” is a charge. Particle-antiparticle pairs include, for example, a positron (+) for an electron (-) and an anti-up quark (-) for an up quark (+). Each has an opposite charge, but the other properties are the same. Having “C symmetry” means that the same physical phenomenon will occur with the same probability even if the charges are opposite.

The “P” in “CP symmetry” is a parity. For example, when a pitching machine throws a curve with the same force and rotation under conditions where there is no effect of wind, etc., the degree of the curve should be the same on the left and right sides. Nevertheless, if, for example, the left side of the curve is bent at a greater angle than the right side, this is called “P-asymmetry breaking” (in reality, however, the left/right, up/down, front/back are all reversed).

Under the same conditions, the movement should be the same on both sides. ……

In the case of quarks, which are the main elementary particles that make up matter, CP symmetry breaking has already been confirmed. However, this alone is only one trillionth of the difference needed to create the current universe. This is where neutrinos come into focus. Thanks to the existence of a large “CP violation” between neutrinos and antineutrinos, the present matter-centered universe could have been created. So what is the difference? It is the “oscillation probability” of neutrino oscillation, which was the reason for Dr. Takaaki Kajita winning the Nobel Prize in Physics.

“Neutrino oscillations”, transforming back and forth between three types

Elementary particle “Neutrino”

Neutrinos are considered to be the “smallest unit of matter,” smaller than the protons and neutrons in the nucleus of an atom, not to mention molecules and atoms. Seventeen subatomic particles have been discovered by 2022, and neutrinos are one of them. Neutrinos are classified into a group called “leptons.” They have similar properties to electrons, which are also leptons, but are very different in that they have no electric charge and are so light that they were long believed to be massless (why they are so light is one of the remaining mysteries).

Neutrinos are both particles and “waves”

To better understand neutrino oscillations, it is first necessary to understand the properties of a “quantum.” It is hard to imagine in our everyday world, but all elementary particles are “quantum,” having both wave and particle properties. For example, atmospheric neutrinos are created when particles falling from space (cosmic rays) collide with particles in the atmosphere, but when they fall to earth from there, they are transmitted as “waves.”

The three waves composition produces a “beat.”

Curiously, the neutrinos travel in three overlapping waves, each of which has a different mass and, therefore, a different frequency (wavelength). This causes the waves to shift slightly as they fly, resulting in a “beat” effect. For example, when two tuning forks are slightly out of pitch, the sound waves interfere with each other and produce a “whirring” sound, and the same thing happens with neutrino waves.

This beat causes neutrinos to move back and forth between three types of neutrinos called “flavors”: electron neutrinos, muon neutrinos, and tau neutrinos. When they are born in the atmosphere, they are muon neutrinos, but as they fly long distances, they repeatedly become tau neutrinos and then back to muon neutrinos again. The name “neutrino oscillation” is derived from the fact that the neutrino transforms in a shaking manner.

Depending on how the three waves “mix,” the type of neutrino changes!

Will this mystery be solved in the near future?

Investigate the difference between neutrino and antineutrino “oscillations”

Antineutrinos “oscillate” as well, but by examining the difference from the neutrino case, “CP symmetry breaking” can be verified. The Super-Kamiokande, which is currently in operation, is also conducting the T2K experiment using an artificial neutrino beam from the J-PARC accelerator at Tokai-mura, Naka-gun, Ibaraki Prefecture, 295 km away. T2K has already reported the results of the CP violation study. The Hyper-Kamiokande project plans to increase the intensity of this J-PARC beam by a factor of 2.5 to reduce errors due to insufficient data and to increase the number of measurements to further improve reliability.

Possible “discovery” in the 2030s.

Based on the results of the experiment so far, it is expected that the CP symmetry breaking, or the difference between neutrinos and antineutrinos, is “close to maximal.” If the breaking is maximal, it will be discovered about three years after the start of the Hyper-Kamiokande experiment, and we can measure the size of the breaking parameter. Even if it is not the maximal, it is expected to be possible to determine whether the CP symmetry is broken or not in 10 years.

The day of discovering “CP symmetry breaking” is not so far. However, research will continue in 2030 and 2040. When those who are interested in this field join Hyper-Kamiokande in the future, there should still be many mysteries left to be solved. Please join us in unraveling the secrets of the universe and elementary particles.

The current best fit parameter is minus 113 degree.

The closer to 90 degrees (or minus 90 degrees),
the greater the “violation”.

CP phase angle, a parameter that indicates the difference between neutrinos and antineutrinos, or the magnitude of “CP symmetry breaking” (not related to the angle in reality). The above is as of 2020. What will it be in the next announcement?