Guide

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

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04

## Were all forces, electromagnetic force and gravity, originally one?

Do you know what kind of “forces” exist in this world?
Electromagnetic force and gravity are the most familiar, but there are also “weak forces” and “strong forces” in the microscopic world.
These four forces are said to have been one unified force when the universe was just born.
The “proton decay” that we will explore at Hyper-Kamiokande is evidence that the three forces other than gravity were originally unified.

Did the universe change and the forces divide?

### Four forces in our world

#### “Gravity,” which is actually weak, “Electromagnetic force,” which also works when we touch things.

It is known that there are “four forces” in this world. For example, “gravity” seems very strong, pulling our bodies to the earth, and creating stars, black holes, and other celestial bodies. However, it is actually the weakest of the “four forces.” This can be explained by the fact that even a tiny magnet sticking to a steel plate will not fall off. The tiny magnet’s magnetic force (electromagnetic force) is stronger than the gravitational force generated by the large earth.

The magnet does not fall because the magnetic force (electromagnetic force) is stronger than the gravity.

“Electromagnetic force” refers to electric power and magnetic force. Electromagnetic forces work when we touch things as well as when we use electrical appliances. Our bodies and materials are made up of numerous atoms, with electrons revolving around the nucleus at the center. When we touch something, negatively charged electrons repel each other, and there is actually a small distance between our hands and the material.

Electromagnetic forces are acting between the hand and the ball.

#### The “strong force” that makes up matter, the “weak force” that changes particles

Electromagnetic forces also work when nuclei (+) and electrons (-) are bound together to form an atom. The nucleus consists of protons (+) and neutrons without charge. The “strong force,” which is stronger than the repulsive electrical force between protons, prevents the nucleus from falling apart. This force connects subatomic particles called quarks together to form protons and neutrons. As the name implies, it is very strong force, but the force range is so short that we do not feel it in the everyday world.

The “weak force” is so named because it is weaker than the electromagnetic force. We usually don’t feel the weak force, but it plays an important role in transforming various particles into other particles, such as causing the beta decay of atomic nuclei.

These “forces” are actually carried by elementary particles. The electromagnetic force is transmitted by “photons (light),” the strong force by “gluons,” and the weak force by “W bosons” and “Z bosons.” (Gravity is thought to be transmitted by gravitons, but this has not yet been discovered.) The force works when matter particles throw such “force-carrying particles” at each other. The reason why the “weak force” is so weak can be attributed to the fact that the “W boson” and “Z boson” are very heavy (the weight of 80 to 90 protons) and difficult to throw.

[The “strong force” and the “weak force” work in the world of particles such as atoms.

### The “Grand Unified Theory” unifies the three forces

#### The universe cools down and the forces divided

Although the “four forces” seem to be completely different today, they were one force when the universe was just born. However, it is believed that as the very hot universe expanded and cooled, the force branched into three: first gravity, then the strong force, and then the electromagnetic force, and the weak force.

#### Separated into the four forces 0.00000000001 seconds after the birth of the universe

The weak force was separated 10-11 (0.00000000001) seconds after the birth of the universe. In no time at all, the force was divided into four.

※Example of index calculation: 102=100、103=1000、10-2=0.01、10-3=0.001

What does it mean that the forces split? The Electroweak Unification Theory clarified the branching of the electromagnetic force and the weak force. The “W bosons” and “Z bosons,” which were initially flying around at the speed of light like “photons,” are no longer free to move when the universe cooled and particles gained mass.

#### At that time, the universe was in an ultra-high-energy state equivalent to 1016 proton.

On the other hand, the separation between gravity and the strong force is still not clearly understood. The theory that unifies the three forces except gravity is called the Grand Unified Theory.

One way to verify the unification of the forces is to create the same high-energy state of the universe as it was at the time of unification. However, before the three forces diverged, the energy state of the universe was 1016 GeV (equivalent to about 1016 of proton energy) in a 10-32 m universe, which is much smaller than the 10-15 m size of a proton. Humankind can not yet create such an ultra-high energy state. So how can we prove this theory? The key is “proton decay,” which we aim to discover and observe at Hyper-Kamiokande.

### Search for “proton decay” to prove theory

#### Can Hyper-Kamiokande measure the lifetime of protons?

The proton decay phenomenon has never been found so far. In the Standard Theory of Particle Physics, the proton is considered unbreakable. However, the Grand Unified Theory predicts the decay of the proton. According to it, the average lifespan of a proton is more than 10 billion times the age of the current universe (about 13.8 billion years). This is a tremendous length of time, but if a sufficient number of protons are collected and observed, there is a possibility that the decay can be observed.

The Super-Kamiokande, which is currently in operation, has collected 22,500 tons of water (7.5×1033 protons) and observed it for more than 25 years, but no decay has yet been observed. This indicates that the proton lifetime is more than 1034 years, which is longer than originally assumed. It is expected to discover the proton decay in the Hyper-Kamiokande, which boasts a volume about 10 times that of the Super-Kamiokande and will have dramatically improved sensitivity.

#### “Proton Decay” changes the conventional concept of elementary particles

The observation of “proton decay” has another significant meaning. The elementary particles that make up matter are classified as “quarks” and “leptons.” It is expected that a new relationship between these two may be revealed. What is this relationship? It is possible that “quarks” and “leptons” are the same subatomic particle, but they look different.

If we find that quarks and leptons are actually the same elementary particle, it would be an important discovery that would greatly advance the Standard Model of elementary particles.

#### What is proton decay [Youtube]

The “u” and “d” represent quarks.

#### Quarks disappear, leptons remain.

This figure shows an example of proton decay. As you can easily see in the video, one of the three quarks is lost at the end. On the other hand, a positron, which is a lepton, is generated. It can be said that a lepton is coming out of a proton (quark).

#### A simpler rule should exist.

The Standard Theory is the foundation of particle physics, but there are many unexplained facts. Why are there 17 subatomic particles? Why do quarks and leptons both have three generations?” “Why does each particle’s mass have such a big difference?” Why do protons and electrons have exactly the same charge?” Aren’t there simpler rules that go beyond the standard theory?

No one knows the answer to when proton decay can be observed or whether it ever really breaks down in the first place. However, we cannot move forward without actually conducting the experiment. “I believe that protons decay,” says Dr. Masato Shiozawa, the co-spokesperson of Hyper-Kamiokande. We hope that readers will look forward to the day when proton decay is discovered.

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