What is an elementary particle?

The Super-Kamiokande observes “neutrinos.”
You may have heard the name “neutrino,”
but may not understand it in detail.
We will introduce topics related to neutrinos briefly.



We consist of
elementary particles.

All matter such as we living things
and the materials around us,
as well as water, air, stars, and the universe,
are made up of elementary particles.

10-7 m(0.0000001m)


For example, most of the human body is water (H2O),
and a water molecule is composed of
two hydrogen atoms (H) and one oxygen atom (O).

10-10 m(0.0000000001m)


When you zoom into an atom, electrons can be seen rotating
around the nucleus. (Strictly speaking, rather than saying
“rotating,” electrons exist with a certain probability.)

10-14 m(0.00000000000001m)


The nucleus is basically made up of protons and neutrons.
(For light hydrogen, the nucleus is made of one proton.)

10-15 m(0.000000000000001m)

Protons / Neutrons

The proton and neutron consist of three elementary particles
called quarks. (The electron is also an elementary particle.)

10-18 m(0.000000000000000001m)

Elementary particles

An elementary particle is defined as a particle
that cannot be (or cannot be considered to be)
further broken down.

Thus far, 17 kinds of elementary particles have been found.
A neutrino is one of these elementary particles.

An elementary particle is defined as a particle that cannot be (or cannot be considered to be) further broken down. Thus far, 17 kinds of elementary particles have been found. A neutrino is one of these elementary particles.



Neutrino properties

When and where are
neutrinos generated.

  • When the universe was born.
  • Inside of the sun and stars
  • When a massive star explosions at its end
  • When cosmic-rays from the universe collide with the earth’s atmosphere
  • In addition, neutrinos are generated from the inside of the earth,
    nuclear reactors, accelerators, and even from bananas.

The universe is
full of neutrinos.

The universe is filled with an average of about 300 neutrinos per one cc. Please imagine that a 500 ml PET bottle contains about 150,000 neutrinos.

Neutrinos are light.

The neutrino is one million times lighter than an electron belonging to the same elementary particle family. Compared with a human body, the size of a neutrino is about one-trillionth of one-trillionth. For example, let’s imagine that the size of a neutrino is about 1 mm, then the human’s size becomes nearly equal to the galaxy.

Neutrinos are tiny.

As the name “neutral (neutral) small (ino) particle” implies, neutrinos are very tiny elementary particles. A neutrino is about 1/100 millionth, 1/100 millionth, and even 1/100 millionth the size of a human being. For example, if you imagine a neutrino to be about 1 mm in size, a human being would be about the size of a galaxy.

※The size of neutrino is assumed to be the interaction cross section of neutrino. (cross section of solar neutrino; 10-44cm2)

Neutrinos pass
through anything.

For example, at this very moment, several hundred trillion neutrinos born in the sun are passing through our bodies every second. Neutrinos from the sun barely collide once while passing through the earth tens of billions of times.

Is a neutrino
another kind of “light”?

It takes about 100,000 years for light inside the sun to move to the surface of the sun. On the other hand, a neutrino comes from the sun to the earth in about 8 minutes. Therefore, if looking at the sun with neutrinos, we can observe the inside of the sun in real-time. Since neutrinos pass through anything, there are things that can be seen only by neutrinos.

Three families
of Neutrino

Electrons and neutrinos are like relatives with or without electric charges. Corresponding to the types of electrons (electron, muon, and tau), there are three types of neutrinos: electron neutrino, muon neutrino, and tau neutrino. There are three types of antineutrinos as well.

Neutrinos change
their type

Neutrinos have the mysterious property of changing their types. For example, a muon neutrino changes to an electron-type neutrino while flying. Since neutrinos occasionally transform back and forth between the different types, such a phenomenon is called “neutrino oscillation.”



Is it thanks to neutrinos
that matter exists?

The birth of the universe.
From elementary particles to protons.
To hydrogen and carbon.

The early universe was a high-temperature soup of particles. As the soup cooled down, elementary particles changed to protons and further changed to light elements such as hydrogen. Such hydrogen gas gathered to form stars. Then, a “nuclear fusion reaction” took place inside the stars, which began to produce heavy elements such as carbon. Our earth, anything around us, and we ourselves are made of substances formed over the history of the universe.

Through the pair annihilation with “antimatter,” the universe may have become a “universe without matter.”

There is a big mystery in the universe’s development.
In the early universe, equal amounts of matter and antimatter were created. Since matter and antimatter annihilate when they are encountered, it could have been a universe where the stars and we didn’t exist. However, the present universe is full of matter such as stars and substances like human beings.

Is it thanks to neutrinos
that matter exists?

The key to solve this mystery may be neutrinos.
It is believed that the slight difference between neutrino oscillations and their antimatter antineutrino oscillations (called “CP violation”) may be a key to the asymmetry of matter. The T2K experiment, using Super-Kamiokande and J-PARC, has verified the difference between neutrino oscillations and antineutrino oscillations.