Elementary Particles and Types of Neutrinos
The universe is composed of atoms, which are made of an electron and a nucleus that contains protons and neutrons. If we go even smaller, we get to elementary particles, which cannot be further reduced.
Elementary particles can be grouped into bosons, which make up the fundamental forces of the universe, and fermions, which compose matter. Fermions include quarks and leptons. The combination of quarks make up protons and neutrons. Leptons include electrons and neutrinos. What makes quarks and leptons different is that quarks interact with the strong nuclear force, while leptons do not.
There are mainly 3 groups of neutrinos: electron neutrino, muon neutrino and the tau neutrino. Each of these have a corresponding anti-matter, which results in 6 types of neutrinos.
Properties and Where Neutrinos are From
Neutrinos are one of the smallest of the elementary particles, and are unaffected by magnetic fields. They rarely react with matter, which means that they carry information from a very long time ago safely, often since their birth. Hundreds of trillions of neutrinos are passing through our bodies every second, but we do not realise because they rarely react. This rarity of reaction also makes it very difficult to detect.
Neutrinos are born in various situations:
- Sun: It is made by the nuclear fusion reaction in the sun.
- Earth’s atmosphere: When cosmic rays hit the Earth’s atmosphere, a “air shower” of smaller particles, including neutrinos, is made.
- Supernova: Neutrinos are released in high-energy phenomena such as a supernova.
- Artificially induced: Neutrinos can be produced by colliding a proton using a particle accelerator.
Brief History of Neutrinos
In 1930, Austrian physicist Wolfgang Pauli first theorised the existence of neutrinos, when he was thinking about a problem about a phenomenon called beta decay.
In 1956, neutrinos that were created inside a nuclear reactor was first detected. During the 1970s, the first neutrinos made in the sun were detected.
In 1987, first neutrinos that were created outside of the solar system, in a supernova, was detected by Japanese researcher Masatoshi Koshiba. It was 160,000 light years away. He received the Nobel Prize in 2002.
In 1996, a facility made to detect neutrinos called the Super Kamiokande begun to run in Japan.
In 1998, the existence of phenomenon called neutrino oscillations (explained later) were proved experimentally using the Super Kamiokande. Japanese researcher Takaaki Kajita received the Nobel Prize in 2015.
In 2005, the construction of a large neutrino detector, Ice Cube, started in Antarctica.
Superkamiokande, Research Goals and Key Results
The Super Kamiokande (SK)
The Super Kamiokande is a large facility designed to detect neutrinos. It is located in Gifu-prefecture, Japan, and is 1000m underground. It is one of the leading facilities for neutrino research, with many international collaborators. It is a large tank filled with 50,000 tons of ultra pure water. The walls are covered with Photomultiplier Tubes (PMT), that detect the light that neutrinos release when they react with the nucleus of the water in the tank.
This facility can detect neutrinos made in the sun, in supernovas and in the atmosphere. There are facilities nearby that artificially induce neutrinos and shoot them in the direction of the SK.
Neutrinos have Mass: Conclusion drawn from the Discovery of Neutrino Oscillations
Researchers noticed that the actual recorded number of neutrinos that went through the Earth was less than what was expected. Researchers realised that this was due to a phenomenon called neutrino oscillations, where neutrinos change types as it moves.
From special relativity, in order for neutrinos to change types, it needs to move at a speed less than the speed of light. Things that move slower than light have mass, so it was shown that neutrinos have mass.
In 1970, a standard model for elementary particles was made. This assumed that neutrinos were massless, so the discovery that they do have mass was important. It is expected to lead to a deeper understanding of the universe.
Astronomical Objects and Phenomena
By analysing the neutrinos that react in facilities like SK, the inner workings of the sun and the supernova can be studied.
The heat generated in the core of the sun takes about 100,000 years to come to the surface, so the sun we see on Earth is actually the light 100,000 years old. However, since neutrinos rarely react with matter, it comes to Earth approximately 8 minutes after it is made in a nuclear fusion reaction. Researchers can gain real time information about the sun.
More about the energy of the cosmic rays can be known by analysing neutrinos made by the collision of cosmic rays with the Earth’s atmosphere.
Proton Decay and the Grand Unified Theory of Forces
Protons have been thought not to decay any further. However, neutrino detectors have the potential to record proton decay. If it is shown that protons decay, the elementary particles can be shown to be all connected to each other. This results in the establishment of the Grand Unified Theory, which can explain the Electromagnetic Force, the Weak Force and the Strong Force altogether.
There have been no records of proton decay yet, and this shows that the lifetime of protons is longer than 10^34 years (the Universe is currently 10^10 years old).
Further Research
The Hyper Kamiokande
Currently, the construction and operation of the Hyper Kamiokande is planned. Its volume is about 10 times larger than the SK. It is expected to start running in 2027. By using this facility, more neutrinos can be detected, resulting in faster data collection and more conclusions.
Researchers continue to detect neutrinos to find out more about:
- The properties of the neutrino
- Workings of astronomical objects and phenomena
- The beginning and workings of the universe
- Proton decay
[1] Kajita, Takaaki. (2016). “Neutrino – The Large Roles of Small Elementary Particles”. https://www.youtube.com/watch?v=aBGWT4D9VCU. Last Accessed: 2019/11/25
[2] Super Kamiokande. “Neutrino and Neutrino Oscillation – What are Neutrinos?”. http://www-sk.icrr.u-tokyo.ac.jp/sk/sk/neutrino.html. Last Accessed: 2019/11/25
[3] Super Kamiokande. “About Super Kamiokande – Research Themes”. http://www-sk.icrr.u-tokyo.ac.jp/sk/sk/research.html. Last Accessed: 2019/11/25
[4] Nakayama, Shoei. “Hyper Kamiokande”.https://indico.fnal.gov/event/9942/session/4/material/slides/0?contribId=53. Last Accessed: 2019/11/25
[5] Shiozawa, Masato. “Hyper Kamiokande Plan”. http://www.hyper-k.org/doc/Hyper-Kamiokande-Shiozawa-190117v0.2-web.pdf. Last Accessed: 2019/11/25
[6] Gallart, Silvia Bravo. “Why Neutrinos Matter”. https://www.youtube.com/watch?v=nkydJXigkRE. Last Accessed: 2019/11/25
[7] PBS Space Time. “Will a New Neutrino Change the Standard Model?”. https://www.youtube.com/watch?v=0mXW1zPlxEE&t=557s. Last Accessed: 2019/11/25
[8][IMAGE] Super Kamiokande. “Super-Kamiokande Photo Gallery”. http://www-sk.icrr.u-tokyo.ac.jp/sk/gallery/index-e.html. Last Accessed: 2019/11/25
[9][IMAGE] Wikipedia. “Standard Model of Elementary Particles”. https://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles_Anti.svg. Last Accessed: 2019/11/25.
[10][IMAGE] Super Kamiokande. “T2K Experiment”. http://www-sk.icrr.u-tokyo.ac.jp/sk/sk/t2k.html. Last Accessed: 2019/11/25.
English 
Japanese 