Neutrinos are the ghosts of the quantum world. They were named so because they almost do not interact with matter at all. Many trillions of neutrinos pass through you and the Earth every second. The neutrino has no charge and it is a fermionic particle, meaning it has a half integer spin. The neutrino is also the least massive of all the particles in the standard model, its mass is approximately 2 x 10-37 kg. As the particle is so light, it was initially believed that the particle had no mass.
Postulation and Detection
The neutrino was first theorized by Wolfgang Pauli in 1930 to solve a problem involving radioactive decay. The process was called the Beta decay. In the process, an atomic nuclei breaks down into a lighter nuclei which occurs due to the release of an electron. The law of conservation of energy is one of the most fundamental laws in the universe but in the case of Beta Decay something mysterious was going on. The energy of the initial atom was not equal to the combined energy of the initial atom and the released electron. It seemed that there was an exception to the conservation of energy in the quantum world. To solve this problem, Wolfgang Pauli proposed an undiscovered particle, which is today called the neutrino. The Neutrino accounted for the lost energy.
How was this particle detected? Neutrinos experience only the gravitational force and the weak nuclear force. As the weak nuclear force is much stronger than gravity, neutrinos experience the weak force to a far greater extent. As we are talking about the weak nuclear forces, we are obviously talking about nuclear reactions. The largest nuclear reactor, the sun, is also the largest source of neutrinos. It is constantly bombarding the Earth with trillions and trillions of neutrinos, but the neutrinos from the sun are hard to detect. The neutrino was first detected in 1955 in a nuclear reactor on Earth.
How are they detected today?
There are many Neutrino detectors in the world. the largest of these detectors is the Super Kamiokande Neutrino Detector in Japan. it is a huge underground tank which contains more than 50,000 tons of water. Now, we know that neutrinos interact via the weak nuclear force. While passing through the 50,000 tons of water, occasionally some neutrino interacts with some molecule of water. The neutrino emits a weak nuclear force carrying particle (the Z or W Boson) which then collides with the water molecule. the water molecule breaks and this reults in the formation of different particles which interact with the surrounding molecules. These interactions are then detected by the detectors.
Neutrinos are one of the most fascinating particles, but the story does not end here. There are 3 types of Neutrinos and they have some mind boggling properties which are discussed in another article.