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Neutrinos, a Ghost in the Particles.

29 Aug, 2020 | Prabin Pyakurel |

Neutrinos, a Ghost in the Particles.

The Strange Ghost.

Have you ever imagined a ghost-like thing passing through your body without ever being noticed by you? There are trillions of particles that pass through your body and you never know it in your lifetime. You might have heard about atoms in school days, probably you might have also heard about Electron, Proton, and Neutron. However, the regular joe will never know about the particles beyond the level of these three. There exist a whole bunch of sub-atomic particles that physicists describe in the chart called “ Standard Model”.

Standard Model, Credits: Daniel Dominguez/CERN,
Neutrinos are denoted by “v”

One of the most strange and useful particles are Neutrinos. They are elusive particles. About 100 trillion of them pass through your body every second and also pass unhinged through the earth to get out of the other side back to space without a trace. If you are lucky, may be one of these will ever interact with your body in your lifetime. The question then is: how do you know something exists when it looks like nothing, weighs nothing and interacts with nothing. 

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Initial Discovery

It all started one fine day, like all good stories do when Rutherford discovered alpha and beta rays during the decay of uranium nuclei. ‘Alpha rays’ is a fancy name for a particle with two neutrons and two protons. These alpha rays were emitted along with a daughter nucleus whose mass is less than the original nucleus. Also, these two emitted bodies had the same kinetic energy. The nucleus could also decay in another way that is the beta decay which emits an electron and daughter nuclei. At least this was how it was thought to be: a two-product process giving out electrons and daughter nuclei with equal kinetic energy.

However, by 1920 it became clear that electrons released during beta decay didn’t all have the same energy which meant that the law of conservation of energy(the holy grail of physics) would be violated. Physicists wouldn’t stand and stare as this little thing broke down such an important law; so Wolfgang Pauli suggested that the beta decay must have three products instead of two. The third one being a light neutral particle which was later named neutrino by Enrico Fermi. Introduction of neutrino meaning “the little one” would solve the problem of missing energy. Fermi proposed the following basic underlying process for the creation of a neutrino. A neutron would decay into a proton, an electron, and a neutrino during beta decay. The proton would remain in the nucleus giving out electron and neutrino with some kinetic energy.

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A neutron decays into Proton, Electron and Neutrino, Picture by Prabin Pyakurel

The final discovery.

However, this was only in theory, nobody had observed this stranger in physics “the neutrino” until Cowan and Reines in 1956  detected antineutrinos* that were emitted by the savannah river nuclear plant. For this discovery, Reines was awarded the Nobel prize in 1995. Later, two more varieties of neutrinos were discovered called the muon neutrino and the tau neutrino. Our original variety of neutrino is called the electron neutrino. Finally, we could detect these ghostly particles but there was another trick up neutrino’s sleeve. Very few of them would be detected even with large apparatus so we couldn’t observe and study them like we wanted to for quite some time.

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Super Kamiokande’s photosensors detect neutrinos as flashes of light. Credit : Asahi Shimbun/Getty

**Antineutrinos are neutrino anti-particles. Every matter particle has an anti-particle like proton has antiproton, the neutron has antineutron, an electron has positron and so on

Read the next article in the series regarding the Neutrinos. Why care about ghosts among the particles?

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