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Neutrinos: The Origin
- JYP Admin
- Mar 30
- 4 min read
Author: Harsha Sah
While well-known telescopes like Hubble and James Webb are out in space, pointing into the depths of the universe, other ‘telescopes’ of similar importance are located underground, awaiting a lucky coincidence. Some of their names are IceCube, Fermilab, Large Hadron Collider, and Super-Kamiokande. What they discover is not as exciting as the Hubble’s Deep Field or Pillars of Creation images, as neither do they provide any stimulating visuals and nor do they incite the wonder which space does. However, what they discover plays an important role in proving the accuracy of our “well-established” physics laws and in cosmology, they hold answers to key questions related to the beginning of the universe and the puzzling dark matter.
The above-mentioned observatories have a very narrow and specialized field of interest: which is detecting a "ghost particle" called the neutrino. The idea of neutrinos was first postulated by Wolfgang Pauli (one of the pioneers of quantum physics) in a 1930 letter addressed to ‘radioactive ladies and gentlemen.’ This letter followed a very disturbing experimental observation which spelled trouble for physics. At that time, it had been observed that during a beta decay, the energy of the parent nucleus and the energy of the daughter nucleus and electron (the β particle) did not balance. This observation directly violated the law of conservation of energy (as well as of linear and angular momentum) which stated that total energy of a system always remained constant, implying that no energy could disappear. This law is one of the foundations of not just physics but has references to all disciplines of science and is considered to be fundamental in nature. If this law was proven to be wrong, then it wouldn’t have been too far stretched to say that we would have to re-write our scientific knowledge from scratch.
As Pauli wrote in his letter, I have hit upon a desperate remedy to save… the law of conservation of energy. His remedy was the proposal of a new particle called ‘neutron’ which was electrically neutral and had a minuscule mass. This was a bold idea considering that this particle had not been detected until then, as acknowledged by Pauli himself. However, its proposed properties justified why it could have gone undetected for so many years.
In 1932, James Chadwick discovered a relatively large (in atomic terms) electrically neutral particle in the atomic nucleus and named it neutron. The word ‘neutrino’ was then coined to Pauli's proposed particle, to distinguish it from Chadwick's neutrons. Enrico Fermi, who had been quite taken by Pauli’s idea from the beginning, later came up with the theory of beta decay which was the first one to incorporate neutrinos within the reaction.
Even though Pauli is quoted to have said, “I’ve done a terrible thing, I’ve invented a particle that cannot be detected,” by the early 1950s a few physicists were already working to detect these elusive particles. These were Fred Reines and his colleague Clyde Cowan (from Los Alamos Lab known for the Manhattan Project), who had planned to build a sensitive detector and set up an experiment to work around this puzzle which was declared to be impossible to solve by many scientists. Although the specific and technical details of their experiment are beyond the scope of this article, they made predictions using the Fermi’s beta decay equation and designed a prototype neutrino detector.
In the earliest plan to verify the existence of neutrinos, they had proposed to detonate an atomic bomb so that the chances of its direct detection increased drastically. Explosion of the bomb would produce an unnaturally high and intense burst of energy, released in the form of ionizing radiation, especially gamma, and all sorts of particles along with antineutrinos. Thankfully, they didn’t go forward with their original plan and carried out their experiment instead using a nuclear fission reactor. Some of the scientific reasons for this change was the advantage of repeating the experiment several times and in addition, the latter equipped with better shielding would have a lower background noise (due to cosmic rays) which would make it easier to isolate neutrinos, when detected.
After performing preliminary experiments, they moved to a better-shielded Savannah River Site (which ironically, was built to help in nuclear weapons’ production). However, in 1956, after months of data collection, analysis, and confirmation of the results, the Savannah Team announced that they had obtained the first experimental evidence of neutrinos’ existence. (This experiment was very aptly named Project Poltergeist.) The Cowan–Reines neutrino experiment was a turning point in neutrinos’ history as neutrinos’ existence was no longer in question and it proved all those who were convinced that neutrinos were untraceable, wrong. The Project Poltergeist concluded a chapter in particle physics, which Pauli had initiated 25 years ago.
However, for neutrinos, their experimental verification was just the beginning. Neutrinos are probably the most bizarre among the elementary particles. So many decades after the experimental verification of the existence of neutrinos, even today our understanding of neutrinos is incomplete, and a complete knowledge of phenomena like neutrino oscillations might provide us with deeper insights into physics beyond the Standard Model, and perhaps the ultimate nature of reality at extremely small length scales and extremely high energy scales.
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