Fermilab recently announced that its 10-years-worth of efforts have yielded first results with its NOvA experiment successfully detecting neutrinos fired from 500 miles away. The experiment also confirmed the oscillations of neutrinos as they travelled with only 33 being detected out of the 201 muon neutrinos fired by Fermilab.
Scientists describe neutrinos as the enigmatic ghosts of the subatomic world and one of the main reasons behind this is that they rarely interact. Neutrinos are of three types: one being the electron type neutrino while the other two are the muon neutrinos and the tau neutrinos.
As far as scientific notations go, scientists use the lower case Greek letter nu to indicate a neutrino, with a subscript to identify its type. One of the properties of a neutrino is that it seems to remember its origin.
So how where neutrinos discovered? The idea of neutrinos was proposed back in 1930 when scientists were baffled about a problem they had with a kind of radioactive decay called beta decay. Beta decay occurs when an atomic nucleus emits an electron and the nucleus changes from one element to another. The emitted electrons were called beta particles. The problem that baffled scientists was that when the energy of the original nucleus was compared to the combined energy of the daughter nucleus and electron, there was a mismatch.
After the decay, energy was missing. If we go by one of the principles of physics that energy can’t be created or destroyed, the loss of energy in case of beta decay was something that refutes the laws of physics. Scientists thought of many possibilities including the one where in the quantum world energy could disappear.
However, in 1930, physicist Wolfgang Pauli proposed that the reason the energy was missing was that because there was an unobserved particle emitted during the beta decay. These particles have come to be called neutrinos.
Fun Fact: The Sun is one of the most biggest sources of neutrinos that we know of and even though it is about 93 million miles away, somewhere around 650 trillion neutrinos from the Sun hit you every second!!!
So the next logical question is how did scientists detect them? In 1955, physicists put a neutrino detector near a nuclear power plant and turned the plant on and off to observe any difference in the amount of neutrinos detected. On, and the detector saw more neutrinos. Off, and it saw less. Scientists have been studying neutrinos since then and a lot of progress has been made in this field. We have seen neutrinos that were created in distant supernovae and even observed neutrinos emitted from the radioactive material inside the Earth.
Fun Fact: Neutrinos are unique for the fact that they can actually change their identity.
Back in 1962 during an experiment, neutrinos were created in tandem with a muon. If one of those neutrinos was then collided into an atomic nucleus, only muons were generated in the collision – no electrons, no tau particles. This observation led to a Nobel Prize in physics in 1988.
Raymond Davis, a chemist by trade knew that neutrinos could interact with chlorine and make argon. One of the other properties about neutrinos is that they interact very weakly and to ensure that neutrinos interact with chlorine to make argon, he would need a huge number of chlorine atoms. This is he did! He took a huge vat and filled it with one hundred thousand gallons of liquid of perchloroethylene – dry cleaning – fluid.
According to his calculations, for every week of operation that he could expect to create ten atoms of argon. The number may sound dramatic considering that the vat contained about 9 million-million, million-million, million atoms of chlorine and just ten of them should converted to argon! This may sound impossible, yet it turned out that Davis and his experiment were just about right.
Instead of 10, Davis ended up with just three atoms of argon and the easiest explanation was that either the prediction or the measurement was wrong. However many follow-on experiments confirmed these results and this came to be called the solar neutrino deficit.
Another source of neutrinos is the cosmic rays, which is a constant pelting of high energy protons from the deepest of space slamming into the atmosphere. Scientists have revealed through their analysis of how cosmic rays interact that each electron-type neutrino should be accompanied by two muon-type neutrinos.
While the solar neutrino deficit could have been due to improper measurement or calculation, it is very difficult to imagine how neutrinos from cosmic rays could occur in any ratio other than 1 electron type to 2 muon type. However, the measurements didn’t really add up. Different experiments observed different results, but it was generally true that there were fewer muon neutrinos than expected. Another mystery had appeared, this one called the atmospheric neutrino problem.
An Italian-born physicist Bruno Pontecorvo hypothesized that it would be possible for the different flavors of neutrinos to oscillate into one another. If Pontecorvo was true, then electron neutrinos could gradually morph into muon neutrinos and then back again to electron neutrinos. At the time only one kind of neutrinos existed, so the proposal was that neutrinos were oscillating into their antimatter equivalents. However, with current knowledge, we know that neutrino oscillation is between the three distinct types.
The first ever time the neutrino oscillation was proved was in 1998 using the SuperKamiokande experiment in Japan. The experiment consists of a huge detector in an underground cavern that is filled with 50,000 tons of water, surrounded by detectors called phototubes.
In rare instances neutrinos would interact in the water and give off a blink of light. Using that blink of light scientists could identify the trajectory of the neutrino.
By separating out neutrinos created in the atmosphere directly above (which was about 12 miles away) from neutrinos created on the other side of the Earth (which was about 8,000 miles away), they proved that it was the neutrinos that travelled a large distance that had changed identity the most.
The SNO experiment in a mine deep under Sudbury Ontario actually proved that Neutrinos were changing their identity.
The Fermilab accelerator near Chicago, the CERN accelerator near Geneva, Switzerland and the KEK accelerator in Japan all fire beams of neutrinos through the Earth to targets hundreds of miles away and they are all looking unravel the mysteries of neutrinos as of now.