Chemistry and Physics

The Search For Vanishing Neutrinos

The construction of the Super-Kamiokande Neutrino Observatory
Sayumi Panditharatne

One night in March 1938 in Italy, a physicist named Ettore Majorana boarded a ship in Palermo to travel to Naples. Somewhere in between, he vanished under mysterious circumstances a few days after anyone had heard from him. Fittingly, one of his research interests, neutrinos, are able to disappear similarly, as Sayumi Panditharatne explains.

In the world of particle physics, neutrinos are one of the things that have baffled physicists for decades. It is believed that deciphering how these peculiar particles originated and understanding their purpose may re-write our understanding of the universe. But what are they really? 

Neutrinos are a type of fundamental matter particle that is extremely hard to detect, yet they are all around us. They are extremely light and travel very fast. Neutrinos are birthed following a process called particle decay; one well known process is called beta-decay. Atoms undergo nuclear decay, which is when an unstable atomic nucleus emits energy in the form of radiation, and transform into other particles, creating neutrinos in the process.  

Everything in the universe is made up of matter. Particles called antimatter particles are very much matters counterpart, except that their quantum properties such as spin and charge are reversed. Just like how matter can form atoms and molecules, antimatter can combine and form antiatoms and antimolecules. When matter and antimatter come in contact with each other, they annihilate each other and nothing but pure energy is left over.  

he was a very brilliant, but also a very, very weird man

Ettore Majorana was probably the greatest scientist that you’ve never heard of. He was a gifted scientist who was one of the leaders of particle physics in his time. A child prodigy with an exceptional ability for mental calculations, everyone who knew him agreed on two things: he was a very brilliant, but also a very, very weird man.

In a way, his sudden and unexplained disappearance at the age of 31 was sad but apt. He wrote small publications on a variety of physics and mathematics subjects. However, he never sought credit for his many theories. In one particular theory he proposed the existence of a particle that is its own antiparticle, which has since been termed ‘Majorana particles’. 

What solidifies this theory is a rare type of beta decay called double beta decay. In normal double beta decay, two neutrons in an atomic nucleus turn into two protons, emitting two electrons and two antineutrinos. On the other hand, neutrinoless double beta decay produces no neutrinos, making the neutrinos ‘disappear’. This would suggest that neutrinos and their antimatter counterparts, antineutrinos, are the same, making neutrinos Majorana particles.  

this could answer some big unknowns of the universe

If neutrinos are in fact Majorana particles, physicists say this could answer some big unknowns of the universe. According to the theory of the Big Bang which offers a speculation on how the universe was created, the Big Bang should have produced equal amounts of matter and antimatter. However, it has been observed that there is more matter than antimatter in the universe. The vanishing neutrinos could be the key to understanding why the universe contained an imbalance of matter and antimatter.  

The subterranean facility of Super-Kamiokande in Japan seems something out of this world. It is a facility constructed under Mount Ikeno near the city of Hida, designed to hunt neutrinos that resulted from a supernova – the explosion of a star. It is essentially a deep tank with 50,000 tons of ultra-pure water and the surface of the tall walls are lined with sensitive orb-like light detectors called photomultipliers. When a neutrino collides with an atom in a water molecule, a charged particle is ejected, which emits a very weak light called the Cherenkov light. This is detected by the photomultipliers.  

What’s more, Japan is set to build an even better facility – the Hyper-Kamiokande. This updated model with ultra-high sensitive photodetectors will enable physicists to observe the distinction between neutrinos and antineutrinos. Another major discovery they hope to make is the extremely rare and never before observed occurrence that is proton decay, to observe if this will also result in neutrinos. The Hyper-Kamiokande will contain a larger volume of water than its predecessor, thereby making the discovery more likely.  

The very thing that makes neutrinos hard to detect is what makes them travel so fast – they weigh almost nothing. Due to their ability to pass through anything, they have the potential to be applied in the speeding up of communications technology. They could also be utilised in locating mineral deposits in the earth by monitoring the neutrino beam, as neutrinos change their spin depending on how far they travel and how much matter they have passed through.

unlike the mystery of Ettore Majorana himself, the mystery of his namesake particle is something they have hope of solving

The hunt for elusive neutrinos is to be an experiment of high importance in the coming future, with its allocated funding increasing annually. Devices are being built in several countries which are more sensitive than the previous generation of technology. The focus will be on investigating the rare type of radioactive decay, determining the mass of the neutrino and explaining the matter-antimatter imbalance.  

Although there is no guarantee that these experiments will fulfil the quest to find out if neutrinos are Majorana particles, physicists are determined that unlike the mystery of Ettore Majorana himself, the mystery of his namesake particle is something they have hope of solving, perhaps one day revealing the secrets that Majorana may have kept to himself. 

Sayumi Panditharatne

Featured image courtesy of Amber Case via Flickr. Image license found here. No changes were made to this image.

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