Neutrinos: Small and Mysterious

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Issue 5, Volume 112

By Aryana Singh 

Chances are that you have heard of protons, electrons, and neutrons in your science classes. Yet many have not heard of neutrinos, the most common particle with mass in the entire universe. Neutrinos are infinitesimal particles that form when other particles undergo changes such as radioactive decay or the breaking and joining of nuclei. Neutrinos are neutral, meaning they lack a charge. They are also classified as elementary particles, ones that are not composed of other particles. A neutrino is so tiny that no one has been able to measure its mass, making it difficult to study. In fact, until the 1990s, they were thought to be massless. However, neutrinos may hold the answers to some of our universe’s biggest questions.

Neutrinos can travel at the speed of light and are unaffected by forces like magnetic fields due to their lack of charge. Thus, neutrinos have been traveling around the cosmos for around 15 billion years unchanged. Because they are formed when other particles undergo changes, neutrinos can be birthed from violent astronomical events, like exploding stars or the daily nuclear fusion in our Sun that provides us with sunlight.

Yet, studying and experimenting with neutrinos is extremely difficult due to their weak interactions with other particles. The IceCube Neutrino Observatory, in particular, currently studies neutrinos inside a block of ice in Antarctica. When neutrinos meet with atoms in the ice, they release small puffs of energy that help record data regarding the energy, direction, and even identity, of the neutrinos. This information can then be used to study astronomical events nearby and broaden our understanding of physics.

So far, studying neutrinos has helped scientists better understand why antimatter was destroyed during the Big Bang. Early in the Big Bang, there were equal amounts of matter and antimatter present. However, a small imbalance developed between the two, which may have caused matter to take over. Though this idea lacks solid evidence, scientists believe neutrinos may have played a role in our universe being made out of matter and not antimatter. All matter had an equivalent amount of antimatter, including neutrinos. Physicists speculate that neutrinos and antineutrinos were not as symmetrical, with more neutrinos than antineutrinos. The T2K Experiment in Japan made some early measurements that may later serve as evidence for this theory.

Long before scientists hypothesized such things, they first had to predict neutrinos’ existence. When conducting a beta decay experiment in 1931 in which extra protons and neutrons were transformed into the other, Austrian theoretical physicist Wolfgang Pauli noticed an unusual energy spectrum to which he proposed the existence of a light neutral particle. Two years later, Italian physicist Enrico Fermi developed a new theory of beta decay in which he coined the term “neutrino.” However, it was not until 1956 that neutrinos were officially discovered. American physicists Clyde L. Cowan and Frederick Reines used a nuclear reactor and a tank of water to prove the existence of neutrinos. The nuclear reactor produced neutrino fluxes that would, in turn, react with protons in the tank of water. These reactions were detected and recorded as part of the discovery, and Cowan and Reines received a Nobel Prize in physics for their groundbreaking discovery.

According to current physics and chemistry principles, there are three different types of neutrinos, each named for the particle they interact with: electron neutrinos, muon neutrinos, and tau neutrinos. A 1998 study led Japanese researchers to discover that neutrinos can change their type as they travel. However, physicists do not know why this is the case.

As if that were not enough to tackle, physicists have also proposed the existence of a fourth type called the sterile neutrino that does not interact with other matter at all. It is possible that the sterile neutrino could provide insight into why neutrinos change their type. That said, discovering the particle will prove to be the most challenging since it does not interact with any matter. Additionally, recent investigations at the Micro Booster Neutrino Experiment involving highly sensitive hardware have shown no signs of the sterile neutrino. But scientists are unconvinced, believing that it is out there and will continue to use large-scale experiments, such as IceCube, to find out all they can. Neutrino physics is a powerful, emerging field of science that could possibly change our entire understanding of the universe.