Invisible, elusive and almost massless — neutrinos are some of the most mysterious particles in the universe. In this post, Alex uncovers what makes them so unusual, how they’re created, and why they matter to modern physics.
Neutrinos – despite many people not even knowing they exist – are the most abundant particles in the universe. Trillions pass through your body every second as you read this. So how do we know they exist, and what even are they?
Theorisation
In the 1920s, at the dawn of nuclear physics, scientists explored a form of radioactive decay known as beta decay. In beta decay (β⁻), a neutron turns into a proton and an electron (plus an antineutrino) within the nucleus. This at first seems to make little sense as we’ve all been taught that neutrons and protons have the same mass, but they don’t. In fact, a neutron is around 0.1% heavier than a proton. This was initially explained by the release of an electron, which seemed to solve this problem of mass. However, there was another problem: energy. The first law of thermodynamics, the basic standard of all physics, states that energy cannot be created or destroyed. This was a problem for early nuclear physicists as, in beta decay, the kinetic energy of the electron released was less than the energy released from the decay, meaning energy had, somewhere, been lost.
Enter Wolfgang Pauli, Austrian-born Nobel-winning quantum physicist. In 1930, he suggested in a letter that an almost undetectable, neutral, very low-mass particle, which he called the ‘neutron’, carried away the missing energy. The issue seemed to have been resolved theoretically and was even used in the famous Fermi theory of beta decay in 1934, but there was still no experimental evidence supporting Pauli’s theory.
Discovery
Following a name-change in 1932 from the ‘neutron’ to the ‘neutrino’ (meaning ‘little neutral particle’), the quest had begun to prove its existence. In the 1930s and ‘40s, the outbreak of WW2 saw huge leaps in the understanding of nuclear physics, unfortunately used for the making of weapons of mass destruction rather than scientific understanding. However, the vast number of neutrinos that were theorised to have been released in the processes of nuclear fission and other radioactive decays made detecting neutrinos more possible than ever before. It was here that an idea was born by physicist Fred Reines in 1951. If a nuclear bomb could be detonated in close proximity to a detector, then the detection of a neutrino would be almost certain. The reaction they planned to use was an inverse beta decay, slightly different to its counterpart but, more importantly, efficient enough that it could be conducted with only a nuclear reactor rather than a bomb. The experiment was conducted in 1953, but the results were almost indistinguishable due to the impact of cosmic rays from space. Thus, it was decided to conduct the experiment underground instead. After moving the setup underground and placing it close to the Savannah River nuclear reactor and developing a new, more sensitive sensor, in 1956, the mysterious particle was detected and the neutrino had finally been discovered.
So, what are they anyway?
Today, neutrinos, including the electron neutrino, are fundamental to our understanding of the universe. They are fundamental particles, like quarks and electrons, meaning that, without bringing in complications like string theory or quantum field theory, they are completely indivisible. Just like these other particles, they also have antimatter counterparts, such as antineutrinos, which are produced in place of regular neutrinos in some beta decay reactions.
Similarly to the more commonly known electron, neutrinos are categorised as leptons in the Standard Model (our current best understanding of particle physics). This means that they only interact with the weak nuclear force responsible for beta decay, and gravity, not even interacting with light! They come in three ‘flavours’ or ‘generations’ – the electron, muon and tau neutrinos, discovered in 1956, 1962 and 2000, respectively. These three neutrinos differ in mass, have no electric charge, and are considered stable. However, neutrinos have a unique property where they can oscillate between flavours. This oscillation is still being researched today and cannot be easily explained here without pages of equations and years of research understanding. So, put simply, neutrinos of different flavours can change into other flavours, seemingly violating several laws of our current physics models.
The Future
Despite them seemingly having no use due to their lack of interactions with other particles, there are still various aspects of neutrinos that make them useful for potential future technologies, whether it be in astronomy, geology or communications. As they are produced from radioactive decay, detectors can be used to check for any nuclear activity that is not allowed by the International Atomic Energy Agency (IAEA), banned under the Nuclear Non-Proliferation Treaty, decreasing the threat of nuclear war on Earth. As well as this, neutrinos have a property where they change their flavour as they pass through certain materials, creating the possibility of essentially ‘x-raying’ the Earth in search for mineral deposits, improving our geological understanding of our planet. Furthermore, unlike radio waves, neutrinos can carry information through the entire planet without being absorbed as they interact so little. This means that they could limit or even entirely eliminate the need for complicated, expensive communication satellites and other infrastructure in the future. Finally, it has been theorised that certain types of neutrinos are produced from the decay of the elusive ‘dark matter’, which is one of the biggest mysteries in science today. Through the detection and analysis of these neutrinos, there is a possibility that we could finally solve this decades-old problem.
References:
https://www.energy.gov/science/doe-explainsneutrinos
https://t2k-experiment.org/neutrinos/a-brief-history/
https://www.britannica.com/biography/Wolfgang-Pauli
https://www.fnal.gov/pub/science/particle-physics/experiments/ neutrinos.html
https://galileospendulum.org/2012/05/12/the-flavor-of-neutrinos/
https://www.businessinsider.com/why-you-should-care-about-neutrinos- 2013-12
Alex is based in Kent and taking part in the bronze award. His interests include Astronomy, Particle Physics and Maths.
