Neutrinos: An Overview
Currently, you’re being pierced by trillions of invisible particles from the cosmos. Believe it or not, this isn’t science fiction, and although these elusive particles seem insignificant to you, they may shed light on one of life’s greatest unknowns: the creation of the universe. This seems like reason enough to learn more, so let’s dive in!
Where do they come from?
The majority of neutrinos come from high energy nuclear reactions in the cosmos. Neutrinos are emitted from any splitting or joining of nuclei, but are most commonly seen in the decay of stars or high energy celestial bodies such as supernovae or quarks. When stars burn, they expel much of their energy through emitting neutrinos since they don’t interact much with matter, and can therefore escape the overwhelming density of the massive nuclear reaction. Since neutrinos can travel without being interrupted by mass, they often retain the high energy and velocity from their creation reaction, this allows them to zip through the universe at near light speed!
How did we find them?
Neutrinos had been theorized a whole 30 years before their discovery in order to offer an explanation for the seeming lack of energy conservation in negative beta decay.
In better understood types of decay, such as alpha decay, the disparity between the initial mass of the particle and the mass of the lighter post-decay particles is found in the kinetic energy of the alpha particle; this upholds the law of conservation of energy. Since the same particles are present after every instance of alpha decay, the kinetic energy of the alpha particle must be the same every occurrence. However, in beta decay, the kinetic energy of the beta particle was consistently observed to be less than what was required for the law of energy conservation. At the time, it seemed like beta decay violated a fundamental physics law, but in truth the law of energy conservation was upheld by the emission of the neutrino.
The then-theoretical neutrino was proposed as a chargeless and presumed massless particle. Being chargeless would explain why it was so difficult to detect, as well as how it could be emitted when the charges of the proton and electron emitted in negative beta decay cancel out.
The particle became a reality in 1956, when “project poltergeist” collected neutrino emission data from a nuclear reactor for 5 months before definitively declaring the detection of neutrinos.
How do we detect a seemingly undetectable particle?
On very rare occasions, a neutrino will interact with an atom and leave a small amount of free electrical charge in its wake. Neutrino detectors pick up on these interactions through recording the charged particles and flashes of light (Cherenkov radiation) associated with neutrino interaction. The detector also includes large tanks of ideally ionizing or scintillating material that amplify these neutrino interactions. It is preferable to put a lot of matter in the neutrinos way to increase the chances of that rare interaction. To limit interference from other cosmic energetic particles hitting earth, neutrino detectors are typically placed underground where earth material protects them from cosmic interference. Neutrinos pass right through earth’s atoms, and occasionally make their mark in the deep underground detector.
What are neutrinos?
Elementary matter particles, specifically leptons, that interact solely through weak nuclear force and gravity, and have neutral electric charge.
Once the particle was revealed, more and more of its nuances were discovered. Neutrinos turned out to be the second most abundant particle in the universe (after photons). This makes sense, as neutrinos are created every time two nuclei break apart or merge, something that has undoubtedly happened countless times since the big bang.
It was also unveiled that neutrinos come in 3 flavors; electron neutrinos, tau neutrinos, and muon neutrinos, discovered in that order.
These flavors are identified not by the properties of the neutrino itself (which are difficult to observe head on due to the rarity of it interacting with matter) but by the particles they interact with. Neutrinos of a certain flavor are more likely to interact with fundamental particles of the corresponding type. For example, in muon decay the unstable elementary muon particle decays into a corresponding muon neutrino and a W-boson. The presence of electrons, muons, or tau particles are picked up by the detector, then neutrino flavors are identified to match.
Perhaps the biggest surprise in identifying neutrino properties was that neutrinos have mass! Another way to think of neutrinos is them being categorized into three different mass states, referred to as mass 1, mass 2, and crazily enough, mass 3! These masses do not directly correspond to the three neutrino flavors, however they do influence the chance of the neutrino interacting as a certain flavor. The energy of the neutrino and the distance it has traveled determines the mix of masses and the rate of oscillation between flavors.
Neutrinos oscillate between the three masses while traveling, existing in a superposition between them. When the neutrino interacts, it commits to a singular flavor, but neutrinos could be any combination of masses while they aren’t being observed.
If neutrinos weren’t bizarre enough, guess what? They have antiparticles! In fact, the first detection of neutrinos was from the emission of electron antineutrinos in the decay of a neutron into a proton.
Antineutrinos are identified from their interaction with antiparticles, in this case the proton being the electron’s antiparticle, as well as their spin to the right in an opposite handedness to neutrinos. Particles and antiparticles are usually differentiated by opposite charges, but since neutrinos are chargeless particles, they are only differentiated from antineutrinos by the ‘direction’ of their spin.
Physicists are currently trying to find more differences between neutrinos and their antiparticles, such as possible differences in oscillation rate.
Why are neutrinos a big deal?
By having mass, neutrinos uproot the standard model, the theory that details our understanding of the forces around us. In the standard model, the higgs boson is what gives all leptons mass (neutrinos are leptons). When particles interact with the higgs field to gain mass, they switch handedness from right to left or left to right. But a right handed neutrino has never been observed, so currently the standard model is incapable of explaining why neutrinos have mass.
The possible indistinguishability between neutrinos and antineutrinos is what hints at neutrino’s ability to explain the origin of our universe.
The difference between a neutrino and an antineutrino is largely determined by the particle’s spin, but since neutrinos have mass they travel slower than the speed of light and our perspective could theoretically ‘catch up to them’ if not ‘move faster’ than them. By changing our perspective, we change the neutrino’s left handed spin to right handed, removing any difference between the neutrino and the antineutrino.
If there ends up being a difference between neutrinos and their antiparticle, neutrinos would be labeled as such and be considered ‘Dirac Particles”.
If neutrinos end up being their own antiparticle, they would be considered ‘Majorana Particles’ and be able to further explain why we have more matter than antimatter in the universe. That does seem like quite a jump, but the phenomena of neutrinoless double beta decay could account for the matter-antimatter imbalance, and that phenomenon is only made possible with Majorana neutrinos
In neutrinoless double beta decay (above), two particles undergo negative beta decay in which two unstable particles simultaneously decay into more stable beta particles and each release an electron and an antineutrino. Because neutrinos are antineutrinos, the two would annihilate each other, resulting in only two electrons being emitted from the decay. This causes a preference for matter over antimatter, as electrons are matter and they’re being emitted without any antimatter to balance them out. With the abundance of neutrinos, this matter-preferring reaction could account for the inequality between matter and antimatter in the universe. And that is a big deal.
Neutrinos are on the forefront of physics research, making us reconsider our previous understanding of how the world works. With this article, hopefully you are one step closer to grasping the puzzles of modern physics, and in doing so perhaps we can marvel the hidden intricate wonders of the world around us in hopes of one day being able to understand it all.
These concepts are mind boggling and complicated. If I didn’t manage to explain a concept effectively, it’s likely that Symmetry Magazine, Even Bananas, or the sources hyperlinked below would have done a better job. Highly recommend checking them out.
Glossary:
Superposition: Something being in between states when not observed, a quantum property
Nuclei decay: when an unstable nucleus emits energy to become more stable
Spin: a particle’s intrinsic angular momentum that comes in integer or half integer multiples, a quantum mechanical property that has no equivalent in classical physics
Leptons: subatomic particles that only interact through electromagnetic, weak, and gravitational force
Cherenkov Radiation: electromagnetic radiation emitted when a particle is moving faster than the speed of light in a medium, emitted when the particle interacts with the surrounding medium and slows the particle’s velocity to below the speed of light. Here is how it is used in neutrino experiments
Scintillating Material: material that can convert radiation into visible light (material that allows us to see Cherenkov Radiation)
Standard Model: our current explanation of the building blocks of matter and how they interact
Fun fact: Since neutrinos can travel without being interrupted by mass, and are often emitted at higher rates before big reactions in space are made visible by traveling photons, their emissions offer means to predict which areas of our sky we should observe to record high energy cosmic behavior such as supernovae.
