Neutrinos, sometimes known as “ghost particles,” are tiny, massless, neutral particles. They are constantly whizzing around us, rarely interacting with anything else, but they may hold the key to understanding why the universe looks the way it does. The neutrino was initially hypothesized to “fix” an issue with existing theories’ inability to represent experimental data, but it has continued to surprise us with new unexpected behaviors that we are still trying to properly understand. To detect such an elusive particle, we build enormous detectors ranging in mass from hundreds to thousands of tones, but sometimes even more, and direct powerful beams of particles at them. By putting massive absorbers between the beams and our detectors, we can ensure that only neutrinos pass through. The more neutrinos you see, the larger the detector and the stronger the beam.

Over time, we have gotten quite good at making neutrino detectors that are not only huge but also very sensitive. When neutrinos interact with a detector, a variety of particles are produced. The more of them you can measure, the more you will learn about the neutrinos with which you interacted. We aim to measure as many neutrinos as possible as accurately as possible in order to understand their behavior. Modern detectors are built to carefully measure as many interaction products as feasible. This enables researchers to establish the neutrino type (also known as the flavor), neutrino energy, and neutrino direction.

It turns out that if you have a large, highly sensitive detector and an intense beam of practically invisible particles, you have created the ideal experiment for searching for a variety of other theoretical particles. From dark matter to heavy neutrinos, millicharged particles to magnetic monopoles, a plethora of speculative particles have been proposed to fill gaps in our understanding of the universe, much like the neutrino’s original necessity. In general, these theoretical particles, like the neutrino, must be difficult to detect (otherwise we would have seen them already), so it makes a lot of sense to sift through the data from our huge and sensitive neutrino detectors for clues about other novel physics.

Some of these particles are highly sought-after theoretical solutions. Dark matter is required to explain the spinning of big galaxies, and heavy neutrinos are required in some theories to balance the neutrino’s very modest mass (among other reasons, to hypothesize their existence). Such “heavy” particles could emerge in the beam just like a conventional neutrino, but since they are heavier, they would travel slower and reach the detector later. We can look for this type of delayed signature by looking for neutrino-like interactions after the neutrino beam has passed. Some particles are expected to decay within detectors rather than interact with them. Such a disintegration would appear disturbingly similar to a neutrino interaction. The distinction is that a neutrino interacts with an atom’s nucleus, and that nucleus is impacted by it either “recoiling” from the interaction or spewing forth protons and neutrons as a result of the disruption. A decaying particle would leave no such signal, and so it may be distinguished from neutrinos if the detector is sensitive enough to detect the difference.

Some detectors can detect the presence of other particles as well. “Millicharged particles” is one example. Such a particle, unlike a neutrino, is not electrically neutral, but has a much lower charge and hence interacts much less with a detector. These would be far fainter than electrons or their heavier counterparts known as muons. In a small detector, their faintness would make them appear to be random noise, but a concerted search in a big neutrino detector would be able to find them (depending on how small their charge is, of course). Other hypothesized particles to seek for include “magnetic monopoles,” which would function like a magnet with one end removed, leaving only a north or south pole. A multitude of theories anticipate such particles, albeit their features are difficult to pin down mathematically; once again, a big, very sensitive detector is likely to be the best chance.

The neutrino was first conceived to solve a problem, and it has only continued to astonish us since its discovery. We developed experiments that are now sensitive to potential solutions to other difficulties in order to understand more about neutrinos. We are prepared for new surprises to be revealed by these detectors.