Neutrino trident process at a muon collider

Deep in the subatomic world, particles known as neutrinos are ghostly travelers. They have almost no mass, carry no electric charge, and rarely interact with anything they pass through. Because they slip through ordinary matter so easily, they are notoriously difficult to catch and study. Yet, understanding these elusive particles is crucial for physicists, as they hold clues to the fundamental forces that shape our universe and might reveal laws of physics that go beyond our current best theories. For decades, scientists have relied on massive detectors placed near particle accelerators or natural sources like the sun to glimpse these particles. Now, a new proposal suggests using a future machine designed to smash muons together—a type of heavy cousin to the electron—to create an intense, focused beam of neutrinos. This beam would offer a pristine, well-understood source of particles, allowing researchers to test the rules of nature with a precision never before possible.

In a recent study, researchers Reinaldo Francener and Victor P. Gonçalves explored what would happen if such a machine, called a muon collider, were built with a dedicated neutrino detector placed far downstream from the collision point. As the muons travel around the collider's ring, they naturally decay into neutrinos and other particles. In the straight sections of the ring, these neutrinos fly out in a tight, high-energy beam. The team focused on a rare event called the "neutrino trident" process. In this scenario, a neutrino from the beam approaches a heavy atomic nucleus in a detector target. Instead of passing right through, the neutrino interacts with the electric field surrounding that nucleus. This interaction is so specific that it causes the neutrino to split, creating a pair of new charged particles—either two muons, two electrons, or two tau particles—while the neutrino itself continues on its way. It is a delicate dance of forces where a single neutrino produces two new particles out of the vacuum of the nucleus's field.

The researchers simulated this process for a proposed detector containing ten kilograms of tungsten, a very dense metal, operating for one year. Their calculations predicted a surprisingly large number of these events. They found that the detector would record roughly ten thousand events where an electron and a positron (the electron's antimatter twin) are created, and nearly six thousand events producing a pair of muons. Even the heavier, more elusive tau particles would appear in about two hundred and sixty events. The study also looked at cases where the two new particles are of different types, such as an electron and a muon, predicting tens of thousands of such mixed pairs. These numbers are significant because previous experiments, which relied on less controlled neutrino sources, have seen only a handful of these events with large margins of error. The sheer volume of events predicted here suggests that a muon collider could turn the trident process from a rare curiosity into a routine measurement tool.

To ensure these predictions were realistic, the team applied strict filters that mimic what a real detector would see. They accounted for the fact that detectors are not perfect and that background noise from other particle interactions could hide the signal. Even after applying these realistic cuts, which remove events that are too messy or low in energy to be useful, the team estimated that more than three thousand five hundred clean muon-pair events would remain. This high count means scientists could measure the properties of the trident process with unprecedented accuracy. Such precision is vital because the rate at which these events occur is tightly linked to the fundamental rules governing how particles interact. If the measured rate differs even slightly from the prediction of the Standard Model—the current best theory of particle physics—it would be a clear sign that new, unknown forces or particles are at work.

The researchers used this potential to look for signs of "New Physics," specifically testing a theory that predicts a new, heavy particle called a Z-prime boson. This hypothetical particle would interact only with certain types of leptons, the family of particles that includes electrons, muons, and taus. By calculating how the presence of this new particle would change the number of trident events, the team mapped out exactly what a muon collider could discover. Their results show that a detector at this facility could explore regions of the theory's parameter space that are currently invisible to other experiments. It could probe for these new particles with masses ranging from very light to much heavier than the known forces of nature. The study concludes that a neutrino detector at a muon collider would not only confirm the Standard Model with high precision but also significantly extend the search for new physics, potentially uncovering particles that have so far remained hidden from our view.