Muon Tridents at FASER and NA64: Motivation, Predictions, and Prospects for Precision Measurements
This paper presents improved theoretical predictions and prospects for the first precision measurement of muon trident scattering at TeV energies using data from the FASER and NA64 experiments, demonstrating their potential to probe the process's energy dependence with percent-level accuracy.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the vast, invisible landscape of particle physics, there are interactions so rare and subtle that they have remained largely hidden for decades. One such interaction involves a high-energy muon, a heavy cousin of the electron, passing near a heavy atomic nucleus. As it skims past the nucleus's powerful electric field, the muon can spontaneously split into three particles: itself, a new muon, and an antimuon. This event, known as the muon trident process, is a fundamental prediction of the theory governing electricity and magnetism. For over fifty years, scientists have tried to catch these events, but they have been incredibly difficult to spot. Previous attempts recorded only a handful of candidates, leaving a significant gap in our understanding. Theoretical predictions for how often these events should happen have also disagreed wildly, with some estimates differing by a factor of ten or more. This uncertainty matters because these interactions are a key part of how high-energy muons lose energy as they travel through matter, a detail crucial for interpreting data from cosmic ray observatories and neutrino experiments.
A team of researchers has now turned their attention to two modern experiments, FASER and NA64µ, to see if they can finally measure this elusive process with the precision it deserves. Located at the Large Hadron Collider and a separate particle accelerator in Europe, these detectors are positioned to catch the intense streams of muons produced by high-energy collisions. The scientists used powerful computer simulations to model exactly what would happen when these muons hit the heavy metal targets inside the detectors. They built a detailed theoretical model that accounts for the complex structure of the atomic nucleus and the subtle effects of quantum mechanics, moving beyond the rough approximations used in the past. Their calculations predict that during the upcoming run of the collider, the FASER experiment will record more than 85,000 of these trident events, while the NA64µ experiment could see nearly nine million. This represents a massive leap forward from the mere dozens of events seen in previous decades.
The study did more than just count potential events; it carefully examined the reliability of the predictions. The researchers compared their new, detailed calculations against older, simpler methods that had been used for years. They found that the older methods, which treated the atomic nucleus as a simple point of charge, significantly overestimated the number of events, especially at the highest energies. By using a more realistic description of the nucleus, which accounts for how its electric charge is spread out, the team refined the expected rates. They also investigated other subtle physical effects, such as the interference between identical particles and the influence of the strong electric field on the interaction. They discovered that these factors, while small, are measurable and depend on the energy of the incoming muon. For instance, the electric field of the heavy nucleus slightly reduces the number of events, an effect that becomes more pronounced at lower energies.
A critical part of the work involved distinguishing the true signal from background noise. In a busy detector, other processes can mimic the appearance of three muons. The researchers simulated these background events, which include muons scattering off other particles or producing showers of electrons and positrons that look like muons. They found that while these backgrounds exist, they are far less common than the trident signal. Furthermore, the team showed that by looking at specific characteristics of the events, such as the angle at which the muons scatter and their combined energy, the signal can be separated cleanly from the noise. This means that the experiments are not just capable of seeing the events, but of measuring them with high precision, potentially reaching an accuracy of one percent.
The implications of this work extend beyond simply counting particles. By measuring the muon trident process across a wide range of energies, from a few hundred billion electron volts up to several trillion, scientists can test the fundamental laws of physics with unprecedented rigor. The study suggests that these experiments will be able to probe the structure of the atomic nucleus itself, as the rate of events is sensitive to how the nucleus is modeled. While the researchers also looked for signs of new, unknown particles that might alter the process, they found that the current data would not be sensitive enough to detect them, effectively ruling out certain theoretical possibilities in this specific energy range. Ultimately, this research transforms the muon trident from a theoretical curiosity into a measurable phenomenon, offering a new and powerful laboratory to explore the deep workings of the universe.
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