Hadronic heavy neutral lepton decays to the limit
This paper investigates the hadronic decay structure of Heavy Neutral Leptons into multi-body final states by proposing a seamless transition between hadronic and partonic descriptions based on invariant mass, highlighting the resulting multi-hadron channels as promising experimental signatures.
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 catalog of the universe, the Standard Model acts as the definitive rulebook, listing every known particle and how they interact. Yet, this rulebook is incomplete. It cannot explain why neutrinos, those ghostly particles that stream through everything, have mass, nor can it account for the overwhelming abundance of matter over antimatter in the cosmos. To fill these gaps, physicists hypothesize the existence of heavy neutral leptons. These are theoretical cousins to the familiar neutrino, but significantly heavier and capable of living long enough to travel measurable distances before decaying. If they exist, they could be the missing keys to understanding the fundamental nature of our universe. The challenge for scientists is not just to find them, but to know exactly what they look like when they finally break apart. Because these particles interact so weakly, they are often produced in high-energy collisions and then travel some distance before vanishing into a shower of other particles. To spot them, researchers must predict precisely what that shower will contain, a task that becomes incredibly difficult when the heavy particle is heavy enough to produce complex clusters of matter rather than simple pairs.
A team of researchers at the Max Planck Institute for Physics and the Technical University of Munich has tackled this prediction problem by refining the map of how these heavy particles decay. For years, physicists have relied on a simplified method to guess what happens when a heavy neutral lepton breaks down. They would calculate the decay into basic building blocks called quarks and then assume these quarks instantly snap together into specific, simple particles like pions or kaons. This approach worked well for lighter particles, but as the heavy lepton's mass increased into the range of a few billion electron volts, the old method began to blur the lines between simple particle pairs and complex, multi-particle clusters. The researchers realized that this transition zone was being handled too roughly, potentially hiding the very signals needed to detect these elusive particles.
To solve this, the team developed a new way to describe the decay process that respects the complex reality of the transition. Instead of forcing a single, rigid switch between the world of simple quarks and the world of complex particles, they proposed a smooth handover based on the energy of the resulting cluster. They treated the decay into broad, short-lived groups of particles as a distinct process, calculating the exact paths these groups take before they settle. Specifically, they included detailed calculations for decays that produce three pions, a common outcome that had been largely overlooked in previous estimates. By mapping out these complex, multi-particle outcomes with greater precision, they found that the old methods were overestimating the speed at which these heavy particles would decay in certain mass ranges. This means the particles might live slightly longer than previously thought, a crucial detail for experiments trying to catch them.
The researchers then tested how these new, more accurate calculations would change the search for these particles in real-world experiments. They simulated the sensitivity of the upcoming SHiP experiment at CERN, which is designed to hunt for heavy neutral leptons. Their results showed that for particles with masses below about two billion electron volts, the detailed, multi-particle description is essential and provides the most accurate picture. However, as the mass increases beyond this point, the simple picture of quarks breaking apart becomes more dominant again. Interestingly, they found that simply adding more complex particle clusters to the search list does not automatically make the experiment more sensitive. In the higher mass ranges, the decay produces such a chaotic mix of many particles that it becomes difficult for detectors to identify a clear signature. The signal gets lost in the noise of high-multiplicity events, meaning that for the heaviest candidates, the cleanest signals will still come from simpler decay modes involving just leptons.
This work does not claim to have found the heavy neutral lepton, nor does it prove that the new method is the final word on the subject. Rather, it offers a more reliable framework for interpreting the data that will soon arrive from major particle physics facilities. By correcting the way scientists calculate the lifetime and decay patterns of these hypothetical particles, the study ensures that when an experiment finally sees a signal, the interpretation will be grounded in a more realistic understanding of how matter transforms at these energy scales. The team acknowledges that their current simulations rely on standard tools that may need further tuning to perfectly match the complex dance of particles in the transition zone. Nevertheless, by clarifying the boundary between simple and complex decays, they have provided a clearer path for the next generation of physicists to follow in their quest to uncover the secrets of the universe.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.