The Large Hadron Collider (LHC) has been a beacon of particle physics discovery for over a decade, but the future of probing the fundamental building blocks of our universe lies in even more powerful machines. Among these, the Future Circular Collider at electron-positron collisions (FCC-ee) stands out as a monumental leap forward, promising unprecedented precision and the potential to uncover physics beyond the Standard Model. A recent groundbreaking study, published in the European Physical Journal C, delves into the exciting possibilities offered by the FCC-ee for searching for elusive heavy neutral leptons, particles that have long been theorized but eluded direct detection. This research isn’t just about pushing the boundaries of our knowledge; it’s about meticulously crafting experimental strategies to find these phantom particles, a quest that could redefine our understanding of mass, neutrinos, and even the very fabric of cosmic evolution. The researchers have meticulously outlined how the FCC-ee, with its immense luminosity and clean collision environment, can sift through vast amounts of data to isolate the faint but distinct signatures of these hypothetical particles, particularly in final states that include a muon, a well-understood cousin of the electron. This focus on muon-inclusive final states is a clever and efficient approach, leveraging the predictable behavior of muons to mitigate background noise and enhance the sensitivity of the search. The implications of finding such particles are profound, potentially shedding light on the universe’s matter-antimatter asymmetry and the puzzling smallness of neutrino masses.
The Standard Model of particle physics, while incredibly successful, is not without its limitations. It does not fully explain phenomena such as dark matter, dark energy, or the tiny, yet non-zero, masses of neutrinos. The concept of heavy neutral leptons (HNLs) offers a compelling avenue for theoretical extensions to the Standard Model. These hypothetical particles, unlike the known light neutrinos, would possess significant mass and interact very weakly with ordinary matter. Their existence could elegantly explain why neutrinos are so light – they might be “diluted” by the presence of these heavier counterparts in a mechanism known as the “seesaw mechanism.” The FCC-ee, with its precisely controlled electron-positron collisions, is uniquely positioned to generate these HNLs at specific energy ranges, allowing physicists to act as cosmic detectives, piecing together evidence from their decay products. The sheer volume of collisions at the FCC-ee will provide an unparalleled statistical power, enabling the search for rare processes that would be practically invisible at current colliders. Imagine sifting through billions upon billions of collisions, looking for a single, specific decay pattern that screams “new physics!” This is the scale of the challenge and the promise of the FCC-ee.
The specific focus of this new study accentuates the strategic brilliance of particle physics experimentation. By targeting final states that include at least one muon, the researchers are exploiting a crucial piece of information. Muons, while heavier than electrons, behave similarly in many particle interactions and have well-understood decay properties. Their presence in a potential HNL decay chain acts as a valuable tag, helping to distinguish genuine signals from the overwhelming background of known particle interactions. This isn’t merely a matter of convenience; it’s a calculated decision to maximize the discovery potential. When an HNL decays, it can produce a variety of daughter particles. If one of these particles predictably manifests as a muon, and the other products can be accounted for by standard physics, then the observation gains significant weight. The FCC-ee’s ability to precisely reconstruct these complex event topologies is paramount to the success of such targeted searches, making it a veritable precision instrument for uncovering the hidden laws of nature.
Heavy neutral leptons are not merely theoretical constructs dreamt up to fill gaps in our understanding. They are motivated by deep theoretical puzzles like the aforementioned neutrino mass problem. If these HNLs exist and participate in interactions that link them to the Standard Model neutrinos, their presence would naturally lead to the suppression of the masses of the neutrinos we observe. The heavier the HNL, the lighter the standard neutrino. The FCC-ee’s energy reach, particularly at specific collision energies designed to resonate with certain particle masses, could be the perfect hunting ground for these elusive particles. The study details specific collision energies and event topologies to look for, akin to a treasure map for particle physicists. This level of detailed simulation and prediction is essential for translating the theoretical possibility of HNLs into a concrete experimental search program.
The FCC-ee is not just another accelerator; it’s a paradigm shift in collider technology. Unlike the proton-proton collisions of the LHC, which generate a complex spray of particles, electron-positron collisions are remarkably clean. This “cleanliness” is a critical advantage when searching for rare and subtle signals. The backgrounds from known physics processes are significantly reduced, allowing for much higher precision measurements and the detection of extremely rare events. This makes the FCC-ee an ideal environment for exploring the high-mass frontier suggested by HNL theories. The ability to precisely measure the energy and momentum of collision products is paramount, and the FCC-ee excels in this regard, providing physicists with highly granular data to scrutinize.
Furthermore, the FCC-ee is designed to operate at unprecedentedluminosity, meaning it can achieve an extremely high rate of collisions. This sheer volume of data is crucial for any search that relies on detecting rare events. Imagine trying to find a specific needle in a haystack; the FCC-ee provides an enormous haystack, but it’s a haystack where the needles are significantly easier to spot due to the cleaner environment. The statistical power gained from such high luminosity directly translates to increased sensitivity for discovering new particles. The researchers have meticulously calculated the expected number of signal events and background events for various HNL masses, demonstrating how the FCC-ee’s capabilities will surpass those of any current or past experiment.
The study delves into sophisticated event reconstruction techniques. When a heavy neutral lepton decays, it will produce a cascade of other particles. Identifying these particles and their properties, such as their momentum and energy, is crucial for reconstructing the event and inferring the properties of the parent particle. The FCC-ee’s detectors are designed with advanced tracking and calorimetry systems to achieve this precision. The paper details how muons, electrons, photons, and other particles produced in these decays will be identified and measured, and how cuts will be applied to select candidate events that are likely to contain an HNL signature. This meticulous attention to detector performance and analysis strategy is what makes such searches feasible.
One of the fascinating aspects of searches for heavy neutral leptons is their potential connection to the baryon asymmetry of the universe. The observable universe is dominated by matter, with very little antimatter. The Standard Model, by itself, does not provide a sufficient explanation for this observed asymmetry. Theories involving HNLs, however, offer compelling mechanisms through which such an imbalance could have been generated during the early epochs of the universe. Discovering HNLs would therefore not only illuminate particle physics but also provide crucial insights into cosmology and the very origin of our existence. The FCC-ee offers a unique window into this fundamental question by potentially revealing the particles responsible for setting the stage for our matter-dominated cosmos.
The researchers meticulously explored different scenarios for the mass ranges of these heavy neutral leptons. The FCC-ee’s tunable collision energies allow for a comprehensive scan across a wide spectrum of potential HNL masses. Depending on the specific theoretical model, HNLs could be considerably heavier than any known lepton. The FCC-ee is designed to probe these high-mass regions, where interactions might be significantly suppressed, making their direct observation exceptionally challenging. The study presents predictions for discovery reach across various hypothetical mass ranges, highlighting the FCC-ee’s potential to either discover these particles or place stringent constraints on their existence, thereby narrowing down the possibilities for new physics.
The inclusion of muons in the envisioned detection channels is a strategic choice with significant implications for background suppression. While electrons are also well-understood, the specific decay signatures involving muons can often offer a cleaner distinction from the dominant standard model processes. The physics of muon production and decay is well-characterized, allowing physicists to build more precise models of expected background events. When the observed data deviates significantly from these predictions and shows a surplus of events with the expected characteristics of an HNL decay, the confidence in a discovery increases dramatically. This analytical approach underscores the blend of theoretical insight and experimental precision that drives modern particle physics.
The methodology presented in the paper involves extensive Monte Carlo simulations. These simulations use powerful computers to model billions of particle collisions, both from known Standard Model processes and hypothetical HNL decays. By comparing the simulated HNL signals with the simulated backgrounds, physicists can estimate how many standard model events would mimic a signal, and thus determine the sensitivity of the experiment. The FCC-ee’s ability to generate these detailed simulations with high fidelity is crucial for designing optimal search strategies and interpreting the results of future data analysis, ensuring no stone is left unturned in the quest for new discoveries.
The study also considered various decay modes of the heavy neutral leptons. While the focus is on muon-inclusive final states, HNLs can decay in multiple ways. The researchers have taken into account different branching ratios – the probabilities of decaying into specific sets of particles – to provide a comprehensive picture of the FCC-ee’s discovery potential. This holistic approach ensures that even if an HNL decays primarily through channels not explicitly focused on, its presence might still be inferred through other correlated signals. The flexibility of the FCC-ee’s detector and analysis framework is essential for capturing these diverse signatures.
The ultimate goal, of course, is discovery. The prospect of finding a heavy neutral lepton would be a monumental achievement in particle physics, opening up new avenues of theoretical exploration and experimental investigation. It could provide the first direct evidence of physics beyond the Standard Model in the lepton sector, with far-reaching consequences for our understanding of fundamental forces and particle interactions. Such a discovery would likely necessitate a revision or extension of our current theoretical frameworks, potentially leading to a more complete and unified picture of the universe at its most fundamental level. The FCC-ee, with its precision and power, is poised to be the instrument where this revolutionary discovery might unfold.
This research represents more than just a theoretical exercise; it is a meticulously planned roadmap for the FCC-ee’s experimental program. The detailed analysis of signal and background, the strategic selection of final states, and the exploration of different HNL mass ranges all contribute to a robust and compelling case for the FCC-ee’s capability to uncover these exotic particles. The scientific community is eagerly anticipating the era of FCC-ee operations, where such focused searches will become a reality, and the whispers of new physics might finally become a resounding chorus of discovery, fundamentally altering our perception of the subatomic world and our place within it. The commitment to precision and the relentless pursuit of the unknown are the hallmarks of this endeavor, promising physics that will resonate for generations.
Subject of Research: Searches for heavy neutral leptons (HNLs) in final states including a muon at the Future Circular Collider at electron-positron collisions (FCC-ee).
Article Title: Searches for heavy neutral leptons at FCC-ee in final states including a muon.
Article References:
Bellagamba, L., Polesello, G. & Valle, N. Searches for heavy neutral leptons at FCC-ee in final states including a muon.
Eur. Phys. J. C 85, 1069 (2025). https://doi.org/10.1140/epjc/s10052-025-14749-y
Image Credits: AI Generated
DOI: https://doi.org/10.1140/epjc/s10052-025-14749-y
Keywords: Heavy neutral leptons, FCC-ee, Standard Model, beyond the Standard Model, particle physics, muon, neutrino mass, collider physics, future colliders.
