Probing Heavy Neutral Leptons through New Gauge Interactions at the Forward Physics Facilities

Deep within the subatomic world, physicists have long suspected that the Standard Model, our best map of how matter and forces interact, is missing a crucial piece. This missing piece is a family of particles known as heavy neutral leptons. Unlike the familiar electrons and protons that make up atoms, these particles are invisible to most detectors because they do not carry an electric charge and rarely interact with ordinary matter. They are theorized to be the heavy cousins of the neutrinos, the ghostly particles that stream through the Earth by the trillions every second. If these heavy neutral leptons exist, they could explain why neutrinos have mass and might even hold the key to understanding the mysterious dark matter that fills the universe. However, because they are so elusive and heavy, finding them requires looking in very specific places and using very clever tricks to catch them before they vanish.

A team of researchers has now proposed a new way to hunt for these particles using the Large Hadron Collider, the world's most powerful particle accelerator located on the border of France and Switzerland. Instead of looking for these particles to be created in the usual way, which is extremely rare, the scientists suggest that a new, unseen force could act as a production line, churning out heavy neutral leptons in vast numbers. This new force would be carried by a heavy particle called a Z-prime, which acts as a mediator, connecting the heavy leptons to the protons smashing together in the collider. Once created, these heavy leptons would travel a long distance before decaying, or breaking apart, into visible particles that detectors can see. The researchers focused their study on the far-forward region of the collider, a narrow tunnel extending hundreds of meters away from the main collision point, where a suite of experiments known as FASER and its future, larger successors are waiting to catch these fleeting signals.

The study, which relies on detailed computer simulations, reveals that this new method of production could open up a vast, previously unexplored territory in the search for heavy neutral leptons. In the traditional view, creating these particles depends entirely on a very weak connection between them and ordinary neutrinos, a process so rare that it makes detection nearly impossible for many types of these particles. The new proposal changes the game by introducing a second, stronger interaction that creates the particles in abundance, while the weak connection still controls how they eventually decay. This separation allows the detectors to see a much larger number of events than previously thought possible. The researchers found that by looking for specific decay patterns, such as the heavy leptons turning into pairs of electrons, muons, or neutral particles that split into light, the current FASER experiment could already see signs of these particles.

The potential for discovery grows significantly when the team looked ahead to the High-Luminosity era of the collider, where the machine will run with much greater intensity. Their simulations show that the proposed successors, FASER2 and FASERν2, could probe regions of the particle's properties that have never been tested before. These future detectors are designed to be much larger and sit further down the beam line, allowing them to catch particles that travel longer distances. The study indicates that with these upgraded facilities, scientists could potentially reach the exact range of properties predicted by the most popular theories explaining why neutrinos have mass. This would mean that the far-forward experiments could finally test the "canonical" idea of how these heavy particles fit into the universe, a goal that has been out of reach for decades.

The researchers examined three different versions of this new force, each with slightly different rules for how it interacts with the particles in the Standard Model. In one scenario, the force connects to all matter equally; in another, it prefers to interact with the heaviest type of lepton, the tau; and in a third, it only talks to the building blocks of protons and neutrons, ignoring the lighter electrons and muons entirely. By running simulations for each of these scenarios, the team mapped out exactly where the experiments would be sensitive. They found that the new production mechanism works best for particles with masses ranging from a few hundred million electron volts up to several billion electron volts. This mass range is particularly interesting because it sits in a gap where many previous experiments have struggled to look.

A key part of the study involved understanding exactly where these particles are born. While the main collisions at the center of the collider produce the vast majority of the signal, the researchers also considered a secondary source. As the beam of protons travels through the machine, it hits a block of dense material designed to stop stray particles. This impact creates a shower of secondary neutrons and other particles that can also produce the heavy leptons. The simulations showed that while this secondary source does contribute, the primary collisions at the heart of the collider are the dominant source of the signal. This confirms that the most powerful way to find these particles is to look at the debris from the main crash, rather than the secondary interactions further down the line.

The researchers also carefully considered the different ways these heavy leptons might decay inside the detector. They looked for signatures where the particle turns into a neutrino and a pair of charged particles, or a neutrino and a neutral particle that immediately splits into two beams of light. They also looked for decays where the heavy lepton transforms into a charged particle and a spray of other hadrons, which are particles made of quarks. By calculating how often these events would happen and how well the detectors could distinguish them from background noise, the team determined the limits of what each experiment could achieve. They found that the detectors are capable of filtering out almost all background noise, leaving a clean signal if the heavy leptons exist.

The results of this study suggest that the far-forward physics program is uniquely positioned to explore a new class of theories. Unlike other searches that rely on a single, weak interaction to both create and destroy the particle, this approach uses a strong force to create the particle and a weak force to destroy it. This separation is the key to the enhanced sensitivity. The simulations show that the upcoming experiments could reach mixing strengths that match the predictions for the origin of neutrino mass for all three types of neutrinos. This is a significant achievement because it means the experiments could test the most fundamental explanation for why neutrinos have mass, a question that has puzzled physicists for decades.

The study concludes that the combination of the new production mechanism and the unique geometry of the forward detectors offers a powerful new window into the subatomic world. By focusing on the forward direction, where particles travel in a tight beam, and by utilizing the high energy of the collider, the FASER program can access a parameter space that is complementary to other searches. The findings provide a strong motivation for the construction of the larger, more sensitive detectors planned for the High-Luminosity era. If these heavy neutral leptons exist within the mass range explored, the next generation of experiments could be the ones to finally reveal them, offering a glimpse into the physics that lies beyond our current understanding of the universe.