Probing first-generation doublet vector-like leptons via single production at the 3 TeV CLIC

The universe is built from a small set of fundamental particles, a collection of building blocks that physicists have cataloged in what is known as the Standard Model. This model has been remarkably successful, predicting the behavior of matter and energy with incredible precision. However, it is not a complete picture. It leaves out gravity, offers no explanation for dark matter, and struggles to explain why the Higgs boson has the mass it does. Because of these gaps, scientists have long suspected that there are heavier, hidden particles waiting to be discovered. One popular idea is the existence of "vector-like leptons." Unlike the familiar electrons and neutrinos that make up our everyday world, these hypothetical particles would have a unique symmetry that allows them to be much heavier without breaking the rules of physics. If they exist, they could help solve some of the deepest mysteries in cosmology and particle physics, acting as a bridge between the known world and the unknown.

To find these elusive particles, researchers must build machines capable of smashing energy together at speeds approaching the speed of light. The Compact Linear Collider, or CLIC, is a proposed machine designed to do exactly this, operating at a collision energy of three trillion electron volts. In a new study, a team of physicists simulated what would happen if such a machine were turned on today, specifically looking for the first generation of these heavy vector-like leptons. They focused on a specific scenario where these new particles appear in pairs of a neutral state and a charged state, mixing slightly with the ordinary electron. The researchers did not just guess; they ran detailed computer simulations to see how these heavy particles would be created, how they would decay, and how their unique signatures could be separated from the background noise of ordinary particle collisions.

The team investigated three distinct ways these heavy particles could reveal themselves. In the first scenario, a heavy neutral particle is produced alongside a neutrino. This heavy particle would quickly decay into a standard electron and a W boson, a carrier of the weak nuclear force. The W boson would then break apart into two jets of ordinary matter. The final result would be a single high-energy electron, two sprays of particles, and a significant amount of missing energy carried away by the invisible neutrino. In the other two scenarios, a heavy charged particle is produced alongside a standard electron. This heavy charged particle would decay into an electron and either a Higgs boson or a Z boson. The Higgs boson would break into two bottom quarks, while the Z boson would split into two jets of ordinary matter. These events would leave behind a signature of two electrons and either two heavy quark jets or two light particle jets.

To make sense of these complex events, the researchers applied a series of strict filters to their simulated data. They looked for electrons with very high energy, far more than what is typically seen in standard collisions. They required that the particle jets match the specific mass of the W, Z, or Higgs bosons, ensuring they were not just random debris. They also looked for specific patterns in how the particles were spaced apart, which would indicate they came from the decay of a heavy parent particle. By applying these cuts, the team was able to strip away the overwhelming background of ordinary events, leaving behind a clean signal that could be attributed to the new physics. They tested these methods across a range of masses, from 1.3 trillion electron volts up to nearly 2.9 trillion electron volts, and for different levels of mixing between the new particles and the electron.

The results of the simulation were encouraging. The team found that the 3 TeV CLIC would be an incredibly sensitive tool for hunting these particles. The neutral particle channel proved to be the most powerful, capable of detecting the new particles even if they mixed very weakly with ordinary matter. At an integrated luminosity of one inverse attobarn, a measure of the total number of collisions, the machine could exclude the existence of these particles up to a mass of 1.3 trillion electron volts if the mixing was as small as 0.013. As the mass increased to 2.9 trillion electron volts, the sensitivity to the mixing parameter rose to about 0.065. With more data, corresponding to five inverse attobarns of collisions, the machine could potentially discover these particles with high confidence across almost the entire mass range studied. The charged particle channels, while slightly less sensitive due to smaller production rates, still offered a powerful way to cross-check the findings, reaching discovery limits of about 2.9 trillion electron volts for the Higgs decay mode and 2.8 trillion electron volts for the Z decay mode.

This study demonstrates that a future linear collider could probe a vast landscape of new physics that is currently out of reach for existing machines. The Large Hadron Collider has already set strict limits on these particles, ruling out masses below roughly 1.22 trillion electron volts for certain types. The CLIC simulations show that by pushing to higher energies and utilizing the clean environment of electron-positron collisions, scientists could explore the region just above these limits and far beyond. The ability to detect particles with such small mixing angles is particularly important, as it suggests that even if these new particles interact very feebly with the world we know, they would not remain hidden forever. The research confirms that the single production of these heavy leptons is a viable and sensitive strategy, offering a clear path forward for the next generation of particle physics experiments.