Dark photons in exotic Higgs boson decays at FCC-ee

The universe is filled with things we can see and measure, from the stars above to the atoms that make up our own bodies. Yet, physicists know that the visible world is only a small fraction of reality. There is a vast, invisible realm known as dark matter, which holds galaxies together but refuses to interact with light or ordinary matter in any way we have yet detected. To understand this hidden sector, scientists look for "portals"—theoretical bridges that might allow particles from our visible world to slip into the dark one. One such possibility involves a new, invisible particle called a dark photon. Unlike the familiar photon that carries light, this dark version would be a messenger for a hidden force, potentially interacting with our world only very weakly and perhaps traveling a short distance before vanishing. If such particles exist, they could explain why the universe is made of matter rather than antimatter, or why neutrinos have mass. The challenge is that these particles are so elusive that they have never been seen directly, leaving scientists to design experiments that can catch a glimpse of their fleeting footprints.

Researchers at Uppsala University in Sweden have proposed a new way to hunt for these dark photons using a future machine called the Future Circular Collider, or FCC-ee. This massive facility, planned for CERN, will smash electrons and positrons together at energies of 240 billion electron volts. The goal of this specific study is to look for a rare event where a Higgs boson, the particle responsible for giving other particles mass, decays into a pair of dark photons. The researchers focused on a scenario where these dark photons are long-lived, meaning they travel a measurable distance away from the point of creation before decaying into pairs of muons, which are heavy cousins of the electron. Because the dark photons would travel a short distance before breaking apart, the resulting muons would appear to emerge from a point in space that is slightly displaced from the main collision point, creating a signature that is almost impossible for ordinary background processes to mimic.

To test this idea, the team ran detailed computer simulations of what would happen if the FCC-ee operated at full capacity, producing more than two million Higgs bosons. They modeled the behavior of the dark photons across a wide range of possible masses and interaction strengths, covering scenarios where the particles travel anywhere from a few micrometers to several meters before decaying. The signal they were looking for was very specific: a collision producing two jets of ordinary matter from a Z boson, accompanied by two pairs of muons that originate from displaced points in the detector. The researchers developed a set of rules to sort through the millions of simulated events, filtering out the overwhelming noise of standard particle interactions. They looked for events where the four muons had the right combined energy to match a Higgs boson, where the muon pairs were light enough to be dark photons, and where the muons appeared to come from a secondary location a few millimeters to two meters away from the collision center.

The results of this simulation were strikingly clean. By applying these kinematic rules, the researchers found that they could eliminate every single background event that the Standard Model of particle physics predicts could mimic this signal. In the simulated data, the background dropped to zero, while a significant portion of the signal events remained. This zero-background scenario is a powerful tool for discovery because it means that even a handful of observed events would be a clear sign of new physics. The study calculated that if the Higgs boson decays into dark photons with a probability as low as one in a thousand, the FCC-ee would be sensitive enough to detect it. The analysis showed that the machine could probe dark photon masses ranging from 0.36 to 10 GeV and kinetic mixing parameters as small as one part in ten million.

The researchers compared their projected sensitivity to the current limits set by existing experiments at the Large Hadron Collider, such as those conducted by the ATLAS and CMS collaborations. They found that their proposed search at the FCC-ee would be competitive with, and in some cases more sensitive than, the best current results, particularly for dark photons with very low masses and long lifetimes. The study highlights that the FCC-ee would be uniquely capable of exploring the "long-lived" regime, where particles travel a measurable distance before decaying, a region that is difficult to access with other methods. The team selected three specific examples to illustrate their findings, showing how the detector would respond to dark photons with masses of 0.36, 1.1, and 7.0 GeV, each with different decay lengths ranging from about 2 millimeters to 148 millimeters. In each case, the selection criteria successfully isolated the signal from the background.

This work represents a prospective study, meaning it is a roadmap for what could be achieved rather than a report on data already collected. The findings are based entirely on simulations of the detector's response and the theoretical behavior of the particles. However, the clarity of the results suggests that the FCC-ee is an ideal environment for this type of search. The ability to reduce the background to zero while retaining high efficiency for the signal indicates that if dark photons exist within the range of masses and couplings studied, the FCC-ee would likely find them. The study concludes that this facility offers a powerful new window into the dark sector, with the potential to significantly constrain the properties of dark photons or discover them, provided they exist within the parameters explored. The research underscores the value of precision measurements and the search for rare, displaced signatures in the quest to understand the hidden forces of the universe.