Constraints on a Long-Lived X17 Vector Boson with FASER
This paper combines FASER's Run 3 searches for dark photons and axion-like particles to constrain the protophobic X17 vector boson, excluding a previously untested low-coupling window while leaving a high-coupling window open.
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 author. For technical accuracy, refer to the original paper. Read full disclaimer
The universe is built on a foundation of known particles and forces, a framework physicists call the Standard Model. For decades, this model has successfully explained how matter behaves, from the smallest atoms to the largest stars. Yet, like any map, it may have missing territories. In recent years, a series of puzzling signals from a laboratory in Hungary has hinted at the existence of a new, invisible particle. This potential discovery involves a tiny, short-lived burst of energy that seems to appear when excited atomic nuclei relax back to a calm state. If real, this particle would be a ghostly messenger, interacting so weakly with ordinary matter that it slips through the cracks of our current understanding, potentially opening a door to a hidden sector of physics.
The mystery began with observations of beryllium and helium nuclei. When scientists fired protons at lithium to create excited beryllium, the nuclei decayed by spitting out pairs of electrons and positrons. The angle at which these particles flew apart did not match the predictions of the Standard Model. Instead, they clustered at a specific angle, suggesting they were born from the decay of a new, heavy particle with a mass of about 17 million electron volts. A similar anomaly appeared in helium nuclei, where the decay products clustered at a different angle, exactly as theory would predict if a single new particle of that specific mass were responsible. This hypothetical particle, dubbed X17, would be a vector boson, a type of force carrier, but one that is "protophobic," meaning it barely interacts with protons, making it incredibly difficult to spot in traditional experiments.
To solve this puzzle, a team of researchers turned their attention to the Large Hadron Collider at CERN, the most powerful particle accelerator on Earth. Specifically, they looked to FASER, a detector situated 480 meters down a tunnel from the main collision point. While the main detectors at the collider catch the violent debris of head-on collisions, FASER is positioned to catch particles that shoot straight forward, traveling hundreds of meters before decaying. The researchers reasoned that if the X17 particle exists, it could be produced in these high-energy collisions, travel the length of the tunnel, and then decay into an electron and a positron right inside the FASER detector. Because the particle is so light and interacts so weakly, it would be long-lived enough to make the journey, unlike most other particles that vanish instantly.
The team combined data from two previous searches conducted by FASER. One search looked for "dark photons," a related type of particle, using data from 2022. The other looked for "axion-like particles," using data from 2022 and 2023. By merging these efforts, the researchers effectively turned the entire magnetized region of the detector into a giant trap for the X17. They analyzed billions of collisions, looking for the distinct signature of a particle decaying into a pair of charged tracks that curved in a magnetic field and deposited a massive amount of energy in a calorimeter. The analysis was rigorous, accounting for every possible source of background noise, from stray neutrinos to random cosmic rays, ensuring that any signal found would be genuine.
The result was a definitive null finding. The researchers found no excess events above the expected background. In the specific region of parameter space where the X17 particle was thought to exist with a mass of 17 MeV, the detector saw nothing. This absence of evidence allowed the team to rule out a specific range of interactions for the particle. They found that if the X17 exists, its coupling to electrons cannot be in a "low-coupling" window that had previously been considered a viable possibility. This window, which represented a range of interaction strengths between 6 × 10⁻⁵ and 1.2 × 10⁻⁴, has now been closed. The only remaining possibility for the X17 to explain the Hungarian anomalies is if it interacts with electrons much more strongly than previously thought, in a "high-coupling" window that FASER was not sensitive enough to probe.
This outcome does not disprove the existence of the X17 particle entirely, but it significantly narrows the search. The experiment has effectively eliminated the "low-coupling" option, a region where the particle would interact very weakly with electrons. The data suggests that if the X17 is real, it must interact with electrons more strongly than the models in that excluded window allowed. The work demonstrates the power of placing detectors far from the collision point to catch these elusive, long-lived particles. While the mystery of the Hungarian anomalies remains, the path forward is now clearer, with the low-coupling options removed from the table and the high-coupling region standing as the only remaining candidate for a protophobic vector boson explanation.
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