Search for Neutrino-Produced Long-Lived Particles Using the SBND Cosmic Ray Tagger Beam Telescope
This paper presents the first search for long-lived particles decaying into electron-positron pairs using data from the standalone SBND Cosmic Ray Tagger Beam Telescope, finding no significant excess above background and setting 90% confidence-level upper limits on the effective production cross section at the level of cm for decay lengths between 0.3 and 100 meters.
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 authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the vast, invisible landscape of particle physics, scientists are constantly searching for cracks in the known rules. The Standard Model is our current best map of the universe, detailing how fundamental particles like electrons and quarks interact. Yet, this map has gaps. It cannot explain why the universe is filled with matter rather than antimatter, or why gravity is so weak compared to other forces. To fill these gaps, physicists hypothesize the existence of "long-lived particles." Unlike the fleeting particles that usually appear and vanish in the blink of an eye inside high-energy collisions, these hypothetical particles would be born, travel a measurable distance—perhaps a few meters or even hundreds of meters—and then decay into visible matter. Finding such a particle would be a direct window into new physics, potentially revealing hidden sectors of the universe that interact with our own only very weakly.
At the Fermi National Accelerator Laboratory in Illinois, a team of researchers recently turned their attention to a unique opportunity to hunt for these elusive travelers. They utilized a specialized detector system known as the Cosmic Ray Tagger, which was originally installed to help calibrate a larger experiment called the Short-Baseline Near Detector. Before the main detector was fully assembled, a subset of this tagging system operated as a standalone telescope between 2017 and 2018. During this time, it collected data from a powerful beam of protons smashing into a target, creating a flood of neutrinos. The researchers used this historical dataset to look for a very specific signature: a particle that appears out of nowhere in the empty space between two detector panels, decays into an electron and a positron, and leaves no trace of its origin.
The search focused on a scenario where a neutrino from the beam strikes a nucleus in the material surrounding the detector, such as the soil or concrete of the building. In this interaction, the neutrino could theoretically "upscatter," transforming into a heavy, invisible particle that flies away from the collision point. This new particle would travel through the air, unimpeded, until it reached the space between the detector's upstream and downstream panels. There, it would decay into a pair of charged particles—an electron and its antimatter twin, the positron. Because the decay happens in mid-air, far from any solid material, the resulting particles would appear as a sudden, isolated burst of energy in the downstream panel, with no corresponding activity in the upstream panel to explain where they came from.
To find this signal, the team had to overcome a significant challenge: the background noise. The detector is constantly bombarded by particles from the neutrino beam and by cosmic rays from space. These interactions often produce muons, which are heavy cousins of electrons, that can mimic the signal the scientists were looking for. The researchers developed a rigorous set of rules to filter out this noise. They required that the upstream panel, which acts as a gatekeeper, saw absolutely no activity. This ensured that any particles detected downstream were not simply passing through from the beam. They also looked for events where two distinct hits occurred in the downstream panel, separated by a specific distance, which would be consistent with the two particles from a decay. Finally, they used a sophisticated computer algorithm to analyze the timing, position, and energy of these hits, comparing the real data against millions of simulated background events to see if the pattern matched a new particle or just a random fluctuation.
After analyzing data corresponding to nearly three hundred quintillion protons hitting the target, the researchers found no evidence of these long-lived particles. The data matched the predictions for known background processes perfectly. There was no mysterious excess of events, no sudden spike in the data that would indicate a new particle had been discovered. While this might sound like a null result, it is a powerful scientific achievement. By finding nothing, the team was able to place strict limits on how often such particles could exist. They determined that if these particles do exist, they must be produced with an effective cross section at the level of 10⁻³⁸ cm² or lower. They also ruled out specific ranges of how far these particles could travel before decaying, effectively narrowing the search space for future experiments.
This work represents the first time the SBND collaboration has used its data to search for physics beyond the Standard Model. Although the detector was not originally designed to catch these specific decays, its precise timing and spatial resolution proved capable of performing a high-quality search. The study demonstrates that even partial detector systems can provide meaningful constraints on new physics, particularly for particles that travel macroscopic distances. While the search did not find the new particles, it has successfully mapped out a region of the universe that is now known to be empty, guiding future experiments toward the most promising places to look next. The hunt for long-lived particles continues, but the path forward is now clearer, defined by the rigorous boundaries set by this careful and thorough investigation.
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