Cosmic-ray neutrons tamed: simulations finally match underground detector data

Deep underground, where physicists hunt for some of the rarest signals in the universe, an invisible enemy stalks every experiment: neutrons spawned by cosmic-ray muons. These uncharged particles can mimic the signatures of neutrinoless double beta decay, dark matter interactions, and even the neutron-oscillation events that next-generation detectors are designed to catch. A new comprehensive study published in The European Physical Journal C by Yijian Jiang, Jie Cheng, Haoqi Lu, Yaoguang Wang and colleagues now delivers the most detailed benchmark yet of how well our simulations actually predict this stubborn background, and the results reveal both dramatic progress and a persistent, puzzling tension.

The team centered their investigation on the Daya Bay Reactor Neutrino Experiment, whose gadolinium-doped liquid scintillator detectors sit under 250 meters-water-equivalent of rock in Experimental Hall 1. Daya Bay had previously performed a precision measurement of muon-induced neutron production with relative systematic uncertainties below 10 percent, making it an ideal yardstick. Earlier comparisons had exposed a troubling gap: default simulations overpredicted the measured neutron yield by roughly 20 percent, while at KamLAND the discrepancy reached 30 percent or more. For future experiments like the Jiangmen Underground Neutrino Observatory, JUNO, which will need exquisitely accurate background models, closing that gap is not a luxury but a necessity.

To dissect the problem, the researchers built a full GEANT4-based simulation of the Daya Bay detector geometry, complete with the surrounding water Cherenkov veto, and tested three representative hadronic physics lists: FTFP_BERT_HP, QGSP_BERT_HP, and QGSP_BIC_HP. These lists differ in two crucial ways. At high energies, hadronic interactions are described by string models, either the Fritiof model or the quark-gluon string model, while at low and intermediate energies the intranuclear cascade is handled by either the Bertini or the binary cascade model. All three lists share the high-precision neutron package, which uses evaluated data-driven cross sections below 20 MeV to describe neutron moderation and capture with high fidelity.

The muon flux itself was modeled with care. Starting from a modified version of Gaisser’s sea-level formula adapted for the Daya Bay site, the team propagated muons through a digitized mountain profile using the MUSIC code, reproducing a simulated muon rate of 1.27 per second per square meter and an average muon energy of 63.9 GeV underground. Both values agree well with the measured flux of 1.21 plus or minus 0.12 per second per square meter and with the angular distributions validated by Daya Bay’s resistive plate chamber telescope system. Roughly 61 percent of muons depositing at least 20 MeV in the scintillator were found to traverse the gadolinium-loaded target, with an average path length of 206.3 centimeters, closely matching the 204.1 centimeters from Daya Bay’s own simulations.

The heart of the study lies in its innovative hybrid correction scheme. The TALYS nuclear reaction code, version 1.8, was used to compute more accurate inelastic cross sections for neutrons, protons, and gamma rays striking carbon-12 below 200 MeV, the energy range where most secondary cascade particles reside. Because a single muon-induced cascade contains many individual neutron-producing vertices, the authors implemented a vertex-level reweighting: each neutron-producing interaction was rescaled by the ratio of the TALYS to the GEANT4 exclusive cross section for that channel and projectile energy. Crucially, the method preserves GEANT4’s final-state kinematics rather than substituting TALYS event-by-event, avoiding violations of momentum balance with the nuclear remnant.

The cross-section comparison itself was revealing. Inclusive inelastic cross sections from the three GEANT4 lists were nearly identical for each projectile type, but TALYS predicted values about 20 percent higher for neutrons and 30 percent higher for protons, while its gamma-ray cross-section peak sat at lower energies with a magnitude roughly 50 percent below GEANT4’s. In exclusive neutron-multiplying channels, the choice of intranuclear cascade proved far more influential than the high-energy string model. The BIC-based list shifted production weight away from neutron-induced reactions, about 32.8 percent versus 37.9 percent for the BERT lists, toward charged pion and proton channels, and favored final states with higher neutron multiplicity.

When the corrected simulations were benchmarked against the Daya Bay measurement of the neutron yield, the improvement was striking. The original GEANT4 lists overestimated the measured value of 10.26 times ten to the minus five per muon per gram per square centimeter by roughly 13 to 20 percent. After TALYS-based reweighting, the deviation for the BERT-based model shrank from about 20 percent to approximately 6 percent, while for the BIC-based model it collapsed from roughly 13 percent to the sub-percent level, about 0.3 percent. The correction systematically reduced yields by 12 to 14 percent, demonstrating that the total neutron yield is acutely sensitive to inelastic cross sections in the 50 to 200 MeV window, where TALYS values run systematically lower than GEANT4’s.

Yet the story is not one of unqualified triumph. The team also compared the tagged neutron multiplicity, the number of neutrons captured on gadolinium following each muon, against Daya Bay data. Here a clear tension persists across every model tested: simulations systematically underproduce single-neutron events while overproducing multi-neutron events. The deficit at multiplicity one, the dominant category, had already been flagged in Daya Bay’s earlier comparison using GEANT4 version 9.2, and the TALYS correction actually worsens it, since the fraction of gamma-carbon reactions contributing to single-neutron events drops in the corrected simulation. Quantitative chi-squared tests confirm that the BIC-based model agrees better with data than the BERT-based one across all multiplicities, but the single-neutron deficit remains the largest residual discrepancy.

The authors trace this deficit to the final-state modeling of highly excited carbon-12 nuclei, pointing to GEANT4’s Fermi break-up and evaporation models as the components most in need of refinement. Their analysis shows that single-tagged-neutron events are predominantly induced by gamma-carbon and neutron-carbon reactions, whereas multi-neutron events arise mainly from secondary neutron interactions. This diagnosis suggests a concrete two-step strategy for future work: first, apply TALYS-based cross-section adjustments to nail down the overall normalization of the cosmogenic neutron yield for background subtraction; second, use the residual multiplicity discrepancies, especially the single-neutron deficit, as quantitative constraints for tuning intranuclear cascade parameters and channel-specific reweighting.

The implications reach well beyond liquid scintillator. The authors emphasize that the same framework applies to water Cherenkov detectors like Hyper-Kamiokande and liquid-argon time-projection chambers like DUNE, where cosmogenic neutrons pose equally serious background threats. The methodology may even transfer to atmospheric neutrino neutral-current interactions, whose final-state particle energies overlap the range probed here. By establishing a reproducible, data-driven benchmark and a clear refinement roadmap, the study transforms cosmogenic neutron modeling from a source of nagging uncertainty into a tractable engineering problem, one that the next generation of rare-event searches will be better equipped to solve.

Subject of Research: Cosmogenic neutron production and hadronic simulation modeling in large underground liquid scintillator detectors

Article Title: Comprehensive study of cosmogenic neutron production in large liquid scintillator detectors

Article References: Jiang, Y., Cheng, J., Lu, H., & Wang, Y. (2026). Comprehensive study of cosmogenic neutron production in large liquid scintillator detectors. The European Physical Journal C, 86(9), Article 1082. https://doi.org/10.1140/epjc/s10052-026-16173-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16173-2

Keywords: cosmogenic neutrons, liquid scintillator, Daya Bay, GEANT4, TALYS, hadronic physics lists, neutron yield, neutron multiplicity, cosmic-ray muons, underground experiments, JUNO, background modeling