The Coherent CAPTAIN-Mills experiment searches for beyond-the-Standard-Model particles produced at the Lujan Center beam stop, including light dark matter, sterile neutrinos, axions, and other prompt particles of interest. These searches rely on the fact that relativistic particles produced in the target can reach the liquid-argon detector before the much larger beam-correlated neutron background. Accurately defining T0, the beginning of this prompt region of interest, is therefore essential for separating potential new physics signals from delayed neutron backgrounds.

This presentation describes a data-driven and simulation-supported method for determining T0 for CCM. Because the shielding surrounding the main CCM detector prevents a direct observation of the earliest particles leaving the target, an auxiliary EJ-301 detector was placed in Flight Path 3, where a gap in the shielding allows prompt gamma rays, referred to as the ‘gamma flash’, and fast neutrons from the Lujan target to be observed. The gamma flash measured in this detector provides an experimental timing reference for the earliest speed-of-light particles produced by the beam. By analyzing approximately 30 million beam-trigger events, this study identifies roughly 4 million gamma flash events and measures the first statistically significant gamma flash onset relative to the beam-trigger signal. Using a conservative 5σ threshold, the FP3 detector onset occurs at (-363 ± 1) ns. After applying the measured timing-delay correction between the FP3 detector and CCM, this corresponds to a CCM prompt-window start time of (-503 ± 2) ns relative to the beam-trigger signal at the DAQ.

To validate the interpretation of the FP3 gamma flash, a high-fidelity Geant4 simulation of the Lujan target, moderators, shielding, Flight Path 3 geometry, and EJ-301 detector was developed. For a simulated exposure of 3 billion protons on target, the EJ-301 energy-deposition timing distribution shows a clear gamma flash arriving before the fastest neutron population. This result supports the hypothesis that the first statistically significant FP3 signals are prompt gamma rays rather than neutrons. Together, the high-statistics FP3 analysis and Geant4 validation strengthen the experimental basis for defining CCM’s prompt region of interest and improve the timing framework used in searches for light dark matter, sterile neutrinos, and other beam-produced particles.