Rates and survivability of PN‑induced neutral‑kaon backgrounds in the HCal visibles search at full luminosity

Determine the production rates of neutral kaons from photo‑nuclear reactions in the Light Dark Matter Experiment that decay to neutral pions and produce electromagnetic showers in the Back Hadronic Calorimeter, and ascertain whether any such events would survive the full selection for the visible dark photon (A′ → e+e−) displaced‑decay analysis in a 10^16 electrons‑on‑target dataset. This quantification is required to establish the residual background risk for the visibles search and to validate that the analysis remains background‑free at the targeted luminosity.

Background

The visibles search in LDMX targets long‑lived dark photons decaying into Standard Model particles, looking for a displaced electromagnetic shower in the Back HCal with minimal energy in the ECal and a low‑momentum recoil electron. A particularly concerning background arises from photo‑nuclear reactions that produce neutral kaons (Ks or KL) which subsequently decay to neutral pions; the resulting photons can create electromagnetic showers in the Back HCal that closely mimic the signal.

While the analysis includes a sequence of calorimetric and tracking‑based vetoes, the collaboration notes that the rates of these specific PN‑induced kaon backgrounds at the ultimate luminosity and their potential to pass all selection cuts remain to be established. Resolving this uncertainty is necessary to confirm that the visibles search can be conducted with negligible background at 10^16 EoT.