Physical mechanism of delayed isolated S1/S2 signals after high‑energy interactions in liquid xenon TPCs

Determine the physical mechanism that produces delayed isolated S1 photoelectron bursts and small S2 signals following preceding high‑energy interactions (with S2 areas larger than 10,000 photoelectrons) in liquid xenon time projection chambers, including XENONnT, which generate accidental coincidence background events in the coherent elastic neutrino–nucleus scattering analysis.

Background

In the XENONnT search for solar 8B neutrinos via coherent elastic neutrino–nucleus scattering, the dominant background arises from accidental coincidences formed by isolated S1 and S2 signals. A major source of these isolated peaks is delayed emission after preceding high‑energy gamma interactions with very large S2 areas.

This delayed activity—often called electron trains—has been observed across many liquid xenon detectors and produces single photoelectron hits and small S2 signals that can pair into spurious events in the region of interest. Although the collaboration employs data‑driven simulations and targeted cuts to mitigate this background, the underlying physical mechanism responsible for these delayed signals remains unresolved, motivating a direct determination of its cause.