Gamma-hadron discrimination based on shower observables is essential for identifying gamma-ray astrophysical sources at the highest energies. In this work, we introduce Ptailα,T, a new discrimination variable for ultra-high-energy photon searches within the framework of a water-Cherenkov detector (WCD) array. The observable extends signal-integrated methods by incorporating the time structure of WCD traces, using cumulative signal distributions. Using simulated proton- and gamma-induced air showers at energies around 1017eV, we evaluate the performance of Ptailα,T and compare it with established WCD-based observables such as Sb, risetime-based variables, and the SWGO-inspired, Ptailα. The new variable attains a background contamination of roughly 2×10−2 at 50% gamma efficiency, improving upon existing WCD-only methods by nearly a factor of five and approaching the performance of an idealized muon-isolating reference. These results demonstrate the effectiveness of exploiting time-resolved signal tails to enhance ultra-high-energy photon searches in sparse surface arrays.
@article{arxiv.2602.12167,
title = {Time-Structured Tail Probabilities for Ultra-High-Energy Gamma-Hadron Discrimination in Water-Cherenkov Arrays},
author = {Ruben Conceição and Pedro J. Costa and Mário Pimenta},
journal= {arXiv preprint arXiv:2602.12167},
year = {2026}
}