Accurate reconstruction of the horizontal vertex (x,y) from the S2 electroluminescence pattern is essential for fiducialization and background rejection in dual-phase argon time projection chambers. In this work, we perform a Geant4-based simulation study using the G4DS framework to investigate how detector geometry, in particular the distance between the top photodetector plane and the gas pocket, impacts S2-based XY reconstruction. A compact dual-phase argon TPC instrumented with seven Hamamatsu R8520-506 PMTs is simulated with electron recoils at 41.5 keV (corresponding to the 83mKr calibration energy), as well as 1.0 keV to probe the low-S2 regime. The PMT array height is scanned from 0 mm to 50 mm, and XY positions are reconstructed using a geometrical solid-angle (GSA) method with the S2 emission modeled by 1 mm-thick slices across the 7 mm gas pocket. The results show a clear non-monotonic dependence of reconstruction bias and resolution on PMT height, driven by the trade-off between S2 light sharing and photon statistics. These findings provide guidance for geometry optimization in future low-threshold dual-phase argon detectors and will be validated with upcoming prototype measurements.
@article{arxiv.2602.08402,
title = {Geometry-driven impact of photosensor placement on S2-based XY reconstruction in a dual-phase argon TPC},
author = {Jilong Yin and Yi Wang},
journal= {arXiv preprint arXiv:2602.08402},
year = {2026}
}