Reconstruction of the effective energy-deposition vertex of muon showers using PMT waveform in a large-scale liquid scintillator detector

Abstract Cosmogenic muon-induced radioactive isotopes pose a significant background source for deep-underground low-background experiments. Although rock overburdens at underground sites substantially attenuate the cosmogenic muon flux, residual muon-induced backgrounds still require active suppression. For future multi-kiloton liquid scintillator (LS) detectors, such as the Jiangmen Underground Neutrino Observatory (JUNO), shower muons contribute to more than 88% of all muon-induced isotopes. Consequently, precise reconstruction of shower vertices is essential for implementing localized spatial vetoes. We propose a novel waveform-based method to reconstruct the shower vertex, defined as the energy-deposition centroid in radius of 2 m. By subtracting the track contributions from non-shower muons in the recorded waveforms, the isolated shower component is extracted. Subsequently, using a photon propagation model and an iterative optimization algorithm, the shower vertex positions are reconstructed. Simulations show that for 68% of events, the single shower vertex resolution is better than 0.16 m, 0.15 m, and 0.26 m along X, Y, and Z respectively. Furthermore, the reconstruction efficiency exceeds 96% when requiring the distance between the reconstructed and true vertices to be less than 3.0 m. This method provides a critical technical foundation for muon-induced background suppression in JUNO and other large-scale LS detectors.