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340 Space-Time Correlated Discriminant to Separate Double-Beta Decay from Solar Neutrinos in Liquid Scintillator

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Description: We present a technique for separating nuclear double beta-decay events from background 8B solar neutrino interactions in a liquid scintillator detector instrumented with photo-detectors with mm space and 100-psec time resolutions. The technique uses position and time information of detected photo-electrons (PEs) to separate directional Cherenkov light from isotropic scintillation light in the reconstruction of the kinematics of candidate events. Here we introduce a Cherenkov-scintillation space time boundary defined as the light cone in the 2-dimensional space of the arrival time and the polar angle of each PE with respect to the axis from the center of the detector to the vertex. The PEs located near the boundary correspond to photons that were emitted early and contain a high fraction of directional Cherenkov PEs. We apply weights derived from the distance to the boundary of each individual PE, which are then used in a spherical harmonics analysis that separates the two-track event topology of double-beta decay from the one-track topology of 8B events. The GEANT4 simulation assumes a detector of 6.5 m radius filled with 130Te-loaded liquid scintillator and surrounded by photo-detectors with time and space resolutions of 100 ps and 3 mm, respectively. The scintillation properties and photo-detector quantum efficiency are modeled after KamLAND. Assuming a fiducial volume of 3 m radius, a photo-coverage of 65% and vertex resolution of 5.2 cm at 2.53 MeV the method of reconstructing event topology predicts factors of 1.3 and 2.3 in background suppression at 90% and 70% signal efficiency respectively. Additionally, the PEs near the Cherenkov-scintillation space-time boundary can be used to reconstruct the directionality of one-electron candidate events, allowing for further 8B background suppression due to the correlation between the direction of the scattered electron and the position of the sun. We find polar and azimuthal angular resolutions of 0.46 and 0.84 radians, respectively. We show the dependence of the topology and directionality reconstructions on photo-coverage and vertex resolution, and discuss directions in detector development that can improve background suppression; however determination of a combined background rejection factor based on topological and directional reconstruction is a subject of further studies using a detailed detector-specific background model.

Andrey Elagin
Runyu Jiang



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