[Paper Review] Space-Time Discriminant to Separate Double-Beta Decay from $^8$B Solar Neutrinos in Liquid Scintillator

Runyu Jiang, A. Elagin|arXiv (Cornell University)|Feb 19, 2019

Neutrino Physics Research40 references4 citations

TL;DR

This paper proposes a space-time discriminant using Cherenkov and scintillation light arrival times and angles to separate neutrinoless double-beta decay (0νββ) events from ⁸B solar neutrino backgrounds in liquid scintillator detectors. By defining a Cherenkov-scintillation space-time boundary and applying weighted spherical harmonics analysis, the method achieves 1.3× and 2.3× background suppression at 90% and 70% signal efficiency, respectively, with additional directional reconstruction enabling further background rejection.

ABSTRACT

We present a technique for separating double beta-decay from $^8$B solar neutrino interactions in a liquid scintillator detector. The technique uses position and time of photo-electrons (PEs) to separate directional Cherenkov light from isotropic sintillation 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 $^8$B events. The Geant-4 simulation assumes a detector of 6.5 m radius filled with $^{130}$Te-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 $σ_{r_v} = $ 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 $^8$B 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.

Motivation & Objective

• To address the challenge of ⁸B solar neutrino background in large-scale liquid scintillator detectors searching for neutrinoless double-beta decay.
• To develop a method that leverages high-resolution time and position measurements of photoelectrons to distinguish two-track 0νββ decay events from one-track ⁸B neutrino events.
• To improve background suppression by exploiting the directional nature of Cherenkov light from electron tracks in ⁸B events.
• To enable directional reconstruction of scattered electrons to further suppress ⁸B background using solar correlation.
• To evaluate the performance dependence on detector parameters such as photo-coverage and vertex resolution for future detector optimization.

Proposed method

• Define a Cherenkov-scintillation space-time boundary in a 2D space of photoelectron (PE) arrival time and polar angle relative to the detector center-to-vertex axis.
• Assign weights to each PE based on its distance to the space-time boundary, emphasizing early-emitted PEs with high Cherenkov content.
• Use a rotationally invariant S-spectrum derived from spherical harmonics to reconstruct event topology from weighted PE distributions.
• Apply maximum likelihood estimation to classify events as two-track (0νββ) or one-track (⁸B) based on topological features.
• Perform directional reconstruction on one-electron events using the weighted angular distribution of PEs to estimate electron scattering direction.
• Utilize the correlation between the direction of ⁸B-scattered electrons and the Sun’s position to suppress remaining background.

Experimental results

Research questions

• RQ1 Can a space-time discriminant based on arrival time and angular distribution of photoelectrons effectively separate 0νββ decay events from ⁸B solar neutrino events in liquid scintillators?
• RQ2 How well can the Cherenkov-scintillation space-time boundary enhance the identification of Cherenkov-dominated early photoelectrons for topological reconstruction?
• RQ3 To what extent does directional reconstruction of scattered electrons improve background suppression in ⁸B-dominated environments?
• RQ4 How do photo-coverage and vertex resolution affect the performance of the topology and directionality reconstruction techniques?
• RQ5 What improvements in background suppression are achievable by enhancing Cherenkov-to-scintillation PE collection efficiency?

Key findings

• The topology reconstruction method achieves a background suppression factor of 1.3 at 90% signal efficiency and 2.3 at 70% signal efficiency in the default detector configuration.
• The polar angular resolution for directionality reconstruction is 0.46 radians, and the azimuthal resolution is 0.84 radians, with a 0.49 radian RMS error for polar angle.
• Photo-coverage beyond 65% provides diminishing returns for topological and directional reconstruction performance.
• Improving the Cherenkov-to-scintillation PE collection ratio leads to approximately 50% higher background suppression at 70% signal efficiency.
• The Cherenkov-scintillation space-time boundary effectively enriches the PE sample with Cherenkov light, enabling robust topological and directional reconstruction.
• The method’s performance is sensitive to liquid scintillator properties and light collection system resolution, suggesting that future detector development should focus on shorter emission wavelengths, longer rise times, and high-time-resolution, red-sensitive photo-cathodes.