Queen's 2015 Nobel Prize in PhysicsChloe Lefebvre has successfully defended her PhD thesis. Her thesis’s title is "Direction Reconstruction in the SNO+ Liquid Scintillator Phase with 2.2g/L PPO”. She carried out her research under the supervision of Prof. Mark Chen.
This thesis demonstrates the recovery of directional Cherenkov information in the SNO+ detector during its scintillator phase, addressing the loss of directionality inherent to fast, high-light-yield liquid scintillators, a capability valuable both for probing low-energy solar fluxes and for constraining the irreducible 8B ve background in the neutrinoless double-beta-decay searches. Using the livetime accumulated with the detector filled with 780 tonnes of LAB doped with 2.2 g/L PPO, the analysis transposes the Correlated Integrated Directionality (CID) method introduced by Borexino, exploiting timing resolution to separate early Cherenkov photons from prompt isotropic scintillation light. Above 5 MeV, directional reconstruction identifies 81 ± 3 8B ve solar candidates against an expected 83 from solar flux calculations, providing a proof of concept for directionality in large liquid scintillator detectors, while extending the analysis down to 2.5 MeV yields 175 +43/-56 (stat.) ± 76 (syst.) solar candidates out of an expected 103 in a sample of 1303 events. Despite limitations in livetime and radiopurity, these results demonstrate that CID can be applied to SNO+ data to extract directional information from the 8B ve solar signal.
