Speaker
Description
Neutral current π⁰ (NCπ⁰) events are a major background for ν$_e$ signal events in T2K neutrino oscillation measurements. These events, characterised by two photons from π⁰ decay, can mimic the detector response of electrons in Super-Kamiokande (Super-K). Previously, there were no dedicated samples to effectively constrain this NCπ⁰ background. To address this, events reconstructed with two Cherenkov rings (the 2-ring π⁰ sample) have been selected, improving the modeling of NCπ⁰ interaction cross-sections. In addition, events reconstructed as single-ring, which were previously rejected from the 1-ring e-like selection, are now included (referred to as the 1-ring π⁰ sample). This adjustment recovers a portion of the ν$_e$ signal events that were previously misidentified and rejected during selection.
The inclusion of these NCπ⁰ samples offers a model-independent constraint on NCπ⁰ backgrounds directly from Super-K data and increases the total ν$_e$ ($\bar{ν}_e$) signal events by 7% (11%) at T2K. Since NC interactions are flavor-independent, these samples provide a large event sample to serve as a cross-check of the total neutrino flux at Super-K, in comparison with the unoscillated flux measured near the neutrino beam production point. Furthermore, NCπ⁰ samples offer unique advantages for sterile neutrino searches. A deficit in charged-current (CC) events without a corresponding deficit in NC events would be a strong signature of sterile neutrino oscillations.
