In the first part of the project implementation, Monte-Carlo (MC) samples of neutral pion decays were studied, and basic selection criteria for signal decay modes were developed. The selection procedures were then optimized to boost the signal acceptance by employing a flexible particle identification algorithm that relies on the specific kinematics of the studied pion decays.
The MC samples were compared to the data collected by the NA62 experiment. It was found that the simulation of the NA62 Straw spectrometer was not satisfactory and did not reproduce the features observed in the data. Therefore, a significant update of the MC simulation of the Straw detector has been carried. The update improved the agreement between the simulation and the experimental data.
The trigger chain used to collect studied neutral pion decays comprises both a hardware di-electron trigger and a software trigger identifying multi-track events. A minimum bias trigger, collected in parallel to the physics triggers, was used to evaluate the time-dependent efficiencies of various signal trigger components.
The central part of the analysis focused on the background processes that can contribute to the signal samples. Apart from the expected processes originating from misidentification of other pion and charged kaon decays, an additional important background process was identified. This process involves a photon conversion into an electron-positron pair in the Straw detector material. A dedicated biasing of the MC simulation was implemented to study this process in detail and provide precise background estimates for the final measurement.
By studying the data set collected by the NA62 experiment, it was estimated that the sample size of pion decays into an electron-positron pair is comparable to the previous best measurement by the KTeV experiment. The data analysis is in the state of finalization, and its results will be published soon.
