Monte Carlo simulation and resolution study of the η → e⁺e⁻ decay in the WASA-at-COSY detector

Abstract:

A comparative study is done on the Mini Drift Chamber (MDC) and the Scintillating Electromagnetic Calorimeter (SEC), two main components of the WASA detector. The purpose of the study is to determine the most effective way to calculate the mass of the η−meson by determining the invariant mass of the final products in the η → e⁺e⁻ decay. When calculating the invariant mass of the lepton pair the distribution from the MDC had a standard deviation of σ_MDC = (4.212 ± 0.080) · 10⁻² GeV/c² and from the SEC σ_SEC = (3.563±0.011)·10⁻² GeV/c². To get a precise measurement, events with a lepton scattering at a polar angle below 30^◦ have to be rejected, and to achieve highest precision, it is favorable to use the SEC for momenta above 0.28 GeV/c and the MDC below, respectively. In this study, a combined method is developed which results in an invariant mass resolution of σ_Mix = (3.341±0.012)·10⁻² GeV/c². Thus, the combined method gives a small improvement compared to using the SEC alone, but a considerable improvement compared to using only the MDC.