A feasibility study of toroidal magnets for eta meson rare decays into charged particles

Two toroidal magnets set at the target, in series with the SPES2 spectrometer, may constitute a large acceptance spectrometer for studying rare and forbidden binary decays of the eta meson. Two types of such magnets are considered, a constant gap height parallel pole face toroid and a constant angular aperture clamped face toroid, of which the characteristics and performances are studied in detail. Ray-tracing calculations, using calculated magnetic field maps and a particle generator based on Monte Carlo simulations, show that such a pair of magnets provides 0.5 to 1% momentum resolution (FWHM) for the decay particles and an overall acceptance within 10-15% for decay events.