Study on Mass Composition of Ultra-High Energy Cosmic Rays by Telescope Array

Abstract: Results from the TA mass composition studies are reviewed. From the analysis of longitudinal development of air showers using the fluorescence detector data we obtained 〈Xmax〉, the average depth of shower maxima, as a function of energy. This is the best estimator for cosmic ray composition because of its strong dependence on primary mass. The data provided by the surface detectors for the ground-level information of showers are also used to improve the FD shower reconstruction accuracy. The obtained Xmax data are compared with expectations from particular composition models, pure proton and iron. The TA data prefers a proton-dominant composition in the energy range log10(E/eV) = 18.3 ∼ 19.5