The Mass Composition of Ultra-high Energy Cosmic Rays Measured by New Fluorescence Detectors in the Telescope Array Experiment

The longitudinal development of an extensive air shower reaches its maximum at a depth, Xmax, that depends on the species of the primary cosmic ray. Using a technique based on Xmax, we measure the cosmic-ray mass composition from analyses of 3.7 years of monocular mode operations with the newly constructed fluorescence detectors of the Telescope Array experiment. The Xmax analysis shows our data to be consistent with a proton dominant composition at energies above 1018.0 eV.