Telescope Array measurement of UHECR composition from stereoscopic fluorescence detection

Abstract

The chemical composition of ultra-high-energy cosmic rays (UHECRs) affects the observable distribution of air-shower Xmax values, the atmospheric slant depth at which the number of secondary shower particles reaches its maximum. The observed Xmax distributions at various primary UHECR energies can be compared with the distributions predicted by detailed detector simulations for any assumed composition and high-energy hadronic interaction model. In this poster, we present measurements of Xmax by the Telescope Array (TA) fluorescence detectors with stereoscopic shower reconstruction. We find that for all hadronic models considered, the data collected since TA operation began in 2007 is consistent with a chiefly light UHECR composition.