Measurements of UHECR Mass Composition by Telescope Array

University of Utah Dept. of Physics and Astronomy & High Energy Astrophysics Institute, 201 James Fletcher Bldg., 115 S 1400 E, Salt Lake City, UT 84112, USA

^* e-mail: whanlon@cosmic.utah.edu

Published online: 17 May 2019

Abstract

Telescope Array (TA) has recently published results of nearly nine years of X_max observations providing its highest statistics measurement of ultra high energy cosmic ray (UHECR) mass composition to date for energies exceeding 10^18.2 eV. This analysis measured agreement of observed data with results expected for four different single elements. Instead of relying only on the first and second moments of X_max distributions, we employ a morphological test of agreement between data and Monte Carlo to allow for systematic uncertainties in data and in current UHECR hadronic models. Results of this latest analysis and implications of UHECR composition observed by TA are presented. TA can utilize different analysis methods to understand composition as both a crosscheck on results and as a tool to understand systematics affecting X_max measurements. The different analysis efforts utilizing fluorescence detector stereo, surface detector and fluorescence detector hybrid, and surface detector-only, currently underway at TA performed to understand composition are also discussed.