Measurement of UHECR Mass Composition Using Telescope Array's TAx4 Detectors

The Telescope Array (TA) in Millard County, Utah, has recently added 257 new surface scintillation detectors and 12 fluorescence telescopes as part of the TAx4 expansion project. Operating in hybrid mode since November 2019, the TAx4 upgrades are designed to enhance TA's capability to detect Ultra-High Energy Cosmic Rays (UHECRs) above 1 EeV, which are rare and not fully understood. Direct detection of UHECRs is impractical due to their low flux, so we instead infer the mass composition by studying the particle cascade that is created when a cosmic ray strikes Earth's atmosphere. A key parameter is Xmax, the slant depth in the atmosphere where the particle shower reaches its maximum size. Analyzing Xmax for UHECRs observed by TAx4 over four years, we find a consistently light composition at high energies when compared to QGSJetII-03 Monte Carlo simulations. This finding is consistent with previous results from TA and can be used to improve our understanding of UHECR origins and acceleration mechanisms.