Composition of Ultra High Energy Cosmic Rays Observed by Telescope Array's TAx4 Detectors in Hybrid Mode*

The Telescope Array (TA) cosmic ray observatory in Millard County, Utah has deployed 257 new surface scintillation detectors (SDs) and 12 new fluorescence telescopes (FDs) as part of the TAx4 expansion project. These detectors have been operating in hybrid mode since November 2019, with the goal of increasing TA's event statistics at the highest energies. TAx4 is designed to detect cosmic rays with energies above 1 EeV, known as Ultra-High Energy Cosmic Rays (UHECRs), which are rare and whose properties are not fully understood. In particular, the mass composition of UHECRs is currently the subject of debate. Since direct detection of UHECRs is infeasible due to their extremely low flux at Earth, information about the primary nuclear species must be inferred through a statistical study of the particle cascades that are created when they strike the atmosphere. A particularly useful parameter for studying composition is Xmax, the atmospheric depth at which the particle shower reaches its maximum size. By learning about UHECR composition, we can better interpret the UHECR energy spectrum, constrain possible astrophysical sources, and learn more about the acceleration mechanisms of nature's most energetic particles. In this talk we will report on the analysis of Xmax for UHECRs observed by TAx4 over a 3 year period. The preliminary hybrid Xmax data indicate a light and unchanging composition at the highest energies, in agreement with previous TA results.