Telescope Array Composition Summary

Abstract

Ultra high energy cosmic ray (UHECR) chemical composition is important to resolving questions about the locations of UHECR sources and propagation models. Because composition can only be deduced by a process of statistical inference via the observation of air shower maxima ($X_{\mathrm{max}}$), UHECR observatories with large data collection rates must be employed to reduce statistical fluctuations. Telescope Array (TA), the largest cosmic ray observatory in the Northern Hemisphere, is designed to answer the question of UHECR composition, as well as other important features of cosmic ray flux, by combining a large array of over 500 scintillation surface detectors spread over 700~km$^{2}$, and three fluorescence detector stations overlooking the array. With eight years of data recorded, results of the measurements of UHECR composition will be presented. UHECR composition is traditionally measured by comparing the first and second moments of the distributions of shower maxima, which evolves with energy, between data and simulations. Reducing statistical fluctuations in the data helps to distinguish between different primary elements in the flux. In the current generation of cosmic ray observatories, UHECR data sets are large enough, and statistical uncertainties are now small enough, that we can safely distinguish between very light primary source flux (i.e., protons) and heavy flux (i.e., iron). Reducing systematic uncertainties is also important though, since large systematic shifts in air shower maxima will influence the interpretation of the data when compared to models. TA therefore employs different methods of measuring \xm, including stereo air fluorescence, air fluorescence-surface counter hybrid, and a new technique using only surface counters. Updated results of TA hybrid composition among the different methods are presented using up to eight years of data. Agreement among all TA hybrid composition results are shown as well as detailed systematic errors which can be further explored by comparing composition results of the different measurement methods. Comparison of TA $X_{\mathrm{max}}$ data are compared to different composition models as well.