Composition of Ultra High Energy Cosmic Rays observed by Telescope Array

The composition of ultra high energy cosmic rays (UHECRs) with energies in excess of 10¹⁸ eV is still unclear over one hundred years after the Nobel prize winning discovery of cosmic rays by Victor Hess in 1912. Evolution of UHECR composition with energy provides important clues as to their origins and allows physicists to rule out theories about their sources, methods of acceleration, and modes of propagation. Questions about the origin of spectral features such as the ankle and the GZK cutoff will be answered by fully understanding the composition of UHECRs. Telescope Array (TA) utilizes hundreds of surface detectors (SDs) constructed with plastic scintillator and dozens of highly light sensitive fluorescence detectors (FDs) to record the passage of extensive air showers in the atmosphere caused by the collision of UHECR primaries and air molecules in the upper atmosphere. TA can measure UHECR mass in a statistical manner by recording energy and depth of air shower maximum, X_max, for many air showers. TA has collected nine years of high quality X_max data which will be presented. TA can measure composition through several different analyses, these methods will also be discussed including a new technique which does not record X_max utilizing machine learning algorithms and SDs.