Determining the Mass Composition of Cosmic Rays Using Shower Universality

The mass composition of ultra-high energy cosmic rays is an important parameter for understanding their origin. Using both fluorescence and surface detectors, The Pierre Auger Observatory measures the depth of shower maximum, $X_{\rm max}$, from which the mass of the primary particle can be inferred. The surface detector measurement of $X_{\rm max}$ is based on the principle of shower universality, and increases the number of cosmic rays by at least a factor of 10 with respect to the fluorescence detector measurement since it is not limited by the duty cycle of the fluorescence telescopes. We present an event-by-event comparison of the $X_{\rm max}$ measurements from both types of detectors for energies above $10^{18.8}$~eV, and a preliminary anisotropy study discriminating by the mass of the primary particle calculated using universality.