Stereo Reconstruction at HiRes

We describe a technique used to reconstruct the energy and Xmax of Ultra High Energy Cosmic Rays (UHECR) observed by the HiRes detector in stereoscopic mode. This technique calculates the relationship between the number of shower particles at a depth of the shower to the signal in either angular or time bins. This relationship is calculated for a given shower segment location. The estimated number of shower particles at a given shower depth is calculated using this relationship from the observed bin signals. The observed longitudinal shower profile is then fit using a technique developed by Martin Block which judiciously removes problematic data points. In simulation, energy resolutions of approximately 12% or better are obtained with high efficiency for energies above 10 18.5 eV. The estimated Xmax resolution for this technique is approximately 35 g/cm 2 or better at these energies.