A Measurement of the Average Longitudinal Development Profile of UHECR Air Showers by the HiRes Experiment

A method of subtracting Cherenkov light component is presented and applied to the HiRes stereo data to measure longitudinal development profiles of extensive air showers. By normalizing and averaging over all showers, an average longitudinal development profile of extensive air showers is obtained. The Gaisser-Hillas, Gaussian-in-Age, and Greisen functional forms are tested as models of the average profile. They all describe the average profile well. Uncertainties of the average profile are discussed. A correlation is found between the profile width and shower maximum depth. The parameter σ in the Gaussian-in-Age model(as the width of profile)is also found to be near constant over shower energies from 10¹⁷ eV to 10²⁰ eV.