Possible discrimination between gamma rays and hadrons using Cerenkov photon timing measurements

Atmospheric Cerenkov technique is an established methodology to study TeV energy gamma rays. However, the challenging problem has always been the poor signal-to-noise ratio due to the presence of abundant cosmic rays. Several ingenious techniques have been employed to alleviate this problem, most of which are centred around the Cerenkov image characteristics. However, there are not many techniques available for improving the signal-to-noise ratio of the data from wavefront sampling observations. One such possible technique is to use the Cerenkov photon arrival times and identify the species dependent characteristics in them. Here, we carry out systematic Monte Carlo simulation studies of the timing information of Cerenkov photons at the observation level. We have parameterized the shape of the Cerenkov shower front as well as the pulse shapes in terms of experimentally measurable quantities. We demonstrate the sensitivity of the curvature of the shower front, pulse shape parameters as well as the photon arrival time jitter to primary species and show their efficiency in improving the signal-to-noise ratio. The effect of limiting the Cerenkov telescope opening angle by using a circular focal point mask, on the efficacy of the parameters has also been studied for each of the parameters. Radius of the shower front, pulse decay time and photon arrival time jitter have been found to be the most promising parameters which could be used to discriminate gamma -ray events from the background. We also find that the efficiency of the first two parameters increases with zenith angle, and efficiency of pulse decay time decreases with increasing altitude of observation. (C) 2001 Elsevier Science B.V. All rights reserved.