Factorial moment studies of Cherenkov images

1. Motivation In this exploratory simulation studies, we use factorial mo ments as a tool to segregrate -ray and proton initiated showers. Extensive Air Showers (EAS) are produced when VHE/UHE primary photons, protons and other high Z nuclei enter atmosphere from the top and produce Cherenkov radiation which can be studied by imaging technique or by measuring lateral distribution. In the Hillas parameterization approach, image is approximated as an ellipse and parmeters like image ”shape” and ”orientation” are calculated. Hillas paramaters like length,width,alpha,distance,size etc. are essentially set of second order moments. Among these parameters ”width” and ”alpha” are known to be very powerful parameters for segregation of -ray initiated cherenkov photon from cosmic ray background. Using this approach, present day -ray telescope have been able to reject cosmic ray background events up to 99.5 % level, while retaining upto 50 % of the -ray events from a point source. Hillas parameters are found to be good classifiers for small images (close to telescope threshold energies) but fail for large images (of higher pri mary energy) as too many pixels are part of the image. Again for GeV energy region telescopes like MAGIC and MACE, Hillas parameters alone are not very attractive. A loss of 50 % of actual image due to parameterization is also a big constraint. Hence the idea of trying factorial moments as a possible tool is a subject matt er of this paper.