OPTIMIZATION OF GAMMA/HADRON SEPARATION UNDER VARIABLE SOURCE INTENSITIES AND ENERGY SPECTRA

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory consists of an array of water Cherenkov tanks that can detect extensive air showers (EAS) generated by astrophysical cosmic rays and gamma rays. One of the greatest challenges in using the HAWC observatory to search for astrophysical gamma-ray sources (e.g. black holes, neutron stars, supernova remnants) is identifying an EAS as a gamma ray shower.  The identification of gamma rays would be very reliable, because the astrophysical flux of charged cosmic rays is 1,000 to 10,000 times larger than the flux of gamma rays.  Charged cosmic rays will create subnuclear particles (i.e. mesons and hadrons) that will, over the course of the shower, decay into subshowers, muons and neutrinos. Using the data collected at this observatory, a method has been devised to separate cosmic rays from gamma rays by looking at the signatures of the subshowers and muons.  A parameter known as the compactness parameter has been defined as the ratio of the number of PMTs hit in the event (nHit) to the measured number of PEs (CxPE) in the PMT with the largest number of recorded PEs outside a 40 m radius from the reconstructed shower core.  Using this parameter, cuts can be made in order to optimize the gamma/hadron separation.  In my research I varied the radius of this CxPE parameter and made cuts based on CxPE radii of 10 m, 20 m, 30 m, 40 m, and 50 m.  I then applied these cuts to an already known source, the Crab Nebula, and plotted the significance of that source as a function of the CxPE radii.  I was able to confirm that the 40 m CxPE radius as predicted by Monte Carlo simulations yields the highest significance.