Improve the Low Energy Sensitivity of the HAWC Observatory

The high altitude water cherenkov gamma-ray observatory (HAWC) has been fully operational since March of 2015 in Mexico at 4,100 meters above sea level on the hillside of the Sierra Negra Volcano. It consists of an array of 300 water cherenkov detectors, each equipped with four photo-multiplier tubes. HAWC operates 24-hours per day with a wide field-of-view (FOV, ∼ 2 sr) and a high duty cycle (∼ 95%). These make it a powerful survey and monitoring experiment for mapping the gamma ray sky at very high energies (VHE, 100 GeV to 100 TeV) and to study sources with varying intensities. Thus HAWC is well suited to detect gamma-ray counterparts of possible flaring sources seen in neutrino events observed by IceCube or gravitational wave events observed by LIGO/Virgo. Extra-galactic sources including active galactic nuclei and gamma ray bursts are characterized by power-law spectra with most of the observed photon flux at 1 Tev and below. This corresponds to the lower energy range for HAWC. To participate in this science it is essential to optimize HAWC’s performance for gamma rays below ∼ 1 TeV. This is a particular challenge as in HAWC gamma rays below ∼ 1 TeV have a low signal-to-noise ratio, the events have limited and incomplete information and the HAWC Monte Carlo simulation does not well model all aspects of the events. Fortunately HAWC data includes a well characterized gamma ray source: the Crab nebula. Thus we use the significance level and the angular resolution of the Crab to quantify our gamma ray detection sensitivity improvements. Two critical factors are involved: the interpretation of HAWC raw detector signals (referred to as data reconstruction) and the rejection of cosmic ray background (referred to as gamma hadron separation). While this thesis focuses on different optimizations for (hadron) background rejection, both factors are addressed. An example of applying one improved analysis on searches for nearby AGNs is presented