Characterization of a Maximum Likelihood Gamma-Ray Reconstruction Algorithm for VERITAS

TL;DR

This paper presents a maximum likelihood gamma-ray reconstruction algorithm for VERITAS that improves angular and energy resolution by using template-based likelihood calculations, surpassing standard methods in accuracy.

Contribution

The paper introduces a novel maximum likelihood reconstruction method for VERITAS that enhances resolution and reduces bias compared to traditional analysis techniques.

Findings

Improved angular and energy resolution over standard methods

Less biased by missing pixel information

Increased computational time per event (80 ms)

Abstract

We characterize the improved angular and energy resolution of a new likelihood gamma-ray reconstruction for VERITAS. The algorithm uses the average photoelectrons stored in templates that are based on simulations of large numbers of showers as a function of 5 gamma-ray parameters: energy, zenith angle, core location (x,y), and depth of first interaction in the atmosphere. Comparing the template predictions of the average photoelectrons in each pixel to observed photoelectrons allows us to calculate the likelihood. By maximizing the likelihood, we find the optimal gamma-ray parameters. The maximum likelihood reconstruction improves on the standard VERITAS analysis which relies on: 1. the weighted average of the axis of elongation in the images to determine the gamma-ray direction and 2. look-up tables that relate the observed energy deposition of Cherenkov photons to the true gamma-ray energy. Not only is the maximum likelihood method more accurate, but it is also not biased by missing pixel information due to the edge of the camera or pixel cleaning. The drawback is that it takes more CPU time (80 ms/event).