Correcting the Energy Reconstruction of Photons Detected in the Electromagnetic Calorimeter

Author:  Johnathon K. Upperman
Institution:  College of William and Mary

The outermost layer of the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility consists of an electromagnetic calorimeter (EC). The EC is composed of alternating layers of scintillator strips and lead sheets and covers a large portion of the angular range. The EC is used to detect particles,such as electrons, photons, and neutrons,that meet a certain energy threshold. When a particle enters the EC, it loses some of its energy and creates a shower of light that is picked up by the scintillators. The location and amount of this detected light is then used to help identify where and with what energy a particle entered the EC. However, this system has some inefficiency built into it; for example, its layers of lead absorb some of the light instead of recording it. Since the EC contains some unavoidable inefficiency and particles do not always deposit all of their energy, the energy recorded in the EC needs to be analyzed periodically to determine what corrections need to be made to identify the true energy of incident photons. Creating the correction helps to better identify photons and to better understand the intrinsic inefficiency in the EC. The focus of this project was to create a correction for the energy of photons detected in the EC and then use this correction to understand EC inefficiency. To do this, a reaction was picked that involves only electrons, protons, and neutral pi mesons that decay into two photons. The invariant mass of the photon pairs, which were considered to have originated from the same decay, was then compared to the known theoretical mass of the neutral pi meson. The known mass was used to identify how the energy of photons needed to be corrected for different energy levels. It was found that a correction function could be created to increase the accuracy of photon reconstruction. The correction function that was discovered varies considerably from a previous correction that was implemented in 2006. The new correction will be used to analyze data that contain photons at 0.3 GeV and greater. With this new correction implemented, the efficiency of assigning energy to incident photons in the EC has increased. Also, we used this information to estimate the overall efficiency of the EC in detecting photons,a process that hasn't been performed previously on CLAS.

The Journal of Young Investigators is not affiliated with the US Department of Energy. This paper was written by a student intern with the Department of Energy and does not constitute a declarative position of either the Department of Energy or the Journal of Young Investigators.