Single Arm Positron Trigger for the
Heavy Photon Search Experiment
Student: Caleb Fogler
School: Old Dominion University
Mentored By: Stepan Stepanyan
The Heavy Photon Search (HPS) is an experiment at the Thomas Jefferson National Accelerator Facility (JLab). Heavy photons (A'), gauge bosons responsible for Dark Matter interactions, would be evidenced by a sharp peak in the invariant mass distribution of electron-positron pairs produced in electron-nucleus scattering (the trident production process). The HPS setup was triggered by two clusters on opposite halves of an electromagnetic calorimeter (ECal) that handled about a 20 kHz trigger rate. That trigger lost some trident events because high energy electrons went between the halves; also, the trigger recorded non-trident events from bremsstrahlung photons. Single Arm Positron (SAP) triggering could save these lost events and eliminate background. The goal of this project was to determine how to lower a SAP trigger rate to acceptable levels, without losing many trident events. ROOT was used to analyze 2.3 GeV pure trident simulations to find optimum cuts to make on ECal clusters and calculate the percentage of lost events. Then analysis was performed on 2.3 GeV real experimental and full simulation data to calculate the rate and validate the simulation. Those same analyses were performed on 4.4 GeV simulations. It was found that with position dependent cluster energy cuts the SAP trigger rate could be reduced below 20 kHz and lost only a few percent of trident events. The rate could be lowered by changing the trigger parameters of the ECal. Triggering proved effective on clusters of x > 110 mm (on the positron side of the ECal) and energies above a linear function but below a maximum. The SAP trigger kept the rate below 20 kHz for both 2.3 and 4.4 GeV beams, with less than 10% loss of events. The SAP trigger provides an acceptable rate and few lost events for HPS.