The Heavy Photon Search beamline and its performance

TL;DR

The paper describes the design, implementation, and performance of the Heavy Photon Search beamline at JLab, focusing on its capabilities for detecting dark photons via a specialized forward spectrometer during engineering runs.

Contribution

It presents the detailed design and performance evaluation of the HPS beamline and detector setup used in the search for heavy photons at JLab.

Findings

Successful operation of the beamline during engineering runs

Achievement of required beam stability and profile for detector proximity

Validation of detector performance in a high-rate environment

Abstract

The Heavy Photon Search (HPS) is an experiment to search for a hidden sector photon, aka a heavy photon or dark photon, in fixed target electroproduction at the Thomas Jefferson National Accelerator Facility (JLab). The HPS experiment searches for the e$^+$e$^-$ decay of the heavy photon with bump hunt and detached vertex strategies using a compact, large acceptance forward spectrometer, consisting of a silicon microstrip detector (SVT) for tracking and vertexing, and a PbWO$_4$ electromagnetic calorimeter for energy measurement and fast triggering. To achieve large acceptance and good vertexing resolution, the first layer of silicon detectors is placed just 10 cm downstream of the target with the sensor edges only 500 $\mu$m above and below the beam. Placing the SVT in such close proximity to the beam puts stringent requirements on the beam profile and beam position stability. As part of an approved engineering run, HPS took data in 2015 and 2016 at 1.05 GeV and 2.3 GeV beam energies, respectively. This paper describes the beam line and its performance during that data taking.