The HPS electromagnetic calorimeter

TL;DR

The paper details the design and performance of the electromagnetic calorimeter used in the Heavy Photon Search experiment, which aims to detect a hypothetical heavy photon via its decay into electron-positron pairs.

Contribution

It presents the construction, calibration, and operational performance of the PbWO4 crystal-based calorimeter in the HPS experiment during 2015-2016.

Findings

Calorimeter successfully provided fast trigger signals.

Effective background reduction through matching with tracking detector.

Stable operation and calibration during data runs.

Abstract

The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called "heavy photon." Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015-2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier.