The Heavy Photon Search Test Detector

TL;DR

The paper addresses the search for a heavy photon A′ in fixed-target electroproduction and presents the HPS Test Run as a feasibility demonstration of the full experimental concept. It describes a compact forward silicon vertex tracker inside a dipole magnet and a PbWO4 crystal calorimeter, designed to identify e+e− pairs from A′ decays via invariant mass and displaced-vertex signatures, with high-rate electronics and a fast trigger to exploit CEBAF's duty cycle. Performance results for the SVT, ECal, and trigger/DAQ indicate mostly satisfactory operation, with momentum and vertex resolutions aligning with design expectations and trigger efficiencies validated against simulations. These findings support readiness for the full HPS program and its exploration of new parameter space for A′ in high-statistics fixed-target studies.

Abstract

The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment's technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e$^+$e$^-$ invariant mass spectrum, above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW0$_{4}$ crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e$^+$e$^-$ pairs requires the first layer of silicon sensors be placed only 10~cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab.

Figures (18)

• Figure 1: Rendering of the HPS Test Run apparatus installed on the beam line.
• Figure 2: Layout of the HPS parasitic run.
• Figure 3: A rendering of the SVT showing the modules on their support plates held by the hinged C-support on the left and the motion levers on the right. The sensors are shown in red and the hybrid readout boards in green. The beam enters from the right through a vacuum box with flanges for services.
• Figure 4: Both halves of the HPS Test Run SVT after final assembly at SLAC. The cooling manifolds and integrated cable runs are clearly seen.
• Figure 5: A prototype module assembly (foreground) with the 50 mrad (left) and 100 mrad (right) module assembly fixtures in the background. A pair of cooling blocks and a spacer block can be seen on the fixtures.