The MUSE Target Chamber Post Veto

TL;DR

The study addresses the proton radius puzzle by enabling a direct comparison of elastic $e p$ and $\mu p$ scattering in MUSE, operating a large-acceptance, vacuum-target spectrometer with a dedicated Target Chamber Post Veto (TCPV) to suppress background from support posts. The TCPV provides two readout pathways (in-chamber SiPMs and external WLS fibers) and is integrated into the trigger system, guided by Geant4 simulations and careful mechanical design to preserve acceptance. Performance results show substantial reductions in trigger rates and strong suppression of post-scatter backgrounds, with the in-chamber readout delivering higher veto efficiency than the WLS-fiber path, thereby enabling high-statistics measurements of proton form factors and radius. The system demonstrates safe, ns-scale veto capability compatible with the LH$_2$ target, supporting MUSE’s goals of testing lepton universality and two-photon exchange effects in the proton structure.

Abstract

The Muon Scattering Experiment (MUSE) was developed to address the proton radius puzzle through simultaneous electron-proton and muon-proton scattering using the Paul Scherrer Institute's PiM1 secondary beamline. MUSE uses a large-solid-angle, non-magnetic spectrometer to detect beam particles scattering from a liquid hydrogen cell contained within a vacuum chamber. Due to the large scattering windows, the structural integrity of the chamber is supported by posts located at small scattering angles. While out of the acceptance, particles in the tails of the beam distribution can strike these posts, causing a significant trigger background. We describe the design and performance of the Target Chamber Post Veto (TCPV) detector installed inside the vacuum chamber to remove these background events at the trigger level.

Figures (22)

• Figure 1: Geant4 schematic of the MUSE setup, with primary detector systems labeled. The target chamber is located at the center of the experimental setup. Target Chamber Post Vetos (TCPVs) are not shown.
• Figure 2: The inside of the target vacuum chamber with the TCPV installed on the downstream support posts. Left: CAD diagram of target vacuum chamber interior from a top-front perspective, with front, top, and bottom chamber parts hidden for TCPV visibility. The two TCPV paddles, with blue and green components, can be seen in front of the target vacuum chamber support posts. The LH$_2$ target cell, shown as a red cylinder, and the target chamber windows, shown in brown, are also depicted. Right: Photograph of the target chamber interior looking downstream, with scattering and exit windows removed. The TCPV paddles are mounted on and supported by the holding frame, which is located at the bottom-center of the image.
• Figure 3: Photograph of two identical TCPV BC404 plastic scintillator paddles, with their WLS fiber outputs. The PCBs that hold the SiPMs are also visible on each end of the scintillator paddles.
• Figure 4: Left: 5.5 mm-diameter, 8.2 mm-long GS-type acrylic plexiglass feedthroughs, prior to installation in the target chamber. Each feedthrough holds two WLS fibers from one TCPV paddle. The WLS fibers are glued using Eljen Technology EJ-500 optical cement resin. Right: WLS Feedthroughs cast in the target chamber flange using Loctite Stycast 2850 FT black epoxy. Here, the WLS fibers seen glowing green during a test of their optical transmission.
• Figure 5: Left: A PCB for the WLS fiber readout is mounted on the target-chamber flange. The Hamamatsu SiPMs that read out the WLS fibers are glued to feedthroughs positioned in special holes in the PCB. The SiPM contacts are soldered to the corresponding PCB pads and sealed with black Loctite Stycast 2850 FT epoxy. Right: TCPV flange installed on the target vacuum chamber upstream wall, between the bars of the strong back. Four LEMO feedthroughs for the in-chamber SiPM signals are visible in the upper half of the flange.The aluminum plate covering the WLS fiber readout, with two read-out LEMO connectors oriented in opposite directions, is in the lower half of the flange.