The MUSE Beamline Calorimeter

TL;DR

This paper reports the design, calibration, and performance validation of a lead-glass calorimeter for the MUSE experiment, built to tag and suppress high-energy photons from initial-state radiation in mixed-species $e+p$ and $\mu+p$ scattering. The detector uses an 8×8 SF5 lead-glass crystal array with PMT readout, CFDs, and QDC/TDC electronics, positioned to monitor beam energy and identify radiative events. A two-step calibration strategy (hardware HV gain matching and software re-alignment) combined with a Geant4-based simulation demonstrates linear energy response and a momentum-dependent resolution compatible with sub-percent radiative-correction control. Data–simulation agreement confirms the calorimeter achieves the needed suppression of the Bremsstrahlung tail, enabling precise Born cross sections and robust tests of lepton universality at MUSE.

Abstract

The MUon Scattering Experiment (MUSE) was motivated by the proton radius puzzle arising from the discrepancy between muonic hydrogen spectroscopy and electron-proton measurements. The MUSE physics goals also include testing lepton universality, precisely measuring two-photon exchange contribution, and testing radiative corrections. MUSE addresses these physics goals through simultaneous measurement of high precision cross sections for electron-proton and muon-proton scattering using a mixed-species beam. The experiment will run at both positive and negative beam polarities. Measuring precise cross sections requires understanding both the incident beam energy and the radiative corrections. For this purpose, a lead-glass calorimeter was installed at the end of the beam line in the MUSE detector system. In this article we discuss the detector specifications, calibration and performance. We demonstrate that the detector performance is well reproduced by simulation, and meets experimental requirements.

Figures (16)

• Figure 1: Geant4 drawing of the MUSE system, showing the calorimeter at the downstream (upper right) end of the detector system.
• Figure 2: Radiative corrections for $ep$ scattering as a function of the minimum electron momentum at one MUSE kinematic setting, calculated using the ESEPP event generator esepp_paper. The radiative correction factor $\delta$ indicates the difference between the experimental and Born cross sections: $\sigma/\sigma_0 = 1 + \delta$. See Ref. Li:2023sxf for details.
• Figure 3: Schematic drawing of a calorimeter crystal. Each crystal is 4 cm $\times$ 4 cm $\times$ 30 cm in dimension. The PMT shown to the left is at the downstream end of the crystal calo_originalChristopher_report.
• Figure 4: Schematic drawing of the calorimeter readout.
• Figure 5: Calorimeter signals from a mixed, negatively charged particle beam at 210 MeV/$c$, after the 379 ns long delay.