A prototype electromagnetic calorimeter for the MUonE experiment: status and first performance results

The MUonE experiment proposes a novel method to determine the leading hadronic contribution to the muon magnetic anomaly, aμ = (g − 2)/2, by precisely measuring the differential cross section of muonic Bhabha scattering using the CERN SPS muon beam directed onto atomic electrons of a light target. The modular detector consists of an array of identical tracking stations, each composed of a light target and silicon strip planes. After the final station, an electromagnetic calorimeter made of PbWO4 crystals with APD readout is followed by a muon filter. The scattered particles are tracked without the use of a magnetic field, and the event kinematics are determined over a large phase space from the correlation of the outgoing particle angles. Ambiguities in regions where the electron and muon have similar deflection angles are resolved by matching the electron track to the calorimeter cluster and/or associating the muon track with hits in the muon filter. The calorimeter provides an independent measurement of the electron energy and is critical for reducing background and assessing systematic errors. Over the past three years, a series of short test beams have been conducted at CERN with a prototype calorimeter to determine calibration requirements. In late 2023, a pilot run was conducted with tracking stations and the calorimeter fully integrated into a common trigger-less readout system. The main motivations for the MUonE calorimeter will be discussed, along with the status and initial performance results.