The POKERINO prototype

The POKER (POsitron resonant annihilation into dark mattER) project aims to perform a missing-energy measurement employing a ∼ 100 GeV positron beam impinging on an active thick target. The beam interaction with this detector could produce feebly interacting massive particles, exiting from it undetected and carrying away a significant fraction of the primary positron energy. The crucial element of the POKER project is a high-resolution PbWO4 electromagnetic calorimeter employed as the active target. A prototype of this calorimeter, POKERINO, consists of a 3 × 3 matrix of PbWO4 crystals with a SiPM-based readout system. In the summer of 2023, the POKERINO's response to high-energy particles was measured for the first time at the H8 beamline of the Super Proton Synchrotron (SPS) at CERN. This manuscript summarises the most important results obtained during the test beam of POKERINO at CERN, highlighting how this study influenced the perspectives of the POKER experimental effort.