The Crilin electromagnetic calorimeter: a fast, highly granular detector for a future Muon Collider

Crilin is an innovative semi-homogeneous electromagnetic calorimeter designed as the baseline solution for the MUSIC detector at the future Muon Collider. Its architecture features multiple layers of Lead Fluoride (PbF 2 ) crystals, each read out by an array of UV-extended SMD Silicon Photomultipliers (SiPMs). The detector is engineered to withstand the demanding conditions of the collision environment: it is compact, longitudinally segmentable with adjustable depth depending on the application, fast, and radiation-tolerant. Crilin’s high-granularity structure, featuring a 13 mm transverse cell size and a 4 X 0 layer depth in the Muon Collider configuration, is essential for disentangling the physics signal from the intense beam-induced backgrounds (BIB). Simulations predict that the BIB consists mainly of photons with low average energy, requiring precise timing and fine segmentation to suppress out-of-time background contributions. To address this, Crilin leverages the fast Cherenkov response of PbF 2 crystals for accurate time-of-arrival measurements, enabling effective background rejection and supporting full 5D particle flow reconstruction with high jet energy resolution. The detector must operate in a harsh radiation environment, facing up to 1 kGy/year of Total Ionizing Dose (TID) and neutron fluences of 10 n 1 M e V e q /cm 2 /year. Extensive radiation hardness studies on both the crystals and the SiPMs have validated the chosen design up to these target conditions. A prototype (Proto-1), composed of two layers of 3 × 3 crystals, has undergone comprehensive testing and demonstrated a timing resolution better than 40 ps for energy deposits down to 1 GeV. Ongoing R&D is now directed toward developing a full-containment prototype, consisting of five layers of 7 × 7 crystal matrices, with beam tests planned for Summer 2026. The main mechanical and electronic upgrades foreseen for this module have been implemented and validated in Proto-2, a 3 × 3 crystal prototype. Recent beam tests of Proto-2 enabled a complete characterization of the electronics and readout chain, and supported the selection of the optimal crystal wrapping material.