Ultra-fast Small Angle Calorimeter for photon detection at KOTO II

KOTO-II at J-PARC is proposed to extend the search for the ultra-rare decay KL → π 0 νν̄ by operating at substantially higher beam intensity than KOTO. In this regime, the Beam Hole Photon Veto (BHPV) must provide excellent timing performance and excellent γ/hadron discrimination to identify background photons and beam-induced activity. Given the time resolution of the lead-aerogel Cherenkov BHPV used in KOTO, a ∼19% nominal beam loss is expected under KOTO-II conditions. As an alternative, we propose a compact and highly granular Small Angle Calorimeter (SAC) based on ultra-fast lead-tungstate crystals read out with fast photomultiplier tubes, aiming to improve timing and reduce accidental veto losses. A SAC prototype consisting of two 3 × 3 PWO-UF crystal layers was tested at the CERN PS T9 beamline with Hamamatsu R9880 and R14755 metal-channel photomultipliers. In parallel, the crystals were characterized with high-resolution X-ray diffraction to quantify their orientation and enable measurements with aligned crystals. We present an overview of the detector R&D and the first beam test results on energy response and time resolution.