Development of an MPPC-based aerogel Cherenkov counter for K/π separation at J-PARC

We report the development and performance of an aerogel threshold Cherenkov counter (BAC) designed for beam K / π separation in the J-PARC E72 experiment at the K1.8BR beamline. The experiment utilizes secondary kaon beams with momenta between 645 and 825 MeV/ c , where a significant pion background ( K / π ∼ 0.12 ) necessitates efficient pion rejection to control trigger rates and data acquisition. The BAC, engineered for integration within the limited space and 0.8 T magnetic field of the Superconducting Hyperon Spectrometer (SHS), incorporates silica aerogel radiators (n = 1.115), a Geant4-optimized parabolic reflector, and Multi-Pixel Photon Counters (MPPCs) for photon detection. Electron-beam tests with 2 GeV/ c electrons at KEK PF-AR confirmed a uniform photoelectron yield across the detector, and Geant4 simulations reproduced the observed distributions to within 10%. Further tests with secondary meson beams at J-PARC demonstrated pion detection efficiencies exceeding 99% over the entire E72 momentum range and robust K / π separation. The BAC maintained stable performance at beam intensities up to 600k pions per spill (2.4 times the nominal E72 intensity) and in the SHS magnetic field. These results establish that the BAC fulfills the stringent requirements for beam K / π separation in the J-PARC E72 experiment.