Feasibility demonstration of a high-momentum, high-purity muon beamline at the J-PARC Hadron Experimental Facility

We have established and characterized a novel high-purity muon beam delivery scheme, designated as the μ20 mode, at the high-momentum hadron beamline (B-line) of the J-PARC Hadron Experimental Facility. This scheme delivers tertiary muons produced via the in-flight backward decays of secondary pions in the straight section of the beamline, inherently suppressing parent hadron contamination. In a systematic performance evaluation, the muon beam achieved a standalone intensity of 1.7 × 103particles/spill with a spatial spread (standard deviation, σ) of approximately 15mm at the downstream detector position.A systematic attenuation analysis utilizing an iron absorber verified a high muon purity of 98.0 ± 0.6% for the 3GeV/c muon beam mode. Crucially, this work marks the first successful feasibility study of a pure multi-GeV/c muon beamline operation at an experimental facility in Japan.