Search for the K−pp bound state via γd→K+π−X reaction at Eγ=1.5–2.4 GeV

A search for the K − p p bound state (the lightest kaonic nucleus) has been performed using the γ d → K + π − X reaction at E γ = 1.5 – 2.4 GeV at LEPS/SPring-8. The differential cross section of the K + π − photo-production off deuterium ( d 2 σ / d cos θ K + l a b / d cos θ π − l a b ) has been measured for the first time in this energy region. A peak structure was searched for in the inclusive missing mass spectrum. A statistically significant peak structure was not observed in the region from 2.22 to 2.36 GeV/c2. The upper limits of the differential cross section of the K − p p bound state production were determined to be (0.17–0.55), (0.55–1.7) and (1.1–2.9) μb at 95% confidence level with the assumed widths of 20 MeV, 60 MeV and 100 MeV.