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^-pp bound state (the lightest kaonic nucleus) has been performed using the γd→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 (d²σ/dcos θK⁺lab/dcos θπ^-lab) 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^-pp 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.