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

A search for the K−ppK−pp bound state (the lightest kaonic nucleus) has been performed using the γd→K+π−Xγd→K+π−X reaction at Eγ=1.5–2.4 GeVEγ=1.5–2.4 GeV at LEPS/SPring-8. The differential cross section of the K+π−K+π− photo-production off deuterium (d2σ/dcosθK+lab/dcosθπ−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−ppK−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.