Indication of a Deeply Bound and Compact K- pp State Formed in the pp → pΛK+ Reaction at 2.85 GeV

We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda K+ reaction at 2.85 GeV to search for a strongly bound compact K- pp(equivalent to X) state to be formed in the pp -> K+ + X reaction. The observed spectra of the K+ missing mass and the p Lambda invariant-mass with high transverse momenta of p and K+ revealed a broad distinct peak of 26-sigma confidence with a mass M-X = 2267 +/- 3(stat) +/- 5(syst) MeV/c(2) and a width Gamma(X) = 118 +/- 8(stat) +/- 10(syst) MeV. The enormously large cross section indicates formation of a compact K- pp with a large binding energy of B-K = 103 MeV, which can be a possible gateway toward cold and dense kaonic nuclear matter.