Indication of a deeply bound and compact K- pp state formed in the pp- -> pLambdaK+ reaction at 2.85 GeV.

We have analyzed data of the DISTO experiment on the exclusive pp → pΛK + reaction at 2.85 GeV to search for a strongly bound compact K - pp(= X) state to be formed in the pp → K + + X reaction. The observed spectra of the K + missing mass and the pA invariant-mass with high transverse momenta of p and K + revealed a broad distinct peak of 26-σ confidence with a mass M X = 2267 ± 3(stat) ± 5(syst) MeV/c 2 and a width Γ 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.