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