The exclusive production cross sections for and mesons have been measured in proton-proton reactions at p lab ϭ3.67 GeV/c. The observed / cross section ratio is (3.8Ϯ0.2 Ϫ0.9 ϩ1.2 )ϫ10 Ϫ3 . After phase space corrections, this ratio is enhanced by about an order of magnitude relative to naive predictions based upon the Okubo-Zweig-Iizuka ͑OZI͒ rule, in comparison to an enhancement by a factor ϳ3 previously observed at higher beam momenta. The modest increase of this enhancement near the production threshold is compared to the much larger increase of the / ratio observed in specific channels of p p annihilation experiments. Furthermore, differential cross section results are also presented which indicate that although the meson is predominantly produced from a 3 P 1 proton-proton entrance channel, other partial waves contribute significantly to the production mechanism at this beam momentum.
We have analyzed data of the DISTO experiment on the exclusive pp --> pLambdaK+ reaction at 2.85 GeV to search for a strongly bound compact K- pp(approximately = X) state to be formed in the pp --> K+ + X reaction. The observed spectra of the K+ missing mass and the pLambda 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/c2 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.
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