Determination of the η(')-proton scattering length in free space.

Taking advantage of both the high mass resolution of the COSY-11 detector and the high energy resolution of the low-emittance proton beam of the cooler synchrotron COSY, we determine the excitation function for the pp→ppη(') reaction close to threshold. Combining these data with previous results, we extract the scattering length for the η(')-proton potential in free space to be Re(a(pη(')))=0±0.43 fm and Im(a(pη(')))=0.37(-0.16)(+0.40) fm.

Determination of the total width of the η' meson.

Taking advantage of both the low-emittance proton beam of the cooler synchrotron COSY and the high momentum precision of the COSY-11 detector system, the mass distribution of the η' meson was measured with a resolution of 0.33 MeV/c2 (FWHM), improving the experimental mass resolution by almost an order of magnitude with respect to previous results. Based on the sample of more than 2300 reconstructed pp → ppη' events, the total width of the η' meson was determined to be Γ(η') = 0.226 ± 0.017(stat) ± 0.014(syst) MeV/c2.

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 --> 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.

Mechanism of near-threshold production of the eta meson.

Measurements of the analyzing power for the pp-->pp eta reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S11(1535) resonance is predominantly due to the exchange of the pi meson between the colliding nucleons.

Rho(0) meson production in the pp-->pppi(+)pi(-) reaction at 3.67 GeV/c.

Total and differential cross sections for the exclusive reaction pp-->pp rho observed via the pi(+)pi(-) decay channel have been measured at p(beam)=3.67 GeV/c. The observed total meson production cross section is determined to be (23.4+/-0.8+/-8) mu b and is significantly lower than typical cross sections used in model calculations for heavy-ion collisions. The differential cross sections measured indicate a strong anisotropy (approximately cos(theta(CM)(rho)) in the rho(0) meson production.