Narrow Results

The experiment KEK-PS E325 measured e⁺e⁻ and K⁺K⁻ pairs produced in 12 GeV p + A reactions, in order to search for in-medium modification of vector mesons. In the e⁺e⁻ spectra, a significant excess on the low-mass side of the ϕ-meson peak is observed in the very low βγ (= p/m) region of the copper target data. On the other hand when the velocity of ϕ mesons is higher or a smaller target, carbon, is employed, the observed ϕ-meson shapes are consistent with the expected shape in vacuum, both in the e⁺e⁻ and K⁺K⁻ spectra. We have also compared the nuclear mass number dependence of ϕ-meson production between the e⁺e⁻ and K⁺K⁻ decay channels. Using these results, the first experimental limits are obtained for in-medium broadening of the partial decay widths.

The pp → ppϕ and quasi–free pn → dϕ reactions have been studied at the Cooler Synchrotron COSY–Jülich, using the internal beam and the ANKE facility. Total cross sections in the pp entrance channel have been determined at three excess energies ∊ in the range of 18.5–75.9 MeV. In case of the pn entrance channel the energy dependence of the cross section up to 80 MeV has been extracted by exploiting the intrinsic momentum of the neutron inside a deuterium target. Taken together with data for ω–production, a significant enhancement of the ϕ/ω ratio for both entrance channels of a factor 8 is found compared to predictions based on the Okubo–Zweig–Iizuka rule.