PARTIAL DECAY WIDTHS OF THE ϕ INTO e+e− AND K+K− PAIRS IN 12 GeV p + A REACTIONS AT KEK-PS E325

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 phi-meson peak is observed in the very low beta gamma (= p/m) region of the copper target data. On the other hand when the velocity of phi mesons is higher or a smaller target, carbon, is employed, the observed phi-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 phi-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.