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 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.