Evidence for In-Medium Modification of the φ Meson at Normal Nuclear Density

Invariant mass spectra of e+e- pairs have been measured in 12 GeV p+A reactions to detect possible in-medium modification of vector mesons. Copper and carbon targets are used to study the nuclear-size dependence of e+e- invariant mass distributions. A significant excess on the low-mass side of the φ meson peak is observed in the low βγ(=β/√(1-β2)) region of φ mesons (βγ<1.25) with copper targets. However, in the high βγ region (βγ>1.25), spectral shapes of φ mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publications on ρ/ω modification, this study has experimentally verified vector meson mass modification at normal nuclear density