Evidence for in-medium modification of the $\phi$ 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 phi meson peak is observed in the low beta gamma (=beta/sqrt(1-beta^2)) region of phi mesons (beta gamma < 1.25) with copper targets. However, in the high beta gamma region (beta gamma > 1.25), spectral shapes of phi mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publication on rho / omega modification, this study has experimentally verified vector meson mass modification at normal nuclear density. 
R. Muto, H. En'yo, M. Naruki, T. Tabaru, S. Yokkaichi, J. Chiba, M.   Ieiri, O. Sasaki, M. Sekimoto, K. H. Tanaka, H. Funaha...