Author/Authors :
R. Muto، نويسنده , , J. Chiba، نويسنده , , H. Enʹyo، نويسنده , , Y. Fukao، نويسنده , , H. Funahashi، نويسنده , , H. Hamagaki، نويسنده , , M. Ieiri، نويسنده , , M. Ishino، نويسنده , , H. Kanda، نويسنده , , M. Kitaguchi، نويسنده , , S. Mihara، نويسنده , , K. Miwa، نويسنده , , T. Miyashita، نويسنده , , T. Murakami، نويسنده , , T. Nakura، نويسنده , , M. Naruki، نويسنده , , M. Nomachi، نويسنده , , K. Ozawa، نويسنده , , F. Sakuma، نويسنده , , O. Sasaki، نويسنده , , et al.، نويسنده ,
Abstract :
We have measured e+e− invariant mass spectra in 12 GeV p+A interactions at the KEK Proton-Synchrotron. The aim of the experiment is to detect possible in-medium modification of vector mesons at normal nuclear density. We used carbon and copper targets to study the nuclear size dependence of the e+e− invariant mass spectra. A significant enhancement on the low-mass side of the ϕ meson peak was observed in the low βγ region (βγϕ<1.25) in the copper data. On the other hand, in the higher βγ region (βγϕ>1.25), both the spectral shapes obtained from carbon and copper targets were consistent with the expected one from the simulation. This observation is consistent with a picture of the ϕ modification in a nucleus, i.e. normal nuclear density, because such an effect should be visible only for slowly moving mesons produced in a larger nucleus.
