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 betagamma(= beta/square root(1-beta(2))) region of phi mesons (betagamma < 1.25) with copper targets. However, in the high betagamma region (betagamma > 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 publications on rho/omega modification, this study has experimentally verified vector meson mass modification at normal nuclear density.
Muto
R
R
RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. muto@riken.jp
Chiba
J
J
En'yo
H
H
Fukao
Y
Y
Funahashi
H
H
Hamagaki
H
H
Ieiri
M
M
Ishino
M
M
Kanda
H
H
Kitaguchi
M
M
Mihara
S
S
Miwa
K
K
Miyashita
T
T
Murakami
T
T
Nakura
T
T
Naruki
M
M
Ozawa
K
K
Sakuma
F
F
Sasaki
O
O
Sekimoto
M
M
Tabaru
T
T
Tanaka
K H
KH
Togawa
M
M
Yamada
S
S
Yokkaichi
S
S
Yoshimura
Y
Y