Evidence for In-Medium Modification of theϕMeson at Normal Nuclear Density

Abstract

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.

Momentum dependence of theφ-meson nuclear transparency

“…The differences come mainly from the divergent descriptions of the secondary φ production processes. Our findings are not inconsistent with [13,15,16]. The solid line represents the φ calculated on the basis of the predicted φ self-energy in nuclear matter [12].…”

“…This large φN cross section was confirmed in BUU transport model calculations [19]. In the low-density approximation, this implies an in-medium φ width of about 110 MeV/c 2 in its rest frame at density ρ 0 for the conditions of the KEK measurements [13]. This is clearly incompatible with the width reported in the KEK experiment.…”

“…Furthermore, their sensitivity to in-medium modifications will be reduced by the profile of the nucleus spanning all densities from zero to normal nuclear matter density ρ 0 = 0.16 fm −3 , which smears out any density-dependent signal [3]. The KEK-PS-E325 Collaboration measured e + e − invariant mass distributions in the φ region in proton-induced reactions on carbon and copper at 12 GeV and reported a mass shift of 3.4% and a width increase by a factor of 3.6 at density ρ 0 for φ momenta around 1 GeV/c [13].…”

“…Dileptons from φ → e + e − /μ + μ − decays experience no strong final-state interactions in a nucleus. Any broadening of the φ line shape in the nuclear matter should be directly testable by examining dilepton mass spectra produced by elementary or heavy-ion beams, provided that the necessary cuts are applied at low φ momenta [13,14]. However, the measurement of such spectra is difficult due to the low branching ratios for leptonic decays.…”

K̄*mesons in dense matter

“…[17] and [18]. In contrast, the KEK team had earlier reported an attractive mass shift of the ρ in [19] and [20]. As explained in detail in Ref.…”