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We have measured invariant mass spectra of electron-positron pairs in the target rapidity region of 12GeV p+A reactions. We have observed a significant difference in the mass spectra below the ω meson between p+C and p+Cu interactions. The difference is interpreted as a signature of the ρ/ω * Present Address: Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan, email: ozawa@cns.s.u-tokyo.ac.jp † Present Address: ICEPP, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan ‡ Present Address: Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan § Present Address: Fujitsu Corporation, 4-1-1, Kamikodanaka, Nakahara, Kawasaki, Kanagawa 211-8588, Japan * * Present Address: RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan † † Present Address: Xaxon Corporation,1-3-19, Tanimachi, Chu-ou, Osaka, Japan ‡ ‡ Present Address: Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 , Japan 1 modification at normal nuclear-matter density. 24.85.+p,25.30.-c Typeset using REVT E X 2 Recently, chiral property of QCD in hot(T = 0) or dense(ρ = 0) matter attracts wide interests in the field of hadron physics. The dynamical breaking of chiral symmetry in the QCD vacuum induces an effective mass of quarks, known as constituent quark mass. In hot and/or dense matter this broken symmetry is subject to restore partially or completely, and hence the properties of hadrons can be modified. To observe such an effect, measurements of in-medium decay of vector mesons, especially in the lepton-pair channel, are highly desirable to obtain directly the meson properties in matter [1].Although many heavy-ion experiments were carried out in CERN-SPS and BNL-AGS to study hot and dense matter, there was only an experiment which could address the mass modification of vector mesons. The CERES/NA45 collaboration measured low-mass electron pair productions in Pb-Au collisions at 158 A GeV [2], and observed an enhancement of e + e − pair yield in the mass range 0.3 < m ee < 0.7 GeV/c 2 over the expected yield from the known hadronic sources in pp collisions. This striking effect could be understood as a consequence of the mass modification of the ρ and ω meson in hot matter.In QCD the mass of vector mesons, mainly determined by the effective mass of quarks, is closely related toqq condensates ( qq ) which is an order parameter of the chiral symmetry of the QCD vacuum. In this framework a significant decrease of qq is expected not only at high temperature but also at normal nuclear density [3]. Using the QCD sum rule, Hatsuda and Lee theoretically predicted in-medium modification of the vector mesons [4]. According to this model, mass decrease at normal nuclear density is 120 ∼ 180 MeV/c 2 for the ω and ρ mesons and 20 ∼ 40 MeV/c 2 for the φ meson. Thus the measurements of such mesons, which are produced and decayed in a nucleus, are of great interest. The present experiment is one of the several experimental efforts [1...

The invariant mass spectra of e+e- pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.

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.