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Evidence for a modification of the r 0 mass in the nuclear medium via the 3 He͑g, p 1 p 2 ͒X reaction is reported here. The tagged photon energies were mostly below the r 0 production threshold on a free nucleon, and the bound nucleon's Fermi momentum was utilized to produce the r 0 . The mass shift observed is dm r 0 160 6 35 MeV͞c 2 and is significantly larger than theoretical model predictions for this nucleus and nuclear matter density. [S0031-9007(97)05040-0] PACS numbers: 24.80. + p, 12.40.Yx, 14.40.Cs At high temperature and nuclear matter density, hadronic matter is expected to undergo a phase transition to quark-gluon plasma. Such transitions are at the heart of the relativistic heavy ion programs at the CERN, RHIC, and GSI facilities. One of the interesting consequences of phase transitions is the mass shift of the light vector mesons ͑r 0 , v 0 , f 0 ͒ in the environment of nuclear matter. The topic of hadron renormalization in the presence of dense nuclear matter has been the focus of intense interest of the theoretical community. For a recent review, see Ref.[1].Briefly, the basis of r 0 modifications in the nuclear medium is supported by several theoretical expectations: chiral perturbation theory ͑xPT͒ leading to chirally effective Lagrangian formalism and scaling [2], quantum chromodynamics sum rules (QSR) [3], and a quark bag model combined with quantum hadrodynamics [4], among other effective theories. There is qualitative agreement among most of these theoretical treatments that the renormalized vector meson mass is reduced in the nuclear medium. The width of the r 0 is also expected to be reduced, but by a smaller ratio [5].While there are numerous theoretical works on the subject, there is little experimental evidence directly supporting such mass modifications. Several heavy ion experiments at CERN, leading to dilepton production, indicate an enhanced production cross section in the lower dilepton invariant mass range of 200 600 MeV͞c 2 [6]. The spectral shapes are not well reproduced by any theoretical model calculation based solely on hadron decay after freezeout. The dominant mechanism is p 1 p 2 annihilation, which fails to account for the excess dilepton production observed by the CERES collaboration [7], if the meson masses (particularly the r 0 ) are fixed at their free values. Better agreement is found if the meson masses are modified according to the scaling hypothesis of Brown and Rho [2], or to the QSR predictions of Refs. [8] and [9], respectively. More conventional mechanisms have also been proposed [10].Even though these agreements are encouraging, several factors are not included in the above-mentioned medium modification calculations which can also affect the dilepton invariant mass spectral shapes. The models produce the dileptons at rest, a highly unlikely condition at such high energies. Other processes which produce dileptons at low invariant mass values have not been included [6]. Furthermore, both the experimental uncertainties and energy resolution need to be improv...

MeV, covering an angular range of 50°р c.m. р180°at the higher energies and 90°р c.m. р180°at the lower energies. Unique features of the spectrometer acceptance were employed to reduce systematic errors. Single-energy phase shift analyses indicate the resulting S 11 and S 31 phases favor the results of the SM95 phase shift analysis over that of the older KH80 analysis. ͓S0556-2813͑98͒06512-1͔

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A large reduction of the ρ 0 mass in the nuclear medium is reported, inferred from dipion photoproduction spectra in the 1 GeV region, for the reaction 3 He(γ,π + π − )X with a 10% duty factor tagged-photon beam and the TAGX multi-particle spectrometer. The energy range covered (800≤Eγ ≤1120 MeV) lies mostly below the free ρ 0 production threshold, a region which is believed sensitive to modifications of light vector-meson properties at nuclear-matter densities. The ρ 0 masses extracted from the MC fitting of the data, m * ρ 0 = 642±40, 669±32, and 682±56 MeV/c 2 for Eγ in the 800-880, 880-960, and 960-1040 MeV regions respectively, are independently corroborated by a measured, assumption-free, kinematical observable. This mass shift, far exceeding current mean-1 field driven theoretical predictions, may be suggestive of ρ 0 decay within the range of the nucleonic field.