Evidence forρ0Mass Modification in the3He(γ

Abstract

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...

Shadowing in nuclear photoabsorption above the resonance region

“…The values of the δm ρ obtained from the fits are given in Table 1: they range from −63 MeV to −163 MeV and the shift in carbon is more than a factor of two bigger than in lead. The values of the δm ρ obtained for the lightest nuclei are significantly larger than most of the theoretical expectations, while are in qualitative agreement with a recent measurement performed via the 3 He(γ, π + π − )X reaction [22,23] which suggests a ∼ 160 MeV reduction of the ρ-mass in 3 He. This reduction is so large that cannot be explained by the mean field picture of nuclear matter [24].…”

Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

“…At low scattering angle one might be able to probe the in-medium ρ-meson spectral function close to the q 0 = q line around q 0 =1 GeV, where Pwave Rhosobars are expected to be important. Very interesting results have also been reported by the TAGX collaboration [158] for the reaction 3 He(γ, π + π − )ppn where, based on a partial wave analysis of the outgoing pions, the data have been interpreted in terms of a large downward shift of the in-medium ρ-meson mass, although no calculations using medium-modified ρ-meson spectral functions are available yet. Another possibility might be provided by extracting phase information on electro-produced ρ-mesons in A(e, e ′ ) reactions at TJNAF [159].…”

Hadron properties in the nuclear medium

“…This was a pioneering experiment which attempted to study in-medium modifications of ρ 0 with elementary reactions. However, the claim of finding a ρ 0 mass shift of −160 ± 35MeV/c 2 in 3 He (Lolos et al, 1998) was met with skepticism due to the inevitable pion rescattering effect even for light targets (the emitted pions being in the resonance region), the small target volume, and the much-larger-than-expected shift.…”