Subthreshold ρ⁰ photoproduction on ³He

A large reduction of the ρ⁰ mass in the nuclear medium is reported, inferred from dipion photoproduction spectra in the 1 GeV region, for the reaction ³He(γ, π⁺π^-)X with a 10% duty factor tagged-photon beam and the TAGX multiparticle spectrometer. The energy range spanned (800≤E_γ≤1120 MeV) lies mostly below the free-ρ⁰ production threshold, a region which is believed to be sensitive to modifications of light vector-meson properties at nuclear-matter densities. The values of the ρ⁰ mass extracted from the Monte Carlo fitting of the data were m*_ρ0=642±40, 669±32, and 682±56 MeV/c² for E_γ in the 800-880, 880-960, and 960-1040 MeV regions, respectively. These values were independently corroborated in a qualitative fashion by a measured, assumption-free, kinematical observable. This mass shift, far exceeding current mean-field-driven theoretical predictions, may be suggestive of a nucleonic source of modification or of a bound state in the nuclear mean field.