Subthresholdρ0photoproduction on3He

A large reduction of the ${\rho }^0$ mass in the nuclear medium is reported, inferred from dipion photoproduction spectra in the 1 GeV region, for the reaction ${}^3\mathrm{He}(\gamma ,{\pi }^{+}{\pi }^{-})X$ with a 10% duty factor tagged-photon beam and the TAGX multiparticle spectrometer. The energy range spanned $(800\mathrm{}<~E_{\gamma }<~1120\hspace{0.5em}\mathrm{MeV})$ lies mostly below the free-${\rho }^0$ 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 ${\rho }^0$ mass extracted from the Monte Carlo fitting of the data were $m_{{\rho }^0}^{*}=642\mathrm{}\pm 40,$ $669\pm 32,$ and $682\pm 56\hspace{0.5em}\mathrm{MeV}{/c}^2$ for $E_{\gamma }$ 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.