In-Medium rho⁰ Spectral Function Study via the 2H, 3He, 12C(gamma,pi+pi-) Reaction

الأصل · EN

We report a helicity analysis of sub-threshold rho⁰ production on 2H, 3He and 12C at low photo-production energies. The results are indicative of a large longitudinal rho⁰ polarization (l=1, m=0) and are consistent with a strong helicity-flip production mechanism. This signature is used to extract in-medium rho⁰ₗ invariant mass distributions for all three nuclei in a manner which is less model-dependent than previous measurements. The results are compared to kinematic and phenomenological models of the rho⁰ spectral function. The 2H and 3He data distributions support the role of N*(1520) excitation in shaping the in-medium rho⁰ₗ invariant mass distribution, while the 12C distributions are consistent with quasi-free rho⁰ₗ production. The data support an in-medium modification of the rho⁰ₗ invariant mass distribution.