Chiral Restoration in a Nuclear Medium

Abstract

Using 500 MeV (d,He π−) pion transfer reactions in recoil free kinematics, pionic 1sstates were populated in the Sn isotopes and their binding energies and widths determined by precision missing mass spectroscopy. Using these data and corresponding ones from iso-scalar light nuclei nuclei, O, Ne and Si, we determined the pion nucleus s-wave strength parameters, b0, b1, ReB0, and ImB0. By comparison of the iso-vector pion nucleon strength, determined from pionic hydrogen X-ray spectroscopy b 1 , with the b1 in a nuclear medium scaled to the density ρ(0), we deduced a scaling factor, the square of the pion decay constant in the vacuum and in nuclear medium, as R = b 1 /b1 = f 2 π(ρ0)/f 2 π = 0.64. Thus from the observed increase of the pion s-wave iso-vector strength in a nuclear medium a reduction of f π, the order parameter of chiral symmetry breaking, is indicated in accordance with theoretical expectations. This finding is supported by recent π and π− scattering experiments. A short outlook is given on a future program at RIBF in RIKEN for precision studies of deeply bound 1s-states in heavy nuclei.