Precision spectroscopy of pionic 1s states of Sn nuclei and evidence for partial restoration of chiral symmetry in the nuclear medium

Abstract

Deeply bound 1s states of $\ddot{I}{€}^{-}$ in ${}^{115,119,123}$Sn were preferentially observed using the Sn($d$,${}^3$He) pion-transfer reaction under the recoil-free condition. The 1s binding energies and widths were precisely determined, and were used to deduce the isovector parameter of the s-wave pion-nucleus potential to be $b_1=-0.115\pm 0.007m_{\ddot{I}}{€}^{-1}$. The observed enhancement of $|b_1|$ over the free $\ddot{I}€N$ value ($b_1^{\mathrm{free}}/b_1=0.78\pm 0.05$) indicates a reduction of the chiral order parameter, $f_{\ddot{I}}^{\ast }€(\ddot{I}{)}^2/f_{\ddot{I}}{€}^2\approx 0.64$, at the normal nuclear density, $\ddot{I}=\ddot{I}{}_0$.

4 pages including 3 postscript figures, RevTeX 4 with multirow.sty, submitted to Physical Review Letters