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 ${\pi }^{-}$ 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 $b_1=-0.115\pm 0.007m_{\pi }^{-1}$. The observed enhancement of $|b_1|$ over the free $\pi N$ value ($b_1^{\mathrm{free}}/b_1=0.78\pm 0.05$) indicates a reduction of the chiral order parameter, $f_{\pi }^{\ast }(\rho {)}^2/f_{\pi }^2\approx 0.64$, at the normal nuclear density, $\rho ={\rho }_0$.

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