Pions in Nuclei --Probes of Chiral Restoration
Paul Kienle
Technische Universitaet Muenchen, Germany and Stefan Meyer Institute for Subatomic Physics Wien, Austria
日時：2005年04月28日（木） 15:00-16:15
場所：大学院講義室（総合研究棟7階745号室）
Spontaneous chiral symmetry breaking, as proposed by Nambu andJona-Lasinio is presented as mechanism which creates the largemass gap between the QCD groundstate and the hadrons with thepion decay constant, fp as order parameter of this phase transition.It is expected that $f_\pi^2$, which determines the isovector strengthof pion-nucleon and -nucleus s-wave interaction, decreases linearlywith the density of a nuclear medium and brings about a partialrestoration of chiral symmetry.Recent experiments are presented to study with high precisionthe s-wave pion-nucleon and pion-nucleus interaction in comparison,using X-ray spectroscopy of pionic hydrogen on one hand and piontransfer reactions to study deeply bound s-states of pions in heavynuclei on the other hand. From the ratio of the thus determinedisovector scattering lengths, we find that the squared pion decayconstants, which are inversely proportional to their respectiveisovector scattering lengths, are modified by a nuclear medium.Our results indicate that the order parameter of symmetry breaking$f_\pi(\rho_0)^2$ is reduced by a factor of ~0.64 in a nucleuswith density $\pho_0$ = 0.17 fm$^{-3}$ in good agreement withtheoretical expectations. Thus we have found evidence for partialrestoration of spontaneously broken chiral symmetry by a nuclearmedium, by studying chiral dynamics.