Chiral restoration from pionic atoms?

We evaluate widths and shifts of pionic atoms using a theoretical microscopical potential in which the pion decay constant $f_\pi$ is changed by an in--medium density dependent one ($f_\pi(\rho)$), predicted by different partial Chiral restoration calculations. We show that the results obtained for shifts and widths are worse than if this modification were not implemented. On the other hand, we argue that in microscopic many body approaches for the pion selfenergy, based on effective Lagrangians, the mechanisms responsible for the change of $f_\pi$ in the medium should be automatically incorporated. Therefore, the replacement of $f_\pi$ by $f_\pi(\rho)$ in the many body derivation of the microscopic potential would be inappropriate.Comment: 10 pages, new comments and references adde

Quark mass dependence of s-wave baryon resonances

We study the quark mass dependence of J(P) = 1/2(-) s-wave baryon resonances. Parameter free results are obtained in terms of the leading order chiral Lagrangian. In the 'heavy' SU(3) limit with m(pi) = M-K similar or equal to 500 MeV the resonances turn into bound states forming two octets plus a singlet representations of the SU(3) group. A contrasted result is obtained in the 'light' SU(3) limit with m(pi) = m(K) similar or equal to 140 MeV for which no resonances exist. Using physical quark masses our analysis suggests to assign to the S = -2 resonances Xi(1690) and Xi(1620) the quantum numbers J(P) = 1/2(-)

η\eta bound states in nuclei

The energies and widths of bound states of the $\eta$ meson in different nuclei are obtained using the results for its selfenergy in a nuclear medium, which is evaluated in a selfconsistent manner using techniques of unitarized chiral perturbation theory. We find bound states in all studied nuclei (from $^{12}{C}$ on) and the half widths obtained are larger than the separation of the levels, what makes the experimental observation of peaks unlikely. We have paid a special attention to the region of nuclei where only the $1s$ state appears and the binding energies are of the order of magnitude of the half width, which would magnify the chances that some broad peak could be observed. This is found in the region of $^{24}{Mg}$ with a binding energy around 12.6 MeV and half width of 16.7 MeV. In heavy nuclei like $^{208}{Pb}$ there are many bound states which would be difficult to disentangle and the deepest state has a binding energy about 21 MeV and half width around 16 MeV. Such an overlapping accumulation of states could be seen as an extension of the continuum of $\eta$ strength into the bound region in $\eta$ production experiments.Comment: 9 pages, Latex, 2 Figure

Self energies of the pion and the delta isobar from the ^3He(e,e'pi^+)^3H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the $^3$He(e,e'$\pi^+)^3$H reaction have been measured in the excitation region of the $\Delta$ resonance to determine the longitudinal ($L$), transverse ($T$), and the $LT$ interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and $\Delta$-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the $\pi^+$ mass of $\Delta m_{\pi^+} = (-1.7^{+ 1.7}_{- 2.1})$ MeV/c$^2$ in the neutron-rich nuclear medium at a density of $\rho = (0.057^{+ 0.085}_{- 0.057})$ fm$^{-3}$. Our data are consistent with the $\Delta$ self energy determined from measurements of $\pi^0$ photoproduction from $^4$He and heavier nuclei.Comment: Elsart, 12 pages and 4 figures, Correspondent: Professor Dr. Dr. h.c. mult. Achim Richter, [email protected], submitted to Phys. Rev. Let