Experimental Projects for Spectroscopy of Pionic Atoms and N*(1535) in Nuclei

Abstract

Chiral symmetry and its spontaneous breakdown play an important role as a link between QCD and nuclear physics in the lowest energy region of strong interaction where the quark and gluon degrees of freedom are hidden and pseudo-scalar mesons of massless Nambu-Goldstone bosons appear.1) Chiral condensate 〈qq〉 is known to parametrize the symmetry breaking, and is theoretically calculated on the plane of temperature and density. The theory tells that beyond certain energy density, the chiral condensate is expected to vanish and the symmetry restores.2) There are several experimental approaches to understand the chiral dynamics especially on its relation to the mechanism of hadron mass generation. One approach utilizes head-on collisions of heavy ions with relativistically high energies in order to reach and explore extremely high temperature region.3) The other takes examination of hadrons in nuclear matter which is regarded as extremely high density material.4)–8) The above theory predicts 30% reduction of the chiral condensate at the normal nuclear density to that in vacuum.9)–11) This article briefly describes two experimental projects to study the chiral dynamics both in the meson and in the baryon sectors,12) namely, “Precision spectroscopy of pionic atoms”13) and “N∗(1535) production and its in-medium spectroscopy”.14) The former pursues high precision systematic spectroscopy of deeply bound pionic atoms∗) in the RI beam factory in RIKEN. The systematic spectroscopy will serve unique and high quality information on the in-medium strong interaction strength between the pion and nuclei, which improves the precision of the quantitative estimation of the chiral condensate at the normal nuclear density.5) The latter, “N∗(1535) production and its in-medium spectroscopy”, has its own particular interest.14),15) According to the theories, N∗(1535) is a candidate of chiral partner of nucleons,12),16),17) and thus, the mass difference of the N∗(1535) and nucleon is reduced as the chiral symmetry restores. In-medium N∗(1535) spectroscopy will provide strong evidence of the hadron mass generation through direct measurement of the mass difference through possible level crossing between N∗(1535)-hole and η modes in the nuclear medium.18),19)