Observing partial restoration of chiral symmetry by using π-mesic atoms

According to the standard model, quarks and leptons acquired their mass by the Higgs mechanism in the course of cooling of the universe after the big bang. Exploration of Higgs particles is going to be started at LHC now. Nucleons, which bear most part of the mass of material now, are considered to have acquired their mass in the process when quarks were confined as the universe had been further cooled. The latter part of the above scenario is ascribed to the spontaneous breaking of chiral symmetry and the chiral condensation just as predicted by Nambu. In the recent years, it has become possible to measure the quantitative difference of the chiral condensation in vacuum from that in nuclei by detailed spectroscopic studies of π-mesons and their nuclear bound states (atomic 1s state). In this paper, the experimental methodology and their results are reviewed. To summarize, the relations between the observed quantities and the chiral condensation for π-mesons in nuclear media are well understood theoretically and the changes of the chiral condensation in nuclei are quantitatively obtained by two high precision observations, i.e., the X-ray spectroscopy of π-mesic hydrogen and the spectroscopy of the deeply bound state of π-mesic Sn atom created by the (d, 3He) reaction. In the analyses of π-mesic atoms, however, the uncertainty of the neutron distribution is one of the major sources of systematic error, which must be overcome. (S. Funahashi)