Deeply bound pionic states in heavy nuclei

The present paper reviews theoretical and experimental studies of a new type of nuclear spectroscopy for the structure of deeply bound π−π− states and their formation processes through “pion-transfer” nuclear reactions. The calculated energy levels of deeply bound pionic states in heavy nuclei using the standard pion–nucleus optical potential are found to be narrow discrete states even for the 1s1s state in 208Pb. These hitherto unexpected results are caused by the repulsive pion–nucleus optical potential which pushes pionic wavefunctions outwards so that the nuclear absorption of the bound pions is weakened. Theoretical studies on pion-transfer reactions were carried out as a methodological guide to produce these deeply bound pionic states experimentally. In particular, various predictions were made on the pion-transfer proton-pickup reactions (n,d)(n,d) and (dd, 3He). Following these theoretical studies successful experiments on high-resolution spectroscopy of (dd, 3He) reactions were carried out at GSI. The experimental procedures and results together with their theoretical implications are also reviewed. The most important feature of deeply bound 1s1s states of π−π− in heavy nuclei is that it provides unique information on the ss-wave isovector π−Nπ−N interaction which is connected to reduction of chiral symmetry breaking in the nuclear medium. Recent experiments on Pb and Sn isotopes have yielded clear evidence for partial restoration of chiral symmetry in the nuclear medium.