Precision spectroscopy of pionic atoms to study pion-nucleus interaction in medium
An established approach for quantitative evaluation of the chiral symmetry breaking in finite density is study of pion-nucleus interaction through the experimental measurement of pionic atoms. Theories predict strength of isovector interaction between pion and nucleus, represented by a parameter b1, is enhanced by nuclear medium effects of the strong interaction, which is related to the partial restoration of the chiral symmetry breaking. So far the value of b1 at finite density was measured at GSI, Germany, through the spectroscopy of deeply-bound pionic atoms. From the comparison with the b1 in vacuum, the partial restoration of chiral symmetry breaking was suggested, while the precision of the obtained b1 is still not enough compared with that in vacuum.
For the further study of b1, we performed a precision measurement of deeply bound pionic states in 121, 116Sn using BigRIPS as a high-precision spectrometer. After the fine tuning of the experimental conditions, we achieved unprecedented resolution and measured high quality excitation spectra of 121, 116Sn near the charged pion emission threshold. In these spectra, the 1s, 2p and 2s pionic states in 121, 116Sn atoms are observed. From the obtained binding energies and widths, we evaluated optical potential parameters.
The detail of the experiment and its preliminary results will be presented.