Partial restoration of chiral symmetry in nucleus observed in pionic atoms*

We have quantitatively evaluated chiral condensate, the order parameter of the chiral symmetry, in nuclear matter and found that it is reduced to 77+-2% in the nuclear medium compared to vacuum. We have performed high-precision spectroscopy experiments of pion tin atoms at the RIKEN RI Beam Factory to derive chiral condensation. The obtained spectra were analyzed to determine the interaction between pions and nuclei. The results show that the s-wave interaction is stronger in the nucleus than in the vacuum. This enhancement is due to the wavefunction renormalization of the medium effect. From this, it is possible to estimate the medium effect inversely and link it to the partial recovery of the chiral symmetry. Taking into account the latest nuclear density distribution, spectroscopic factors, and residual interactions, the magnitude of chiral condensate in the nuclear medium is quantitatively evaluated and successfully estimated with errors for the first time in the world in a high-density medium. Future plans will also be discussed.