Chiral symmetry restoration: pionic atoms
- Laura Serksnyte ()
Physicists have been hunting for evidence of chiral symmetry restoration for decades. While the QCD Lagrangian is invariant under the chiral symmetry (assuming massless quark), its ground state is not which results in spontaneous chiral symmetry breaking. Indeed, the vacuum state of QCD is not an empty space but filled with quark-antiquark pairs. The expectation value of such chiral condensate is an order parameter of the chiral symmetry, which is expected to decrease at high temperatures or high matter densities where the chiral symmetry is partially restored. One way to look for chiral symmetry restoration is the spectroscopy of meson–nucleus bound systems such as pionic atoms. The pion-nucleus interaction is modified due to the medium effects in nuclear matter. Such interaction can be described by employing optical potential and fitting it to binding energies and widths of the measured pionic atom states. The level of chiral symmetry restoration can then be obtained by comparing the resulting optical potential isovector parameter b1 to the one estimated in vacuum. The talks in this section aim to discuss the underlying theoretical motivation and experimental design of such pionic atom experiments; and to present available measurements, including the new publication on Sn pionic atoms.