A (2 + 1)-flavor lattice study of the pion quasiparticle in the thermal hadronic phase at physical quark masses

We investigate the properties of the pion quasiparticle in the thermal hadronic phase of (2 + 1)-flavor QCD on the lattice at physical quark masses at a temperature T = 128 MeV. We find that the pion quasiparticle mass ω0 = 113(3) MeV is significantly reduced relative to the zero-temperature pion mass m0π = 128(1) MeV, by contrast with the static screening mass mπ = 144(3) MeV, which increases with temperature. On the other hand the pion quasiparticle decay constant does not change much compared to the corresponding zero-temperature decay constant. The difference of the vector- and axialvector spectral functions serves as an order parameter of chiral symmetry restoration. By analyzing this quantity we conclude that chiral symmetry restoration is already at an advanced stage in the spectral function.