Chiral dynamics in the low-temperature phase of QCD

Understand

We investigate the low-temperature phase of QCD and the crossover region with two light flavors of quarks.

• The chiral expansion around the point $(T,m=0)$ in the temperature vs.
• quark-mass plane indicates that a sharp real-time excitation exists with the quantum numbers of the pion.
• An exact sum rule is derived for the thermal modification of the spectral function associated with the axial charge density; the (dominant) pion pole contribution obeys the sum rule.