  We investigate the properties of the pion quasiparticle in the
low-temperature phase of two-flavor QCD on the lattice with support from chiral
effective theory. We find that the pion quasiparticle mass is significantly
reduced compared to its value in the vacuum, in contrast to the static
screening mass, which increases with temperature. By a simple argument, the two
masses are expected to determine the quasiparticle dispersion relation near the
chiral limit. Analyzing two-point functions of the axial charge density at
non-vanishing spatial momentum, we find that the predicted dispersion relation
and the residue of the pion pole are simultaneously consistent with the lattice
data at low momentum. The test, based on fits to the correlation functions, is
confirmed by a second analysis using the Backus-Gilbert method.
