QuarkPropagator[p] is the massless quark propagator.
QuarkPropagator[{p, m}] gives the quark propagator with mass m.
QP can be used as an abbreviation of QuarkPropagator.
Overview, GluonPropagator, QuarkGluonVertex.
QuarkPropagator[p, Explicit -> True]\frac{i \gamma \cdot p}{p^2}
QuarkPropagator[{p, m}, Explicit -> True]\frac{i (m+\gamma \cdot p)}{p^2-m^2}
QP[{p, m}]
Explicit[%]\Pi _q(p)
\frac{i (m+\gamma \cdot p)}{p^2-m^2}