Matching heavy-quark fields in QCD and HQET at three loops

Understand

The relation between the heavy-quark field in QCD and the corresponding field in HQET is derived up to three loops, and to all orders in the large-$\beta_0$ limit.

• The corresponding relation between the QED electron field and the Bloch--Nordsieck one is gauge invariant to all orders.
• We also prove that the $\bar{\text{MS}}$ anomalous dimension of the QED electron field depends on the gauge parameter only at one loop.