We will work in units with c=1=ℏ. The 4-potential Aμ with upper index is always defined as
Aμ = (Φ,A).
1) Lowering the index of the 4-potential depends on the sign convention
(+,−,−,−)resp.(−,+,+,+)
for the Minkowski metric ημν. This Minkowski sign convention is used in
Ref. 1 (p. xix) and Ref. 2 (p. xv)resp.Ref. 3 (eq. (1.9)).
The 4-potential Aμ with lower index is
Aμ = (Φ,−A)resp.Aμ = (−Φ,A).
Maxwell's equations with sources are
dμFμν = jνresp.dμFμν = −jν.
The covariant derivative is
Dμ = dμ+iqAμresp.Dμ = dμ−iqAμ,
where q=−|e| is the charge of the electron.
2) The sign convention for the elementary charge e is
e = −|e| < 0resp.e = |e| > 0.
This charge sign convention is used in
Ref. 1 (p. xxi) and Ref. 3 (below eq. (58.1))resp.Ref. 2.
References:
M.E. Peskin and D.V Schroeder, An Introduction to QFT.
A. Zee, QFT in a nutshell.
M. Srednicki, QFT.
This post imported from StackExchange Physics at 2014-06-14 12:53 (UCT), posted by SE-user Qmechanic