FeynCalc manual (development version)

EtaSign

EtaSign is an option for SFAD, GFAD, CFAD and other objects representing propagators. It specifies the default sign of the iηi \eta prescription in the propagators, e.g. for standard Feynman propagators the value 1 corresponds to 1p2m2+iη\frac{1}{p^2-m^2 + i \eta}, while the value -1 sets 1p2m2iη\frac{1}{p^2-m^2 - i \eta}.

See also

Overview, SFAD, CFAD, GFAD.

Examples

Notice that if the sign of iηi \eta is already specified in the propagator, e.g.

CFAD[{q, {m^2, 1}}]

1(q2+m2+iη)\frac{1}{(q^2+m^2+i \eta )}

then this specification always overrides the EtaSign option. Hence

CFAD[{q, {m^2, 1}}, EtaSign -> -1]

1(q2+m2+iη)\frac{1}{(q^2+m^2+i \eta )}

still has the positive iηi \eta.