  We provide the geometrical interpretation for the Becchi-Rouet-Stora-Tyutin
(BRST) and anti-BRST symmetry invariance of the Lagrangian density of a four (3
+ 1)-dimensional (4D) interacting U(1) gauge theory within the framework of
superfield approach to BRST formalism. This interacting theory, where there is
an explicit coupling between the U(1) gauge field and matter (Dirac) fields, is
considered on a (4, 2)-dimensional supermanifold parametrized by the four
spacetime variables x^\mu (\mu = 0, 1, 2, 3) and a pair of Grassmannian
variables \theta and \bar\theta (with \theta^2 = \bar \theta^2 = 0, \theta
\bar\theta + \bar \theta \theta = 0$). We express the Lagrangian density and
(anti-)BRST charges in the language of the superfields and show that (i) the
(anti-)BRST invariance of the 4D Lagrangian density is equivalent to the
translation of the super Lagrangian density along the Grassmannian direction(s)
(\theta and/or \bar\theta) of the (4, 2)-dimensional supermanifold such that
the outcome of the above translation(s) is zero, and (ii) the anticommutativity
and nilpotency of the (anti-)BRST charges are the automatic consequences of our
superfield formulation.
