W. Kummer et Dv. Vassilevich, Renormalizability of the open string sigma model and emergence of D-branes, J HIGH EN P, 2000(7), 2000, pp. NIL_227-NIL_252
Rederiving the one-loop divergences for the most general coupling of the op
en string sigma model by the heat kernel technique, we distinguish the clas
sical background field from the mean field of the effective action. The lat
ter is arbitrary, i.e. does not fulfil the boundary conditions. As a conseq
uence a new divergent counter term strongly suggests the introduction of an
other external one-form field (beside the usual gauge field), coupled to th
e normal derivative at the boundary. Actually such a field has been propose
d in the literature for different reasons, but its full impact never seems
to have thoroughly investigated before. The beta function for the resulting
renormalizable model is calculated and the consequences are discussed, inc
luding the ones for the Born-Infeld action. The most exciting property of t
he new coupling is that it enters the coefficient in front of the normal de
rivative in Neumann boundary conditions. For certain values of the backgrou
nd fields this coefficient vanishes, leading to Dirichlet boundary conditio
ns. This provides a natural mechanism for the emergence of D-branes.