Triangle Seminars

Abstract:
String perturbation theory in the presence of D-branes is usually described
in terms of conformal field theory (CFT) on worldsheets with boundaries. In
this formalism, closed string scattering amplitudes in the presence of
static Dp-branes exhibit infrared divergences for p=0 and p=1, so the
worldsheet CFT description breaks down.
For p=0, it is known that these divergences are due to D0-brane recoil. A
systematic framework to take recoil into account is the worldline formalism,
where fixed boundary conditions are replaced by dynamical D0-brane
worldlines. In this formalism, the divergences that plague the worldsheet
CFT are automatically cancelled in a non-trivial way. The amplitudes derived
in the worldline formalism can be reproduced by deforming the CFT with a
specific bilocal recoil operator.
For p=1, the divergences are due to local recoil of D1-branes, which
(classically) end up displaced a finite distance from their original
position. The quantum version of this phenomenon can be viewed as a simple
geometric manifestation of the absence of spontaneous symmetry breaking in
1+1 dimensions. Through a Dirac-Born-Infeld analysis, it is possible to
resum these divergences in a way that yields finite, momentum-conserving
amplitudes.