Loop Amplitudes in Supergravity by Canonical Quantization

Abstract

Dirac's approach to the canonical quantization of constrained systems is applied to $N = 1$ supergravity, with or without gauged supermatter. Two alternative types of boundary condition applicable to quantum field theory or quantum gravity are contrasted. The first is the `coordinate' boundary condition as used in quantum cosmology; the second type is scattering boundary conditions, as used in Feynman diagrams, applicable to asymptotically flat space-time. The first yields a differential-equation form of the theory, dual to the integral version appropriate to the second. Here, the first (Dirac) approach is found to be extremely streamlined for the calculation of loop amplitudes in these locally supersymmetric theories. By contrast, Feynman-diagram methods have led to calculations which are typically so large as to be unmanageable. Remarkably, the Riemannian quantum amplitude for coordinate boundary conditions in $N = 1$ supergravity (without matter) is exactly semi-classical, being of the form $exp(-I/\hbar)$, where $I$ is the classical action, allowing for the presence of fermions as well as gravity on the boundaries. Even when supermatter is included, typical one-loop amplitudes are often very simple, sometimes not even involving an infinite sum or integral. Specifically, the boundary conditions considered for a number of concrete one-loop examples are set on a pair of concentric 3-spheres in Euclidean 4-space. In the non-trivial cases the amplitudes appear to be exponentially convergent.