Boosting Wigner's nj-symbols

TL;DR

This paper analyzes SL(2,C) Clebsch-Gordan coefficients in Lorentzian EPRL spin foam amplitudes, decomposing them into SU(2) nj-symbols and integrals over boosts, leading to a simplified state sum model with potential for analytical and numerical evaluation.

Contribution

It introduces a novel decomposition of Lorentzian spin foam amplitudes into SU(2) nj-symbols and boost integrals, simplifying calculations and providing new insights into the structure of the amplitudes.

Findings

Derived analytical expressions for edge amplitudes.

Proposed a simpler model for lowest order amplitude calculations.

Estimated large spin scaling behavior of the model.

Abstract

We study the SL(2,C) Clebsch-Gordan coefficients appearing in the lorentzian EPRL spin foam amplitudes for loop quantum gravity. We show how the amplitudes decompose into SU(2) nj-symbols at the vertices and integrals over boosts at the edges. The integrals define edge amplitudes that can be evaluated analytically using and adapting results in the literature, leading to a pure state sum model formulation. This procedure introduces virtual representations which, in a manner reminiscent to virtual momenta in Feynman amplitudes, are off-shell of the simplicity constraints present in the theory, but with the integrands that peak at the on-shell values. We point out some properties of the edge amplitudes which are helpful for numerical and analytical evaluations of spin foam amplitudes, and suggest among other things a simpler model useful for calculations of certain lowest order amplitudes. As an application, we estimate the large spin scaling behaviour of the simpler model, on a closed foam with all 4-valent edges and Euler characteristic X, to be N^(X - 5E + V/2). The paper contains a review and an extension of results on SL(2,C) Clebsch-Gordan coefficients among unitary representations of the principal series that can be useful beyond their application to quantum gravity considered here.