Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering

TL;DR

This paper develops a simplified pure connection formalism for gravity, deriving Feynman rules and computing graviton scattering amplitudes, revealing parity-violating effects in certain theories.

Contribution

It introduces a more straightforward connection-based approach to gravity, reducing complexity and enabling calculations of graviton scattering with novel parity-violating insights.

Findings

Reproduces known GR graviton scattering amplitudes

Identifies non-zero parity-violating amplitudes in alternative theories

Shows high-energy preference for negative helicity gravitons

Abstract

We continue to develop the pure connection formalism for gravity. We derive the Feynman rules for computing the connection correlation functions, as well as the prescription for obtaining the Minkowski space graviton scattering amplitudes from the latter. The present formalism turns out to be simpler than the metric based one in many aspects. Simplifications result from the fact that the conformal factor of the metric, a source of complications in the usual approach, does not propagate in the connection formulation even off-shell. This simplifies both the linearized theory and the interactions. For comparison, in our approach the complete off-shell cubic GR interaction contains just 3 terms, which should be compared to at least a dozen terms in the metric formalism. We put the technology developed to use and compute the simplest graviton-graviton scattering amplitudes. For GR we reproduce the well-known result. For our other, distinct from GR, interacting theories of massless spin 2 particles we obtain non-zero answers for some parity-violating amplitudes. Thus, in the convention that all particles are incoming, we find that the 4 minus, as well as the 3 minus 1 plus amplitudes are zero (as in GR), but the amplitudes with 4 gravitons of positive helicity, as well as the 3 plus 1 minus amplitudes are different from zero. This serves as a good illustration of the type of parity violation present in these theories. We find that the parity-violating amplitudes are important at high energies, and that a general parity-violating member of our class of theories "likes" one helicity (negative in our conventions) more than the other in the sense that at high energies it tends to convert all present gravitons into those of negative helicity.