A no-go study on gravitational helicity in connection variables
We study the gravitational helicity using the covariant phase space method. Starting from the covariant phase space of general relativity expressed in terms of connection variables, we construct the symplectic form and identify a duality transformation based on the internal Hodge dual. In the complex self-dual Ashtekar variables the duality becomes a simple $U(1)$ phase rotation. We show, however, that this rotation is not a symmetry.We also show that the expression $\int_\Sigma\Sigma^{+AB}\wedge A^+_{AB}$ inspired by the electromagnetic helicity is neither a Noether charge nor a moment map.The duality of general relativity under a rotation of the curvature two-form is a symmetry of the equations of motion but not of the Lagrangian, and carries no Noether charge. An explicit computation shows that the mass and the NUT charge are not symplectically conjugate, so even in the reduced phase space of stationary solutions this duality has no moment map. This study therefore provides counter examples to the existence of a full-theory gravitational helicity in the covariant phase space approach.