arXiv subjects

Xin-Hao Zhou

Publications and source records attributed to Xin-Hao Zhou.

6 recordsLinked to original sources

arXiv2026

Spacelike reduction of the gravitational topological terms and associated helicity densities

Helicity in gravity is a multifaceted concept, and several inequivalent definitions exist in the literature. In this work, we reduce the topological Pontryagin and Nieh-Yan terms to a constant time spacelike hypersurface. Remarkably, upon applying the SVT decomposition at the linearized level, we obtain four distinct helicity densities. The helicity density arising from the Nieh-Yan term is termed the spin-1 and spin-2 gravitomagnetic helicity density, depending on whether it originates from vector or tensor modes. Meanwhile, the helicity density arising from the Pontryagin term is termed the spin-1 and spin-2 gravito-current helicity density, according to the corresponding mode contributions. We study the mode and multipole expansions of these helicity functionals and apply them to leading order Newtonian two-body systems, weak field boosted Kerr, a locally defined slowly varying gyratonic pp-wave model, and a linearized gravitational Hopfion.

gr-qc↗

arXiv2026

Spinning bulk-to-boundary correlators in the massless theories with Poincaré symmetry

We classify the bulk-to-boundary correlators for general integer-spin $s$ operators in a Poincaré-invariant theory by imposing suitable fall-off conditions near future/past null infinity. Any bulk-to-boundary correlator is a linear superposition of a set of basic tensor structures fixed by the little group \text{ISO}(2) of massless particles. We map the independent tensor structures to all possible non-crossing double-line diagrams. A further mapping of the double-line diagrams to circular diagrams shows that all independent tensor structures are tensor products of loop diagrams. By extrapolating the bulk-to-boundary correlators to boundary-to-boundary correlators, we find a rich structure for general spin-$s$ operators. Furthermore, we show that the extrapolated operator lies in a type Ib spin-$s$ multiplet representation of Carrollian conformal field theory (CCFT). This is a net representation that generated by the Wigner translation generators.

hep-th↗

arXiv2026

Reduction of topological invariants on null hypersurfaces

Gravitational helicity flux density represents the angular distribution of helicity flux in general relativity. In this work, we explore its relationship to the reduction of topological invariants at future null infinity. Contrary to initial expectations, the Pontryagin term, which contributes to the gravitational chiral anomaly, is not related to the gravitational helicity flux density. Instead, the Nieh-Yan term, another topological invariant within the teleparallel equivalent of general relativity (TEGR), can reproduce this flux density. We also reduce these two topological invariants to null hypersurfaces describing near-horizon geometry. In this near-horizon context, the Pontryagin term yields a non-trivial quantity that may characterize a Carrollian fluid helicity relevant to near-horizon physics.

hep-th↗

arXiv2025

Electromagnetic helicity flux density for radiative systems

We show that the helicity flux density is distinguished from magnetic helicity by analysing Hopf solitons. The electromagnetic (EM) helicity flux and the magnetic helicity are Chern-Simons terms at different hypersurfaces. We find the helicity flux density for a point charge moving with an acceleration, extending the Liénard-Wiechert angular distribution of radiant power. We also derive the multipole expansion of the helicity flux density, generalizing the Larmor's formula for the radiant power. These formulae have been applied to discuss the helicity flux density in several toy models such as circular and helical motion as well as soft bremsstrahlung. We also comment on the potential applications of the EM helicity flux density to pulsar systems.

hep-th↗

arXiv2025

Electromagnetic helicity flux operators in higher dimensions

The helicity flux operator is a fascinating quantity that characterizes the angular distribution of the helicity of radiative photons or gravitons and it has many interesting physical consequences. In this paper, we construct the electromagnetic helicity flux operators which form a non-Abelian group in general dimensions, among which the minimal helicity flux operators form the massless representation of the little group, a finite spin unitary irreducible representation of the Poincaré group. As in four dimensions, they generate an extended angle-dependent transformation on the Carrollian manifold. Interestingly, there is no known corresponding bulk duality transformation in general dimensions. However, we can construct a topological Chern-Simons term that evaluates the minimal helicity flux operators at $\mathcal{I}^+$.

hep-th↗

arXiv2024

Quantum flux operators in higher spin theories

We construct Carrollian higher spin field theories by reducing the bosonic Fronsdal theories in flat spacetime to future null infinity. We extend the Poincaré fluxes to quantum flux operators which generate Carrollian diffeomorphism, namely supertranslation and superrotation. These flux operators form a closed symmetry algebra once including a helicity flux operator which follows from higher spin super-duality transformation. The super-duality transformation is an angle-dependent transformation at future null infinity which generalizes the usual electro-magnetic duality transformation. The results agree with the lower spin cases when restricting to $s=0,1,2$.

hep-th↗