arXiv subjects

Wen-Bin Liu

Publications and source records attributed to Wen-Bin Liu.

13 recordsLinked to original sources

arXiv2026

Covariant variation and its applications

We define a covariant variation of tensor fields by combining its Lie derivative with the metric variation. This operator preserves the metric, contractions, and Hodge duality, but its commutator is not closed due to an anomaly. We derive its algebraic and geometric properties, and compare it with the Kosmann derivative. Combining the covariant variation with Kosmann derivative gives total covariant variation for the fields with both spacetime and Lorentz structure, all of which belong to the metric Lie derivative. Moreover, we introduce families of extended operators which contain the affine connection, Lie derivative, and covariant variation. From the anomaly of the covariant variation along the superrotation, an electromagnetic helicity flux appears at null hypersurfaces in four dimensions. We also apply the covariant variation and its anomaly to tensor fields in arbitrary spacetime dimensions, and especially focus on the $p$-forms in $d=2p+2$ dimensions.

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arXiv2025

Extrapolating the massive fields to future timelike infinity

It is well-known that future timelike infinity ($i^+$) in four-dimensional Minkowski spacetime is conformal to the unit three-dimensional hyperboloid ($H^3$). We asymptotically expand massive fields with spin $0,1,2$ near $i^+$ and extrapolate them onto this hyperboloid. These fields oscillate with a frequency equal to their mass and exhibit a universal asymptotic decay $τ^{-3/2}$. The fundamental fields are free and encode the outgoing scattering data. They are local operators defined on the boundary $H^3$ with which we construct the Poincaré charges. The Poincaré algebra can be extended to $\text{MDiff}(H^3)\ltimes C^{\infty}(H^3)$ using smeared operators associated with energy and angular momentum densities. For spinning fields, a spin operator must be included to close the algebra. The extended algebra shares the same form as the five-dimensional intertwined Carrollian diffeomorphism and reduces to the BMS algebra at $i^+$ by restricting the choice of test functions and vectors.

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 $\mathcal{I}^+$.

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arXiv2024

Quantum flux operators in the fermionic theory and their supersymmetric extension

We construct quantum flux operators with respect to the Poincaré symmetry in the massless Dirac theory at future null infinity. An anomalous helicity flux operator emerges from the commutator of the superrotation generators. The helicity flux operator corresponds to the local chiral symmetry which is the analog of superduality in the gauge theories. We also find its relation to the non-closure of the Lie transport of the spinor field around a loop. We discuss various algebras formed by these operators and constrain the test functions by the requirement of eliminating the non-local terms and satisfying the Jacobi identities. Furthermore, we explore their $\mathcal{N}=1$ supersymmetric extension in the Wess-Zumino model. There are four kinds of quantum flux operators, which correspond to the supertranslation, superrotation, superduality and supersymmetry, respectively. Interestingly, besides the expected supertranslation generator, a helicity flux operator will also emerge in the commutator between the superflux operators. We check that our flux algebra can give rise to the super-BMS and super-Poincaré algebras with appropriate choice of parameters. In the latter reduction, we find the helicity flux reduces to behaving like a $R$ symmetry generator in the commutator with the superflux. For completion, we derive the $R$ flux which also includes a charge flux for complex scalar besides the helicity flux for spinor field.

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arXiv2024

On the definition of Carrollian amplitudes in general dimensions

Carrollian amplitude is the natural object that defines the correlator of the boundary Carrollian field theory. In this work, we will elaborate on its proper definition in general dimensions. We use the vielbein field on the unit sphere to define the fundamental field with non-vanishing helicity in the local Cartesian frame which is the building block of the Carrollian amplitude. In general dimensions, the Carrollian amplitude is related to the momentum space scattering matrix by a modified Fourier transform. The Poincaré transformation law of the Carrollian amplitude in this definition has been discussed. We also find an isomorphism between the local rotation of the vielbein field and the superduality transformation.

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↗

arXiv2024

Feynman rules and loop structure of Carrollian amplitudes

In this paper, we derive the Carrollian amplitude in the framework of bulk reduction. The Carrollian amplitude is shown to relate to the scattering amplitude by a Fourier transform in this method. We propose Feynman rules to calculate the Carrollian amplitude where the Fourier transforms emerge as the integral representation of the external lines in the Carrollian space. Then we study the four-point Carrollian amplitude at loop level in massless $Φ^4$ theory. As a consequence of Poincaré invariance, the four-point Carrollian amplitude can be transformed to the amplitude that only depends on the cross ratio $z$ of the celestial sphere and a variable $χ$ invariant under translation. The four-point Carrollian amplitude is a polynomial of the two-point Carrollian amplitude whose argument is replaced with $χ$. The coefficients of the polynomial have branch cuts in the complex $z$ plane. We also show that the renormalized Carrollian amplitude obeys the Callan-Symanzik equation. Moreover, we initiate a generalized $Φ^4$ theory by designing the Feynman rules for more general Carrollian amplitude.

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arXiv2024

Holographic dictionary from bulk reduction

We propose a holographic dictionary which comes from reducing the bulk theories in an asymptotically flat spacetime to its null infinity. A general boundary theory is characterized by a fundamental field, an infinite tower of descendant fields, constraints among the fundamental field and its descendants as well as a symplectic form. For the Carrollian diffeomorphisms, we can construct the corresponding Hamiltonians which are also the fluxes from the bulk, and whose quantum operators realize this algebra with a divergent central charge. This central charge reflects the propagating degrees of freedom and can be regularized. For the spinning theory, we need a helicity flux operator to close the algebra which relates to the duality transformation.

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arXiv2023

Quantum flux operators for Carrollian diffeomorphism in general dimensions

We construct Carrollian scalar field theories in general dimensions, mainly focusing on the boundaries of Minkowski and Rindler spacetime, whose quantum flux operators form a faithful representation of Carrollian diffeomorphism up to a central charge, respectively. At future/past null infinity, the fluxes are physically observable and encode rich information of the radiation. The central charge may be regularized to be finite by the spectral zeta function or heat kernel method on the unit sphere. For the theory at the Rindler horizon, the effective central charge is proportional to the area of the bifurcation surface after regularization. Moreover, the zero mode of supertranslation is identified as the modular Hamiltonian, linking Carrollian diffeomorphism to quantum information theory. Our results may hold for general null hypersurfaces and provide new insight in the study of the Carrollian field theory, asymptotic symmetry group and entanglement entropy.

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arXiv2023

Symmetry group at future null infinity III: Gravitational theory

We reduce the gravitational theory in an asymptotically flat spacetime to future null infinity. We compute the Poincaré flux operators at future null infinity and construct the supertranslation and superrotation generators. The generators are shown to form a closed symmetry algebra by including a generalized gravitational duality operator. We could regard all the generators as the Hamiltonians with respect to the symmetry transformation in the boundary field theory. Our construction of the generators may relate to the BMS fluxes defined in the literature by adding counterterms to the Bondi mass and angular momentum aspects.

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arXiv2023

Symmetry group at future null infinity II: Vector theory

In this paper, we reduce the electromagnetic theory to future null infinity and obtain a vector theory at the boundary. We compute the Poincaré flux operators which could be generalized. We quantize the vector theory, and impose normal order on the extended flux operators. It is shown that these flux operators generate the supertranslation and superrotation. When working out the commutators of these operators, we find that a generalized electromagnetic duality operator should be included as the generators to form a closed symmetry algebra.

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arXiv2023

Symmetry group at future null infinity I: Scalar theory

We reduce the massless scalar field theory in Minkowski spacetime to future null infinity. We compute the Poincaré flux operators, which can be generalized and identified as the supertranslation and superrotation generators. These generators are shown to form a closed symmetry algebra with a divergent central charge. In the classical limit, we argue that the algebra may be interpreted as the geometric symmetry of a Carrollian manifold, i.e., the hypersurface of future null infinity. Our method may be used to find more physically interesting Carrollian field theories.

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arXiv2022

High-accuracy three-dimensional surface detection in smoothed particle hydrodynamics for free-surface flows

In this study, we investigate high-accuracy three-dimensional surface detection in smoothed particle hydrodynamics for free-surface flows. A new geometrical method is first developed to enhance the accuracy of free-surface particle detection in complex flows. This method detects free-surface particles via continuous global scanning inside the sphere of a particle through a cone region whose vertex corresponds to the particle position. The particle is identified as a free-surface particle if there exists a cone region with no neighboring particles. Next, an efficient semi-geometrical method is proposed based on the geometrical method to reduce the computational cost. It consists of finding particles near the free surface via position divergence and then detecting these particles using the geometrical method to identify free-surface particles. The accuracy and robustness of the proposed method are demonstrated by performing tests on several model problems.

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