arXiv subjects

Sruthi A. Narayanan

Publications and source records attributed to Sruthi A. Narayanan.

15 recordsLinked to original sources

arXiv2026

Soft Algebras via Bulk Double Soft Limits

Soft and collinear limits of "celestial amplitudes" give rise to infinite dimensional symmetry algebras for two-dimensional (2D) "celestial" conformal field theories (CFTs). A small subset of these operators generates the action of the entire set recursively via the commutation relations. Insertion of this subset into celestial CFT correlators gives the boundary version of the first three terms in a soft expansion of the corresponding 4D bulk gravitational scattering amplitude. In this paper, we find that a bulk analog of this, which would allow the entire soft expansion of a gravitational amplitude to be determined via just the first three terms, does not follow trivially from the celestial algebras. We show how the interplay of soft and collinear limits results in subtleties for bulk amplitudes that do not show up in the boundary description. We also briefly comment on this construction in gauge theory.

hep-th↗

arXiv2025

Memory Correlators and Ward Identities in the 'in-in' Formalism

The symmetries of asymptotically flat spacetimes impose constraints on observables at infinity. The consequences of this have been extensively explored for S-matrix elements, where soft theorems are known to be equivalent to Ward identities for asymptotic symmetries. However, recently there has been interest in broader classes of asymptotic observables. Here, we consider soft graviton insertions in the 'in-in' formalism. We derive a Ward identity for supertranslations and compute two point functions for the soft charges for 'in-in' correlators. We find that the connected memory correlators are non-trivial in this set up and can be straightforwardly inferred from the average null energy (ANEC) correlators using observations from celestial Conformal Field Theory (cCFT).

hep-th↗

arXiv2025

Marginal Operators from Celestial Diamonds

For a given conformal field theory (CFT), one can deform it via the addition of a marginal operator to the spectrum. In two dimensions, when the added operator has conformal weights $h=\bar{h}=1$, conformal symmetry is not broken and the resulting theory is a distinct CFT. Studying such marginal operators for celestial CFTs allows for a geometric understanding of the space of allowed boundary theories dual to quantum field theories (QFT) in bulk asymptotically flat spacetimes. In traditional holographic examples, a marginal deformation on the boundary corresponds to a vacuum transition in the bulk theory. We affirm this in celestial CFTs which requires a general definition of marginal operators as composite celestial operators via pairs that live at distinct corners of celestial memory and Goldstone diamonds.

hep-th↗

arXiv2025

Marginality from Leading Soft Gluons

Marginal operators in a d-dimensional conformal field theory (CFT), those with conformal dimension $Δ=d$, give us information about the space of related theories. This can be incredibly useful when trying to develop an intrinsic definition of a holographic CFT. Recent work has established that in the case of celestial CFTs, those dual to asymptotically flat spacetimes, marginal operators can be constructed as shadow transforms of soft scalars. In this work, rather than starting from a bulk theory of scalars, we start with Yang-Mills and construct marginal operators from soft gluons via light transforms. We obtain the local metric and connections for the associated conformal manifold by studying the two- and three-point functions of these marginal operators. We then discuss the implications of this for broadening our understanding of operator product expansions of soft operators in celestial CFTs.

hep-th↗

arXiv2025

Collinear Corrections to the Cachazo-Strominger Soft Theorem

Soft theorems describe the behavior of scattering amplitudes when one or several external particles are taken to be energetically soft. In tree-level gravity there are universal soft theorems for the three leading orders in the soft expansion, and they can be shown to be equivalent to Ward identities of asymptotic symmetries. While the leading and subleading symmetries are understood as supertranslations and superrotations respectively, the precise symmetry interpretation of the sub-subleading soft theorem is still a matter of investigation. The form of the sub-subleading soft graviton theorem was elucidated by Cachazo and Strominger using a BCFW expansion of graviton amplitudes. In this work we show that consistency with results based on asymptotic charges requires a careful treatment of collinear singularities in the amplitude, giving rise to collinear corrections to the usual Cachazo-Strominger soft theorem.

hep-th↗

arXiv2024

Celestial Gluon Amplitudes from the Outside In

We show that, given a two-dimensional realization of the celestial OPE in self-dual Yang-Mills, we can find a scalar source around which scattering amplitudes replicate correlation functions computed from the 2D `gluon' operators in a limit where a dynamic massless scalar decouples. We derive conditions on the two-dimensional three-point correlation function so that such a source exists and give two particular examples of this construction, one in which gluons are constructed from vertex operators in the semiclassical limit of Liouville theory and another in which the soft gluons arise from generalized free fields. Finally, we identify a bulk dual to the level of the boundary Kac-Moody algebra and discuss moving beyond the decoupling limit.

hep-th↗

arXiv2024

Approaching the conformal WZW behavior in the infrared limit of two-dimensional massless QCD: a lattice study

Two-dimensional QCD with $N_c$ colors and $N_f$ flavors of massless fermions in the fundamental representation is expected to exhibit conformal behavior in the infrared governed by a $u(N_f)$ WZW model with level $N_c$. Using numerical analysis within the lattice formalism with exactly massless overlap fermions, we show the emergence of such behavior in the infrared limit. Both the continuum extrapolated low-lying eigenvalues of the massless Dirac operator and the propagator of scalar mesons exhibit a flow from the ultraviolet to the infrared. We find that the amplitude of the conserved current correlator remains invariant under the flow, while the amplitude of the scalar correlator approaches $N_f$-independent values in the infrared.

hep-lat↗

arXiv2023

Spectrum of two-dimensional $su(2)$ gauge theories coupled to massless fermions in integer representations

The spectra of two-dimensional $su(2)$ gauge theories coupled to a single massless Majorana fermion in integer representations, $J$, are numerically investigated using the Discrete Light-Cone Hamiltonian. One of our aims is to explore the possible presence of massless states for $J>2$ in spite of the absence of a continuous symmetry. After comparing to existing results for $J=1$ (adjoint fermions), we present results for $J=2,3,4$. As expected, for $J=2$ there are no massless states but in contrast to the $J=1$ theory, the lightest state is a boson. We find exact massless modes in the bosonic and fermionic sector for all values of total momentum for $J=3$ and $J=4$ and, in each sector, the number of massless modes grows with the value of the total momentum. In addition to the spectrum, we present results on the particle number and momentum fraction distributions and argue for a separation of bulk states from edge states.

hep-th↗

arXiv2022

Deforming Soft Algebras for Gauge Theory

Symmetry algebras deriving from towers of soft theorems can be deformed by a short list of higher-dimension Wilsonian corrections to the effective action. We study the simplest of these deformations in gauge theory arising from a massless complex scalar coupled to $F^2$. The soft gauge symmetry '$s$-algebra', compactly realized as a higher-spin current algebra acting on the celestial sphere, is deformed and enlarged to an associative algebra containing soft scalar generators. This deformed soft algebra is found to be non-abelian even in abelian gauge theory. A two-parameter family of central extensions of the $s$-subalgebra are generated by shifting and decoupling the scalar generators. It is shown that these central extensions can also be generated by expanding around a certain non-trivial but Lorentz invariant shockwave type background for the scalar field.

hep-th↗

arXiv2022

Soft Scalars and the Geometry of the Space of Celestial CFTs

Known examples of the holographic dictionary in asymptotically Anti-de Sitter spacetimes equate moduli spaces of bulk vacua with conformal manifolds in the dual quantum field theory. We demonstrate that the same identification holds for gravity in asymptotically flat spacetimes in any dimension, in accord with expectations derived from the celestial conformal field theory (CCFT) formalism. Soft limits of moduli scalars described by the sigma model are universal, and relate to parallel transport of $S$-matrix observables over the moduli space of bulk vacua. The leading "soft moduli operator" is the shadow transform of a dimension $Δ=d$ marginal operator $M(x)$. The universal form of the soft limit guarantees that $M(x)$ acts as a marginal deformation in the CCFT$_d$, and coherent states of the soft scalars correspond to finite deformations along the conformal manifold. This manifold typically has curvature, which is captured by the antisymmetric double-soft theorem and which reflects the Berry curvature in CCFT$_d$. We also compute the Mellin-transformed four-point function in the sigma model and compare to a formula of Kutasov for the curvature of the conformal manifold.

hep-th↗

arXiv2021

State-Operator Correspondence in Celestial Conformal Field Theory

The bulk-to-boundary dictionary for 4D celestial holography is given a new entry defining 2D boundary states living on oriented circles on the celestial sphere. The states are constructed using the 2D CFT state-operator correspondence from operator insertions corresponding to either incoming or outgoing particles which cross the celestial sphere inside the circle. The BPZ construction is applied to give an inner product on such states whose associated bulk adjoints are shown to involve a shadow transform. Scattering amplitudes are then given by BPZ inner products between states living on the same circle but with opposite orientations. 2D boundary states are found to encode the same information as their 4D bulk counterparts, but organized in a radically different manner.

hep-th↗

arXiv2020

The Soft $\mathcal{S}$-Matrix in Gravity

The gravitational $\mathcal{S}$-matrix defined with an infrared (IR) cutoff factorizes into hard and soft factors. The soft factor is universal and contains all the IR and collinear divergences. Here we show, in a momentum space basis, that the intricate expression for the soft factor is fully reproduced by two boundary currents, which live on the celestial sphere. The first of these is the supertranslation current, which generates spacetime supertranslations. The second is its symplectic partner, the Goldstone current for spontaneously broken supertranslations. The current algebra has an off-diagonal level structure involving the gravitational cusp anomalous dimension and the logarithm of the IR cutoff. It is further shown that the gravitational memory effect is contained as an IR safe observable within the soft $\mathcal{S}$-matrix.

hep-th↗

arXiv2020

Massive Celestial Fermions

In an effort to further the study of amplitudes in the celestial CFT (CCFT), we construct conformal primary wavefunctions for massive fermions. Upon explicitly calculating the wavefunctions for Dirac fermions, we deduce the corresponding transformation of momentum space amplitudes to celestial amplitudes. The shadow wavefunctions are shown to have opposite spin and conformal dimension $2-Δ$. The Dirac conformal primary wavefunctions are delta function normalizable with respect to the Dirac inner product provided they lie on the principal series with conformal dimension $Δ= 1+iλ$ for $λ\in\mathbb{R}$. It is shown that there are two choices of a complete basis: single spin $J=\frac{1}{2}$ or $J=-\frac{1}{2}$ and $λ\in\mathbb{R}$ or multiple spin $J=\pm\frac{1}{2}$ and $λ\in\mathbb{R}_{+\cup 0}$. The massless limit of the Dirac conformal primary wavefunctions is shown to agree with previous literature. The momentum generators on the celestial sphere are derived and, along with the Lorentz generators, form a representation of the Poincare algebra. Finally, we show that the massive spin-$1$ conformal primary wavefunctions can be constructed from the Dirac conformal primary wavefunctions using the standard Clebsch-Gordan coefficients. We use this procedure to write the massive spin-$\frac{3}{2}$, Rarita-Schwinger, conformal primary wavefunctions. This provides a prescription for constructing all massive fermionic and bosonic conformal primary wavefunctions starting from spin-$\frac{1}{2}$.

hep-th↗

arXiv2019

Uplifting AdS3/CFT2 to Flat Space Holography

Four-dimensional (4D) flat Minkowski space admits a foliation by hyperbolic slices. Euclidean AdS3 slices fill the past and future lightcones of the origin, while dS3 slices fill the region outside the lightcone. The resulting link between 4D asymptotically flat quantum gravity and AdS3/CFT2 is explored in this paper. The 4D superrotations in the extended BMS4 group are found to act as the familiar conformal transformations on the 3D hyperbolic slices, mapping each slice to itself. The associated 4D superrotation charge is constructed in the covariant phase space formalism. The soft part gives the 2D stress tensor, which acts on the celestial sphere at the boundary of the hyperbolic slices, and is shown to be an uplift to 4D of the familiar 3D holographic AdS3 stress tensor. Finally, we find that 4D quantum gravity contains an unexpected second, conformally soft, dimension (2, 0) mode that is symplectically paired with the celestial stress tensor.

hep-th↗

arXiv2016

A Latitude-Dependent Analysis of the Leptonic Hypothesis for the Fermi Bubbles

The Fermi Bubbles are giant Galactic structures observed in both gamma-rays and microwaves. Recent studies have found support for the hypothesis that the gamma-ray and microwave emission can both be understood as arising from a hard cosmic-ray electron population within the volume of the Bubbles, via inverse Compton scattering and synchrotron radiation respectively. The relative rates of these processes are set by the relative energy density of the interstellar radiation field and the magnetic field within the Bubbles; consequently, under the hypothesis of a common origin, the combination of the gamma-ray and microwave measurements can be used to estimate the magnetic field within the Bubbles. We revisit the consistency of this hypothesis on a latitude-by-latitude basis, using data from Fermi, WMAP and Planck; estimate the variation of the electron spectrum within the Bubbles; and infer bounds on the magnetic field within the Bubbles as a function of distance from the Galactic plane. We find that while the microwave and gamma-ray spectra are generally consistent with the leptonic hypothesis for few-microGauss magnetic fields, there appears to be a preference for spectral hardening in the microwaves at mid-latitudes (especially in the |b|~ 25-35 degree range) that is not mirrored in the gamma rays. This result may hint at a non-leptonic contribution to the gamma-ray spectra; however, the discrepancy can be reconciled in purely leptonic models if the cutoff energy for the electrons is lower in this latitude range and the spectrum below the cutoff is harder.

astro-ph.HE↗