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1. Systematics of U-spin sum rules for systems with direct sums

   https://doi.org/10.1007/JHEP12(2024)072  

   Gavrilova, Margarita; Schacht, Stefan (December 2024, Journal of High Energy Physics JHEP)

   A<sc>bstract</sc> A rich mathematical structure underlying flavor sum rules has been discovered recently. In this work, we extend these findings to systems with a direct sum of representations. We prove several results for the general case. We derive an algorithm that enables the determination of allU-spin amplitude sum rules at arbitrary order of the symmetry breaking for any system containing a direct sum of the representations 0 ⨁ 1. Potential applications are numerous and include, for example, higher order sum rules for CP-violating charm decays with an arbitrary number of final states. 

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2. AdS bulk locality from sharp CFT bounds

   https://doi.org/10.1007/JHEP11(2021)164  

   Caron-Huot, Simon; Mazáč, Dalimil; Rastelli, Leonardo; Simmons-Duffin, David (November 2021, Journal of High Energy Physics)

   A bstract It is a long-standing conjecture that any CFT with a large central charge and a large gap ∆ gap in the spectrum of higher-spin single-trace operators must be dual to a local effective field theory in AdS. We prove a sharp form of this conjecture by deriving numerical bounds on bulk Wilson coefficients in terms of ∆ gap using the conformal bootstrap. Our bounds exhibit the scaling in ∆ gap expected from dimensional analysis in the bulk. Our main tools are dispersive sum rules that provide a dictionary between CFT dispersion relations and S-matrix dispersion relations in appropriate limits. This dictionary allows us to apply recently-developed flat-space methods to construct positive CFT functionals. We show how AdS 4 naturally resolves the infrared divergences present in 4D flat-space bounds. Our results imply the validity of twice-subtracted dispersion relations for any S-matrix arising from the flat-space limit of AdS/CFT. 

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3. Dispersive CFT sum rules

   https://doi.org/10.1007/JHEP05(2021)243  

   Caron-Huot, Simon; Mazáč, Dalimil; Rastelli, Leonardo; Simmons-Duffin, David (May 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract We give a unified treatment of dispersive sum rules for four-point correlators in conformal field theory. We call a sum rule “dispersive” if it has double zeros at all double-twist operators above a fixed twist gap. Dispersive sum rules have their conceptual origin in Lorentzian kinematics and absorptive physics (the notion of double discontinuity). They have been discussed using three seemingly different methods: analytic functionals dual to double-twist operators, dispersion relations in position space, and dispersion relations in Mellin space. We show that these three approaches can be mapped into one another and lead to completely equivalent sum rules. A central idea of our discussion is a fully nonperturbative expansion of the correlator as a sum over Polyakov-Regge blocks . Unlike the usual OPE sum, the Polyakov-Regge expansion utilizes the data of two separate channels, while having (term by term) good Regge behavior in the third channel. We construct sum rules which are non-negative above the double-twist gap; they have the physical interpretation of a subtracted version of “superconvergence” sum rules. We expect dispersive sum rules to be a very useful tool to study expansions around mean-field theory, and to constrain the low-energy description of holographic CFTs with a large gap. We give examples of the first kind of applications, notably we exhibit a candidate extremal functional for the spin-two gap problem. 

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4. Mass sum rules of the electron in quantum electrodynamics

   https://doi.org/10.1007/JHEP09(2020)067  

   Rodini, S.; Metz, A.; Pasquini, B. (September 2020, Journal of High Energy Physics)

   null (Ed.)

   A bstract Different decompositions of the nucleon mass, in terms of the masses and energies of the underlying constituents, have been proposed in the literature. We explore the corresponding sum rules in quantum electrodynamics for an electron at one-loop order in perturbation theory. To this aim we compute the form factors of the energy-momentum tensor, by paying particular attention to the renormalization of ultraviolet divergences, operator mixing and scheme dependence. We clarify the expressions of all the proposed sum rules in the electron rest frame in terms of renormalized operators. Furthermore, we consider the same sum rules in a moving frame, where they become energy decompositions. Finally, we discuss some implications of our study on the mass sum rules for the nucleon. 

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5. Quantifying the orbital-to-spin moment ratio under dynamic excitation

   https://doi.org/10.1063/5.0198326  

   Emori, Satoru; Maizel, Rachel E.; Street, Galen T.; Jones, Julia L.; Arena, Dario A.; Shafer, Padraic; Klewe, Christoph (March 2024, Applied physics letters)

   The orbital component of magnetization dynamics, e.g., excited by ferromagnetic resonance (FMR), may generate “orbitronic” effects in nanomagnetic devices. Yet, distinguishing orbital dynamics from spin dynamics remains a challenge. Here, we employ x-ray magnetic circular dichroism (XMCD) to quantify the ratio between the orbital and spin components of FMR-induced dynamics in a Ni80Fe20 film. By applying the XMCD sum rules at the Ni L3,2 edges, we obtain an orbital-to-spin ratio of 0.108 ± 0.005 for the dynamic magnetization. This value is consistent with 0.102 ± 0.008 for the static magnetization, probed with the same x-ray beam configuration as the dynamic XMCD experiment. The demonstrated method presents a possible path to disentangle orbitronic effects from their spintronic counterparts in magnetic media. 

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