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$Ξ_{b} \to Ξ$ form factors from lattice QCD and standard-model predictions for $Ξ_{b} \to Ξ μ^{+} μ^{-}$ and $Ξ_{b} \to Ξ γ$ decays

Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

Phys. Rev. D 113, 114521 – Published 22 June, 2026

DOI: https://doi.org/10.1103/4rg4-sd18

Abstract

We present the first lattice QCD determination of the $Ξ_{b} \to Ξ$ vector, axial-vector, and tensor form factors, which are relevant for the theory of rare decays including $Ξ_{b} \to Ξ ℓ^{+} ℓ^{-}$ and $Ξ_{b} \to Ξ γ$ . The calculation is performed with $2 + 1$ flavors of domain-wall fermions at three different lattice spacings and pion masses in the range from approximately 430 to 230 MeV. The bottom quark is implemented using an anisotropic clover action. Three-point functions with a wide range of source-sink separations and model averaging are used to extract the ground-state contributions. We fit the dependence of the form factors on the momentum transfer, the pion mass, and the lattice spacing using modified $z$ expansions that account for subthreshold branch cuts, and apply dispersive bounds and asymptotic behavior constraints to achieve controlled uncertainties in the full semileptonic kinematic region. Using our form factor results, we present standard model predictions for the $Ξ_{b}^{-} \to Ξ^{-} γ$ and $Ξ_{b}^{-} \to Ξ^{-} μ^{+} μ^{-}$ branching fractions and two angular observables.

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Phys. Rev. D 113, 114521– Published 22 June, 2026

Vol. 113, Iss. 11 — 1 June 2026

Received 25 March 2026
Accepted 27 May 2026

DOI: https://doi.org/10.1103/4rg4-sd18

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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$Ξ_{b} \to Ξ$ form factors from lattice QCD and standard-model predictions for $Ξ_{b} \to Ξ μ^{+} μ^{-}$ and $Ξ_{b} \to Ξ γ$ decays

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Ξb→Ξ form factors from lattice QCD and standard-model predictions for Ξb→Ξμ+μ− and Ξb→Ξγ decays

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$Ξ_{b} \to Ξ$ form factors from lattice QCD and standard-model predictions for $Ξ_{b} \to Ξ μ^{+} μ^{-}$ and $Ξ_{b} \to Ξ γ$ decays