nucl-th digest · May 7, 2018

[Submitted on 3 May 2018] (cross-list from hep-ph)

Pion-pole contribution to hadronic light-by-light scattering in the anomalous magnetic moment of the muon

Martin Hoferichter🇺🇸 · Bai-Long Hoid🇩🇪 · Bastian Kubis🇩🇪 · Stefan Leupold🇸🇪 · Sebastian P. Schneider🇩🇪

The $π^{0}$ pole constitutes the lowest-lying singularity of the hadronic light-by-light (HLbL) tensor, and thus provides the leading contribution in a dispersive approach to HLbL scattering in the anomalous magnetic moment of the muon $(g - 2)_{μ}$ . It is unambiguously defined in terms of the doubly-virtual pion transition form factor, which in principle can be accessed in its entirety by experiment. We demonstrate that, in the absence of a direct measurement, the full space-like doubly-virtual form factor can be reconstructed very accurately based on existing data for $e^{+} e^{-} \to 3 π$ , $e^{+} e^{-} \to e^{+} e^{-} π^{0}$ , and the $π^{0} \to γ γ$ decay width. We derive a representation that incorporates all the low-lying singularities of the form factor, matches correctly onto the asymptotic behavior expected from perturbative QCD, and is suitable for the evaluation of the $(g - 2)_{μ}$ loop integral. The resulting value, $a_{μ}^{π^{0} -pole} = 62. 6_{- 2.5}^{+ 3.0} \times 1 0^{- 11}$ , for the first time, represents a complete data-driven determination of the pion-pole contribution with fully controlled uncertainty estimates. In particular, we show that already improved singly-virtual measurements alone would allow one to further reduce the uncertainty in $a_{μ}^{π^{0} -pole}$ .

Comments:: 7 pages, 4 figures; journal version
Subjects:: High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); High Energy Physics — Lattice (hep-lat); Nuclear Theory (nucl-th)
arXiv:: 1805.01471 [pdf]

PRL(2018)·200 citations

Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory

High-energy nuclear collisions produce a nonequilibrium plasma of quarks and gluons which thermalizes and exhibits hydrodynamic flow. There are currently no practical frameworks to connect the early particle production in classical field simulations to the subsequent hydrodynamic evolution. We build such a framework using nonequilibrium Green's functions, calculated in QCD kinetic theory, to propagate the initial energy-momentum tensor to the hydrodynamic phase. We demonstrate that this approach can be easily incorporated into existing hydrodynamic simulations, leading to stronger constraints on the energy density at early times and the transport properties of the QCD medium. Based on (conformal) scaling properties of the Green's functions, we further obtain pragmatic bounds for the applicability of hydrodynamics in nuclear collisions.

Comments:: 7 pages, 4 figures, v2: typos corrected and minor changes, version accepted for publication in Phys. Rev. Lett., see also our companion paper arXiv:1805.00961 for the extensive details, for the code of linear kinetic theory propagator KoMPoST used for this study see https://github.com/KMPST/KoMPoST ; v3 updated references, published version
Subjects:: High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:: 1805.01604 [pdf]

PRL(2019)·252 citations

Monday·May 7, 2018

Pion-pole contribution to hadronic light-by-light scattering in the anomalous magnetic moment of the muon

Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory

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