No abstract

We employ a mathematical framework based on rational approximants in order to calculate the pseudoscalar-pole piece of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon, a HLbL;P µ . The method is systematic and data based, profiting from over 13 different collaborations, and able to ascribe, for the first time, a systematic uncertainty which provides for the model independence. As a result, we obtain a HLbL;P µ = 94.3(5.3) × 10 −11 , which uncertainty is well below the one foreseen at future experiments measuring the (gµ − 2).

The η and η 0 transition form factors in the spacelike region are analyzed at low and intermediate energies in a model-independent way through the use of rational approximants. The slope and curvature parameters of the form factors as well as their values at zero and infinity are extracted from experimental data. The impact of these results on the mixing parameters of the η-η 0 system and the pseudoscalar-exchange contributions to the hadronic light-by-light scattering part of the anomalous magnetic moment a μ are also discussed.

The transition form factor is analyzed for the first time in both space- and time-like regions at low and intermediate energies in a model-independent approach through the use of rational approximants. The experimental data provided by the A2 Collaboration in the very low-energy region of the dielectron invariant mass distribution allows for the extraction of the most precise up-to-date slope and curvature parameters of the form factors as well as their values at zero and infinity. The impact of these new results on the mixing parameters of the – system, together with the role played by renormalization dependent effects, and on the determination of the couplings from and radiative decays is also discussed.

In this work we study the axial contributions to the hadronic light-by-light piece of the muon anomalous magnetic moment using the framework of resonance chiral theory. As a result, we obtain a HLbL;A µ = 0.8 +3.5 −0.8 · 10 −11 , that might suggest a smaller value than most recent calculations, underlining the need of future work along this direction. In particular, we find that our results depend critically on the asymptotic behavior of the form factors, and as such, emphasizes the relevance of future experiments for large photon virtualities. In addition, we present general results regarding the involved axial form factors description, comprehensively examining (and relating) the current approaches, that shall be of general interest. 1 arXiv:1910.02881v2 [hep-ph] 9 Dec 2019 1.2 Axial-vector contributions to the muon anomalous magnetic moment Although the Landau-Yang theorem [64,65] forbids the annihilation of a spin-one particle into a pair of real photons, axial-vector exchange contributions to the HLbL piece of a µ are still possible, since at least one photon is off-shell in both axial-γ * -γ * vertices in such a contribution. Still, the Landau-Yang theorem imposes non-trivial requirements on the symmetry structure of the involved form factors, as we will see.Early estimates of the corresponding contributions were carried out both in the extended Nambu-Jona-Lasino model by Bijnens, and by Hayakawa, Kinoshita and Sanda using Hidden Local Symmetry Lagrangians [38,39]. The first group obtained a HLbL;A µ = (2.5 ± 1.0) · 10 −11 , which includes the ballpark value 1.7 · 10 −11 , given by the second group. Melnikov and Vainshtein [45] derived operator product expansion (OPE) constraints on the hadronic light-by-light (HLbL) tensor and built a model where these were saturated by dropping 2 This arises from a SM µ = 1.16591783(35) [15], a SM µ = 1.16591820.4(35.6) [16], a SM µ = 1.16591830(48) [15]. 3 Remarkable progress in the evaluation of the HLbL part of aµ on the lattice has been achieved recently [60-62], as well (see Ref.[63] for a review on this topic).