The hadronic light-by-light contribution to the muon’s anomalous magnetic moment

Abstract

In view of the current 3 -4 σ deviation between theoretical and experimental values for the muon's anomalous magnetic moment, we review the ongoing efforts in constraining the hadronic light-by-light contribution to a µ by using dispersive techniques combined with a dedicated experimental program to obtain the required hadronic input.

Pseudoscalar transition form factors and the hadronic light-by-light contribution to the anomalous magnetic moment of the muon from holographic QCD

“…This turns out to lead to an over-estimation of the single-virtual TFF and, as a consequence, gives a result for a µ that is about 10% higher. With full numerical evaluation, the bottom-up-holographic results span the range a π 0 µ = 5.9(2) · 10 −10 , which turns out to be well in line with the recent result obtained with the DRV interpolator fitted to the world data for π 0 [7] and also with the lattice results of Ref. [15].…”

“…All these models are completely fixed, once f π and the mass of the ρ meson are given. Comparing with the experimental data reviewed in [7], which include preliminary low-energy data for F π 0 (Q 2 , 0) from BESIII, we find a surprisingly good agreement for the bottom-up models, whereas the Sakai-Sugimoto model performs well only at the lowest values of Q 2 . Compared to the interpolators used in Ref.…”

“…Compared to the interpolators used in Ref. [12] and the new one proposed by Danilkin, Redmer, and Vanderhaeghen (DRV) in [7], the holographic models unanimously predict a milder dependence on the difference Q 2 1 − Q 2 2 at fixed sum Q 2 1 + Q 2 2 , which should be testable by future experiments at BESIII. In fact, the behavior found in the recent lattice extrapolation of Ref.…”

“…3 of Ref. [7], which includes preliminary data from BESIII down to 0.3 GeV 2 . At the lowest available Q 2 value, all holographic results are within the experimental error bar, while ) in the top-down holographic SS model and the three bottom-up models HW1, HW2, and SW considered here (all in the chiral limit), with IR coefficients as defined in (22) and Λ 2 ≡ −1/α; the UV behavior is described byF = F/F ∞ , where F ∞ is the LO QCD result (25).…”

“…F (0, 0) is determined by the known decay rates into two real photons. Experimental data for F π 0 (Q 2 , 0) in the region Q 2 1 GeV 2 , where the bulk of the pion-pole contribution to a µ arises [5], have been obtained in particular by the CELLO collaboration [6] and recently by BESIII [7]. In the calculation of a µ , a model of the double-virtual TFF is needed, for which data in the relevant momentum region are still lacking.…”