Puzzles in the hadronic contributions to the muon anomalous magnetic moment

Abstract

We summarize recent developments in the Standard-Model evaluation of the anomalous magnetic moment of the muon aμ , both in the hadronic-light-by-light and hadronic-vacuum-polarization contributions. The current situation for the latter is puzzling as we are confronted with multiple discrepancies that are not yet understood. We present updated fits of a dispersive representation of the pion vector form factor to the new CMD-3 data set and quantify the tensions with the other high-statistics e + e - → π + π - experiments in the contribution to aμ in the energy range up to 1 GeV, as well as in the corresponding contribution to the intermediate Euclidean window. See ref. [1] for an extended version of this contribution.

Hadronic Light-by-Light Corrections to the Muon Anomalous Magnetic Moment

“…The most recent results from the CMD-3 experiment for ππ production show a significant deviation from all other previous experimental results, including CMD-2 [33]. Much like recent lattice HVP results, dispersive estimates of HVP with ππ contributions coming only from this new set of measurements also reduce the tension between the SM prediction and a exp µ [32], which has further added to the confusion. Consistency checks from analyticity and unitarity constraints on the pion vector form factor have not shed much light on the discrepancies between experiments [32].…”

“…Much like recent lattice HVP results, dispersive estimates of HVP with ππ contributions coming only from this new set of measurements also reduce the tension between the SM prediction and a exp µ [32], which has further added to the confusion. Consistency checks from analyticity and unitarity constraints on the pion vector form factor have not shed much light on the discrepancies between experiments [32]. Now we go on to the HLbL scattering, which gives a contribution (see Figure 3) of 92 (18) × 10 −11 [10].…”

“…It is less influenced both by effects due to the finite size of the lattice (large distance) and the non-zero size of lattice spacings (short distance). This quantity has allowed for more clear comparisons between dispersive and lattice estimates [30][31][32], which have so far resulted in confirmations of the phenomenology-lattice tension. The most recent results from the CMD-3 experiment for ππ production show a significant deviation from all other previous experimental results, including CMD-2 [33].…”