Reevaluation of the Hadronic Contributions to the Muon g-2 and to alpha(MZ)

TL;DR

This work reevaluates the hadronic vacuum polarization contributions to the muon g-2 and to α(MZ^2) using new π+π− data from KLOE, comprehensive BABAR multi-hadron data, improved missing-channel estimates, and a unified HVPTools framework with four-loop perturbative QCD. The refined analysis yields smaller hadronic contributions, which increases the discrepancy between the SM prediction and the measured muon g-2, and modestly shifts the electroweak fit's preferred Higgs mass upward by about 7 GeV. A thorough treatment of inter-channel correlations raises the final uncertainty, and remaining tensions—particularly between KLOE and BABAR—underscore the need for new cross-section measurements and improved g-2 and hadronic light-by-light determinations. The results have important implications for precision tests of the SM and for constraining the Higgs sector via electroweak fits.

Abstract

We reevaluate the hadronic contributions to the muon magnetic anomaly, and to the running of the electromagnetic coupling constant at the Z-boson mass. We include new pi+pi- cross-section data from KLOE, all available multi-hadron data from BABAR, a reestimation of missing low-energy contributions using results on cross sections and process dynamics from BABAR, a reevaluation of all experimental contributions using the software package HVPTools, together with a reanalysis of inter-experiment and inter-channel correlations, and a reevaluation of the continuum contributions from perturbative QCD at four loops. These improvements lead to a decrease in the hadronic contributions with respect to earlier evaluations. For the muon g-2 we find lowest-order hadronic contributions of (692.3 +- 4.2) 10^-10 and (701.5 +- 4.7) 10^-10 for the e+e- based and tau-based analyses, respectively, and full Standard Model predictions that differ by 3.6 sigma and 2.4 sigma from the experimental value. For the e+e- based five-quark hadronic contribution to alpha(MZ) we find Delta_alpha_had[5](MZ)=(275.7 +- 1.0) 10^-4. The reduced electromagnetic coupling strength at MZ leads to an increase by 7 GeV in the most probable Higgs boson mass obtained by the standard Gfitter fit to electroweak precision data.

Figures (10)

• Figure 1: Cross section of $e^+e^-\xspace\to\pi^+\pi^-\xspace$ versus centre-of-mass energy for different energy ranges. Shown are data from TOF E_55, OLYA barkovE_54, CMD barkov, CMD2 cmd203cmd2new, SND snd DM1 E_58, DM2 E_59, KLOE kloe08kloe10, and BABAR babarpipi. The error bars show statistical and systematic errors added in quadrature. The light shaded (green) band indicates the HVPTools average within $1\,\sigma$ errors.
• Figure 2: Comparison between individual $e^+e^-\xspace\to\pi^+\pi^-\xspace$ cross section measurements from BABAR babarpipi, KLOE 08 kloe08, KLOE 10 kloe10, CMD2 03 cmd203, CMD2 06 cmd2new, SND snd, and the HVPTools average. The error bars show statistical and systematic errors added in quadrature.
• Figure 3: Relative local averaging weight per experiment versus centre-of-mass energy in $e^+e^-\xspace\to\pi^+\pi^-\xspace$. See Figs. \ref{['fig:xsecpipi']} and \ref{['fig:comppipi']} for references.
• Figure 4: Cross section of $e^+e^-\xspace\to\pi^+\pi^-\xspace\pi^0\xspace$ versus centre-of-mass energy. Shown are data from ND nd, DM1 dm13pi, SND snd3pi CMD cmd3pi, CMD2 cmd23pi and BABAR babar3pi. The error bars show statistical and systematic errors added in quadrature. The light shaded (green) band indicates the HVPTools average within $1\,\sigma$ errors.
• Figure 5: Cross section versus centre-of-mass energy of $e^+e^-\xspace\to 2\pi^+\xspace 2\pi^-\xspace$ (left) and $e^+e^-\xspace\to\pi^+\pi^-\xspace 2\pi^0\xspace$ (right), and for linear (top) and logarithmic ordinates (bottom). The open circles show data from BABAR babar4pibabar2pi2pi0, which dominate in precision. The other measurements shown are taken for the four charged pions final state from ND E_64, M3N m3n4pi, MEA mea4pi, CMD E_72, DM1 E_73PhysLett81B, DM2 E_74E_74pE_75, OLYA E_70, CMD2 cmd2_2pi2pi0 and SND snd_2pi2pi0, and for the mixed charged and neutral state from ND E_64, M3N E_66, DM2 E_74E_74pE_75, OLYA ol, and SND snd_2pi2pi0. The error bars show statistical and systematic errors added in quadrature. The shaded (green) bands give the HVPTools averages.