(g-2)_mu and alpha(M_Z^2) re-evaluated using new precise data (1105.3149v2)

Abstract: We update our Standard Model predictions for g-2 of the muon and for the hadronic contributions to the running of the QED coupling, Delta alpha_had^5(M_Z^2). Particular emphasis is put on recent changes in the hadronic contributions from new data in the 2pi channel and from the energy region just below 2 GeV. In particular, for the e+e- -> pi+pi- contribution we include the recent `radiative return' data from KLOE and BaBar. We also include the recent BaBar data on other exclusive channels. We make a detailed study of the effect of replacing the measurements of the inclusive cross section, sigma(e+e- -> hadrons), by the sum of the exclusive channels in the energy interval 1.43 < sqrt{s} < 2 GeV, which includes a QCD sum-rule analysis of this energy region. Our favoured prediction for the muon anomalous magnetic moment is (g-2)/2 = (11659182.8 +- 4.9)*10^-10 which is 3.3 sigma below the present world-average measurement. We compare our g-2 value with other recent calculations. Our prediction for the QED coupling, obtained via Delta alpha_had^5(M_Z^2) = (276.26 +- 1.38)*10^-4, is alpha(M_Z^2)^-1 = 128.944 +- 0.019.

• The paper recalculates (g-2)μ with enhanced precision, revealing a 3.3σ discrepancy that hints at potential physics beyond the Standard Model.
• It integrates new e⁺e⁻ → π⁺π⁻ data from KLOE and BaBar to reconcile direct scan and radiative return measurements effectively.
• The study refines Δα(MZ²) through detailed energy region assessments, thereby supporting improved electroweak fits and constraining new physics scenarios.

Standard Model Predictions for $(g-2)_{\mu}$ and $\Delta \alpha(M_Z^2)$ with Enhanced Precision

The paper provides an essential update to calculations within the Standard Model (SM) concerning the muon anomalous magnetic moment $(g-2)_{\mu}$ and the QED coupling constant running, $\Delta \alpha^{(5)}_{\rm had}(M_Z^2)$. The focus is on incorporating the latest precise data that particularly benefits from recent measurements in the $e^+e^- \to \pi^+\pi^-$ channel and in the energy interval just below 2 GeV. The results presented reflect new insights gained from both direct scan experiments and analyses of radiative return measurements from the KLOE and BaBar experiments.

Key Findings and Methodologies

1. Muon Anomalous Magnetic Moment $(g-2)_{\mu}$: The paper reports a SM prediction for the muon anomalous magnetic moment as $(g-2)/2 = (11\,659\,182.8 \pm 4.9) \cdot 10^{-10}$. This value is in a notable $3.3\,\sigma$ disagreement with the latest experimental world average, a discrepancy underscoring potential physics beyond the Standard Model (BSM).
2. Data Incorporation: The recalibration primarily includes data from the $2\pi$ channel with results from both KLOE and BaBar. The authors conducted a detailed assessment to balance the existing direct scan measurements and recently available precise radiative return data. This reconciliation is crucial as it showed some tension between datasets from different experiments.
3. Energy Region Analysis: A rigorous evaluation was carried out for the energy region between 1.43 and 2 GeV. The inclusion of new data from BaBar on multiple hadronic channels allows this paper to tackle previously uncertain exclusive channel contributions more confidently. The results benefitted from improved inclusive-exclusion compilations and sum-rule analyses, offering a rather fine percent-scale accuracy for certain channels.
4. Sum-Rule Analysis: For ensuring compatibility with the results obtained from perturbative QCD (pQCD), a sum-rule analysis was utilized. The findings suggest the updated, detailed dataset aligns more closely with the globally accepted pQCD calculations and the world average for $\alpha_s$.
5. QED Coupling Running: The evaluation of the hadronic contribution to the running of $\alpha(M_Z^2)$ is reported as $$\Delta\alpha_{\rm had}^{(5)}(M_Z^2) = (276.26 \pm 1.38) \cdot 10^{-4}$$. This precise determination aids in the precision fits for electroweak parameters crucial in constraining the Higgs boson mass and exploring BSM physics.

Implications and Future Directions

The results have significant implications in the field of precision electroweak measurements and the quest for new physics. The continued discrepancy between the theoretical predictions and experimental measurements of $(g-2)_{\mu}$ suggests areas where the SM could be expanded or refined. The increased precision of $\Delta \alpha(M_Z^2)$ directly feeds into the electroweak fit analysis, potentially impacting the interpretation of Higgs boson search results and other precision observables.

Future work may further decrease uncertainties through additional measurements from ongoing and forthcoming experiments like those at VEPP-2000, and through continued improvements in the theoretical modeling, including lattice QCD contributions to low-energy instances like the hadronic light-by-light scattering effect on $(g-2)_{\mu}$. This advancement underscores the paper's synthesis of rapidly evolving experimental data with state-of-the-art theoretical models, a trend poised to persist with upcoming technological developments.