Abstract
We reevaluate the hadronic vacuum polarisation contributions to the muon magnetic anomaly and to the running of the electromagnetic coupling constant at the Z-boson mass. We include newest e^+e^- rightarrow mathrm{hadrons} cross-section data together with a phenomenological fit of the threshold region in the evaluation of the dispersion integrals. The precision in the individual datasets cannot be fully exploited due to discrepancies that lead to additional systematic uncertainty in particular between BABAR and KLOE data in the dominant pi ^+pi ^- channel. For the muon (g-2)/2, we find for the lowest-order hadronic contribution (694.0 pm 4.0)cdot 10^{-10}. The full Standard Model prediction differs by 3.3sigma from the experimental value. The five-quark hadronic contribution to alpha (m_Z^2) is evaluated to be (276.0pm 1.0)cdot 10^{-4}.
Highlights
1 Introduction uncertainty-squared are given by the π +π −(γ ) final state,1 while this channel amounts to only 12% of the hadronic contribution to α(s) at s = m2Z [1]
6 Using the KLOE combination [27] we find for aμhad,LO[π π ] between the π +π − threshold and 1.8 GeV a value of 506.6 ± 2.4, which is to be compared with 506.7±2.3 as obtained from the HVPTools combination
In light of this discrepancy, which is not fully captured by the local uncertainty rescaling procedure, we add as additional systematic uncertainty half of the full difference between the complete integrals without BABAR and KLOE, respectively, and we place the central value of the aμhad,LO[π π ] contribution half-way between the two results
Summary
The integration of data points belonging to different experiments with their own data densities requires a careful treatment to avoid biases and to properly account for correlated systematic uncertainties within the same experiment and between different experiments, as well as within and between different channels. The uncertainties on the combined dataset, the data integration and the phenomenological fit are computed using large numbers of pseudo-experiments. These are generated taking into account all measurement uncertainties and their correlations. Where results from different datasets are locally inconsistent, the combined uncertainty is rescaled according to the local χ 2 value and number of degrees of freedom following the PDG prescription [6]. Such inconsistencies are currently limiting the precision of the combination in the dominant π +π − channel as well as in the K + K − channel (see discussions below). Closure tests with known distributions have been performed in the dominant π +π − channel to validate both the combination and integration procedures
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