Reevaluation of the hadronic contributions to the muon g−2 and to \(\alpha (M^{2}_{Z})\)

The European Physical Journal C, Jan 2011

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 π + π − 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 τ-based analyses, respectively, and full Standard Model predictions that differ by 3.6σ and 2.4σ from the experimental value. For the e + e −-based five-quark hadronic contribution to \(\alpha(M_{Z}^{2})\) we find \(\varDelta \alpha_{\mathrm{had}}^{(5)}(M_{Z}^{2}) =(274.9\pm1.0)\cdot10^{-4}\). The reduced electromagnetic coupling strength at M Z leads to an increase by 12 GeV in the central value of the Higgs boson mass obtained by the standard Gfitter fit to electroweak precision data.

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Reevaluation of the hadronic contributions to the muon g−2 and to \(\alpha (M^{2}_{Z})\)

M. Davier 1 A. Hoecker 0 B. Malaescu 1 2 Z. Zhang 1 0 CERN, 1211, Geneva 23, Switzerland 1 Laboratoire de l'Acclrateur Linaire , IN2P3/CNRS, Universit Paris-Sud 11 , Orsay, France 2 Now at CERN, 1211, Geneva 23, Switzerland 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 + 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) 1010 and (701.5 4.7) 1010 for the e+ebased and -based analyses, respectively, and full Standard Model predictions that differ by 3.6 and 2.4 from the experimental value. For the e+e-based five-quark hadronic contribution to (MZ2 ) we find h(5a)d(MZ2 ) = (274.9 1.0) 104. The reduced electromagnetic coupling strength at MZ leads to an increase by 12 GeV in the central value of the Higgs boson mass obtained by the standard Gfitter fit to electroweak precision data. 1 Introduction The Standard Model (SM) predictions of the anomalous magnetic moment of the muon, a, and of the running electromagnetic coupling constant, (s), are limited in precision by contributions from virtual hadronic vacuum polarisation. The dominant hadronic terms can be calculated with a combination of experimental cross section data, involving e+e annihilation to hadrons, and perturbative QCD. These are used to evaluate an energy-squared dispersion integral, ranging from the 0 threshold to infinity. The integration kernels occurring in the dispersion relations emphasise low photon virtualities, owing to the 1/s descend of the cross section, and, in case of a, to an additional 1/s suppression. In the latter case, about 73% of the lowest order hadronic contribution is provided by the + ( ) final state,1 while this channel amounts to only 13% of the hadronic contribution to (s) at s = MZ2 . In this paper, we reevaluate the lowest-order hadronic contribution, ahad,LO, to the muon magnetic anomaly, and the hadronic contribution, had(MZ2 ), to the running (MZ2 ) at the Z-boson mass. We include new + crosssection data from KLOE [1] and all the available multihadron data from BABAR [29]. We also perform a reestimation of missing low-energy contributions using results on cross sections and process dynamics from BABAR. We reevaluate all the experimental contributions using the software package HVPTools [10], including a comprehensive reanalysis of inter-experiment and inter-channel correlations. Furthermore, we recompute the continuum contributions using perturbative QCD at four loops [11]. These improvements taken together lead to a decrease of the hadronic contributions with respect to our earlier evaluation [10], and thus to an accentuation of the discrepancy between the SM prediction of a and the experimental result [12]. The reduced electromagnetic coupling strength at MZ leads to an increase in the most probable value for the Higgs boson mass returned by the electroweak fit, thus relaxing the tension with the exclusion results from the direct Higgs searches. 1Throughout this paper, final state photon radiation is implied for all hadronic final states. Fig. 1 Cross section of e+e + versus centre-of-mass energy for different energy ranges. Shown are data from TOF [17], OLYA [18, 19], CMD [18], CMD2 [2023], SND [24], DM1 [25], DM2 [26], KLOE [1, 13], and BABAR [2]. The error bars show statistical and systematic errors added in quadrature. The light shaded (green) band indicates the HVPTools average within 1 errors 2 New input data The KLOE Collaboration has published new + cross section data with + invariant mass-squared between 0.1 and 0.85 GeV2 [1]. The radiative photon in this analysis is required to be detected in the electromagnetic calorimeter, which reduces the selected data sample to events with large photon scattering angle (polar angle between 50 and 130), and photon energies above 20 MeV. The new data are found to be in agreement with, but less precise than, previously published data using small angle photon scattering [13] (superseding earlier KLOE data [14]). They hence exhibit the known discrepancy, on the resonance peak and above, with other + data, in particular those from BABAR, Fig. 2 Comparison between individual e+e + cross section measurements from BABAR [2], KLOE 08 [13], KLOE 10 [1], CMD2 03 [20], CMD2 06 [2123], SND [24], and the HVPTools average. The error bars show statistical and systematic (...truncated)


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M. Davier, A. Hoecker, B. Malaescu, Z. Zhang. Reevaluation of the hadronic contributions to the muon g−2 and to \(\alpha (M^{2}_{Z})\), The European Physical Journal C, 2011, pp. 1515, Volume 71, Issue 1, DOI: 10.1140/epjc/s10052-010-1515-z