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Register a new userUpdated estimate of the muon magnetic moment using revised results from e+e- annihilation
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Michel Davier
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A new evaluation of the hadronic vacuum polarization contribution to the muon magnetic moment is presented. We take into account the reanalysis of the low-energy ee annihilation cross section into hadrons by the CMD-2 Collaboration. The agreement between ee and $tau$ spectral functions in the $pipi$ channel is found to be much improved. Nevertheless, significant discrepancies remain in the center-of-mass energy range between 0.85 and $1.0 {rm GeV}/c^2$.The deviations from the measurement at BNL are found to be $(22.1 pm 7.2 pm 3.5 pm 8.0)~10^{-10}$ (1.9 $sigma$) and $(7.4 pm 5.8 pm 3.5 pm 8.0)~10^{-10}$ (0.7 $sigma$) for the ee- and $tau$-based estimates, respectively, where the second error is from the nonhadronic contributions and the third one from the BNL measurement. Taking into account the $rho^- - rho^0$ mass splitting determined from the measured spectral functions increases the $tau$-based estimate and leads to a worse discrepancy between the two estimates.
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The BaBar collaboration has an extensive program of studying hadronic cross sections in low-energy e+e- collisions, accessible via initial-state radiation. Our measurements allow significant improvements in the precision of the predicted value of the muon anomalous magnetic moment. These improvements are necessary for illuminating the current ~3.6 sigma difference between the predicted and the experimental values. We have published results on a number of processes with two to six hadrons in the final state. We report here the results of recent studies with final states that constitute the main contribution to the hadronic cross section in the energy region between 1 and 3 GeV, as e+e- to K+K-, pi+pi-, and e+e- to 4 hadrons.
The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^prime$ were set by the NA64 experiment for the mass region $m_{A^prime}lesssim 250$ MeV, by analyzing data from the interaction of $2.84cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^prime$ production via secondary positron annihilation with atomic electrons, we extend these limits in the $200$-$300$ MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^+$ beam efforts.
The usual interpretation of Bose-Einstein correlations (BEC) of identical boson pairs relates the width of the peak in the correlation function at small relative four-momentum to the spatial extent of the source of the bosons. However, in the tau-model, which successfully describes BEC in hadronic Z decay, the width of the peak is related to the temporal extent of boson emission. Some new checks on the validity of both the tau-model and the usual descriptions are presented.
Bose-Einstein correlations of pairs of identical charged pions produced in hadronic Z decays are analyzed for both two- and three-jet events. A parametrization suggested by the tau-model is used to investigate the dependence of the Bose-Einstein correlation function on track multiplicity, number of jets, and transverse momentum.
Data from e+e- annihilation into hadrons collected by the JADE experiment at centre-of-mass energies between 14 GeV and 44 GeV are used to study the four-jet event production rate as a function of the Durham jet algorithms resolution parameter ycut. The four-jet rate is compared to QCD next-to-leading order calculations including resummation of large logarithms in the next-to-leading logarithmic approximation. The strong coupling measured from the four-jet rate is as(MZ)=0.1159+-0.0004(stat)+-0.0012(expt)+-0.0024(had)+-0.0007(theo) in agreement with the world average.
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