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The precise measurement of the cross section e(+)e(-) -> pi(+)pi(-)(gamma) from threshold to an energy of 3 GeV, using events with Initial State Radiation (ISR) collected with the BABAR detector, is presented. The ISR luminosity is determined from a study of the leptonic process e(+)e(-) -> mu(+)mu(-)gamma(gamma), and the method is tested by the comparison with the next-to-leading order (NLO) QED prediction. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the BABAR pi pi cross section measured from threshold to 1.8 GeV is (514.1 +/- 2.2(stat) +/- 3.1(syst)) x 10(-10). Other results on ISR multihadronic cross sections from BABAR are presented.
We present an update of the measurement of αS(m2τ) from ALEPH τ hadronic spectral functions. We report a study of the perturbative prediction(s) showing that the fixed-order perturbation theory manifests convergence problems not presented in the contour-improved calculation. Potential systematic effects from quark-hadron duality violations are estimated to be within the quoted systematic errors. The fit result is αS(m2τ) =0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution, the αS(m2Z) determined from τ data is the most precise one to date, in agreement with the corresponding N3LO value derived from Z decays.
Le Modèle Standard (MS) de la physique de particules avec les théories de jauge pour les interactions forte et électrofaible n'a pas encore été mis en défaut par les données expérimentales. Dans les recherches de nouvelle physique, au delà du MS, deux approches sont en général suivies : les recherches exploratoires à la frontière de haute énergie et des tests de précision à des énergies plus basses. Le but de cette thèse est dans le cadre de la deuxième approche, d'obtenir et utiliser des données précises des annihilations e+e− en hadrons, à des énergies de l'ordre 1 GeV. Ces données représentent un ingrédient important pour les tests du MS impliquant la polarisation du vide, comme par exemple la comparaison du moment magnétique du muon avec la théorie, ainsi que pour des tests de QCD et applications. Les différentes parties de cette t...
We propose a new iterative unfolding method for experimental data, making use of a regularization function. The use of this function allows one to build an improved normalization procedure for Monte Carlo spectra, unbiased by the presence of possible new structures in data. We are able to unfold, in a dynamically stable way, data spectra which can be strongly affected by fluctuations in the background subtraction and simultaneously reconstruct structures which were not initially simulated. This method also allows one to control the amount of correlations introduced between the bins of the unfolded spectrum, when the transfers of events correcting the systematic detector effects are performed.
An updated measurement of α_s(m^2_τ) from ALEPH τ hadronic spectral functions is presented. We report a study of the perturbative prediction(s) showing that the fixed-order perturbation theory manifests convergence or principle problems not presented in the contour-improved calculation. Potential systematic effects from quark-hadron duality violations are estimated to be within the quoted systematic errors. The fit result is α_s(m^2_τ) = 0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution, the α_s(m^2_z) determined from τ data is one of the most precise to date, in agreement with the corresponding N^3LO value derived from Z decays.
The ISR method has been largely exploited by the BABAR experiment, for measuring numerous channels of the cross section e+e- into hadrons. For the pi+pi-(gamma) and K+K-(gamma) channels, BABAR has pioneered the method based on the ratio between the hadronic mass spectra and the mu+mu-(gamma) one. This method allows to cancel many systematic uncertainties in the ratio, hence the precise measured cross sections. Many multihadronic channels have also been studied using the ISR method, and cross sections have been published. These experimental results have also been exploited for phenomenological studies, like the determination of the hadronic contribution to the anomalous magnetic moment of the muon (g-2)_mu.
In a recent paper \cite{benayoun} M.Benayoun {\it et al.} use a specific model to compare results on the existing data for the cross section of the process $e^+e^-\rightarrow \pi^+\pi^-$ and state conclusions about the inconsistency of the BABAR results with those from the other experiments. We show that a direct model-independent comparison of the data at hand contradicts this claim. Clear discrepancies with the results of Ref. \cite{benayoun} are pointed out. As a consequence we do not believe that the lower value and the smaller uncertainty obtained for the prediction of the muon magnetic anomaly are reliable results.
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