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Mesures précises de sections efficaces e^+e^− → Hadrons : tests du Modèle Standard et applications en QCD

Abstract : The Standard Model (SM) of particle physics with gauge theories for strong and electroweak interactions is still unchallenged by experimental data. Looking for new physics beyond the SM, two approaches are traditionally followed: exploratory searches at the high energy frontier and precision tests at lower energies. The scope of this thesis is within the second approach, obtaining and using accurate data on e+e− annihilation into hadrons at energies of the order 1 GeV. These data represent a very valuable input for SM tests involving vacuum polarization, such as the comparison of the muon magnetic moment to theory, and for QCD tests and applications. The different parts of this thesis describe four aspects of my work in this context. (1) Measurements of cross sections as a function of energy necessitate the unfolding of data spectra from detector effects. I propose a new iterative unfolding method for experimental data, with improved capabilities compared to existing tools. 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. (2) The experimental core of this thesis is a study of the process e+e− to K+K− from threshold to 5 GeV using the initial state radiation (ISR) method (through the measurement of e+e− to K+K− gamma) with the BABAR detector. All relevant efficiencies are measured with experimental data and the absolute normalization comes from the simultaneously measured mu mu gamma process. I have performed the full analysis which achieves a systematic uncertainty of 0.7 % on the dominant phi resonance. The charged kaon form factor is obtained showing a fast decrease beyond the phi and some distinct structures in the 1.7-2.5 GeV region where vector resonances are known to exist. The energy behaviour at large energies is compared to QCD predictions. Also presented are results on e+e− to pi+pi− from threshold to 3 GeV for which I performed the unfolding and obtained the final results which are compared to existing data. (3) The prediction of the muon magnetic moment (expressed through its 'anomaly', i.e. the deviation from the Dirac value of the gyromagnetic ratio equal to 2) is done using the SM. Our work concerns only the hadronic vacuum polarization contribution obtained from e+e− data through a dispersion integral. As the same information can in principle be obtained for data on hadronic tau decays, we first update the comparative use of the two sources of input data and find a reduced difference between the corresponding evaluations. At the same time, the new tau-based estimate of the muon magnetic anomaly is found to be 1.9 standard deviations lower than the direct measurement. Then the new precise BABAR data are included in a combined analysis using tools (featuring improved data interpolation and averaging, more accurate error propagation and systematic validation) I developed. With the new data, the discrepancy between the e+e− and tau-based results for the dominant two-pion mode is further reduced, from previously 2.4 sigma to 1.5 sigma, in the dispersion integral, though significant local discrepancies in the spectra persist. We obtain for the e+e−-based evaluation a_μ^had LO = (695.5 +- 4.1)*10^-10, where the error accounts for all sources. The largest uncertainty for the SM prediction is still from hadronic vacuum polarization, but it is now below the experimental error. The present comparison between the direct measurement and our (e+e−-based) prediction shows an interesting hint for new physics (3.2 sigma effect). (4) QCD sum rules are powerful tools for obtaining precise information on QCD parameters, such as the strong coupling αS. This study should involve the complete measurement of e+e− to hadrons up to about 2 GeV. Since BABAR has not yet completely measured all hadronic processes, I worked on a similar situation using the spectral functions from tau decays measured by ALEPH. I discuss to some detail the perturbative QCD prediction obtained with two different methods: fixed-order perturbation theory (FOPT) and contour-improved perturbative theory (CIPT). The corresponding theoretical uncertainties are studied at the tau and Z mass scales. The CIPT method is found to be more stable with respect to the missing higher order contributions and to renormalisation scale variations. It is also shown that FOPT suffers from convergence problems along the complex integration contour. The reliability of a class of renormalon-based models for the Adler function is studied, in the context of the CIPT vs. FOPT comparison. It is found that these models are not enough constrained to be able to distinguish the best perturbative method to be used for the determination of α_S(m_τ^2). The α_S determination is revisited and a very precise result on α_S(m_τ^2) is obtained using CIPT (0.344 +- 0.005 (exp) +-0.007 (th)). When evolved to the Z mass, this value agrees with α_S(m_Ζ^2) directly measured from the Z width. This result is the most precise test of the α_S running in QCD.
Keywords : ISR g-2 alpha_S unfolding
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Submitted on : Tuesday, September 21, 2010 - 11:49:06 AM
Last modification on : Wednesday, September 16, 2020 - 4:20:01 PM
Long-term archiving on: : Wednesday, December 22, 2010 - 2:47:37 AM


  • HAL Id : tel-00519719, version 1




B. Malaescu. Mesures précises de sections efficaces e^+e^− → Hadrons : tests du Modèle Standard et applications en QCD. Physique des Hautes Energies - Expérience [hep-ex]. Université Paris Sud - Paris XI, 2010. Français. ⟨tel-00519719⟩



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