Measurement of the e+ e- → pi+ pi- 2pi0 and e+ e- → pi+ pi- 3pi0 cross sections using initial state radiation at BaBar
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Abstract
The muon anomalous magnetic moment g_μ-2 is one of the most precisely measured quantities in particle physics. Yet, its measured value deviates from the prediction by the Standard Model of Particle Physics by approximately three standard deviations.
The cross section of the process e+ e- → π+ π- 2π0 is one of the main contributors to the uncertainty of the Standard Model prediction of g_μ-2. Therefore an improved understanding of this process is fundamental to gaining closer insight into the g_μ-2 puzzle.
The cross section e+ e- → π+ π- 3π0 has never been measured accurately, yielding another uncertainty of g_μ-2. This also hinders the analysis of e+ e- → π+π-2π0 to which it presents a background.
This thesis closes both gaps by analyzing the channels e+ e- → π+ π- 2π0 and e+ e- → π+ π- 3π0.
The analyses are performed on data taken at the BaBar experiment, which operated at SLAC National Accelerator Laboratory between 1999 and 2008. It gathered a total integrated luminosity of approximately 500 fb^-1 at center-of-mass energies around 10.58 GeV, the rest mass of the ϒ(4S) resonance.
This data is used in the present thesis work via the initial state radiation technique, which enables cross section measurements over a continuous energy range.
In comparison to energy scan experiments, this results in small and consistent systematic uncertainties over the full range. Due to the extremely high luminosity, ISR processes, even though suppressed by the fine-structure constant α, are produced in large numbers, leading to small statistical uncertainties.
This effects the opportunity of measuring the aforementioned cross sections with unprecedented accuracy. At the inception of this thesis, the goal was a systematic accuracy of less than 5% for the analysis of the process e+ e- → π+ π- 2π0 in its peak region. The final result achieves 3.1% accuracy, considerably exceeding the original goal. The analysis of the channel e+ e- → π+ π- 3π0 was not planned at the beginning, and now 25 to 32% accuracy have been reached. In both cases the contribution to g_μ-2 is evaluated as well as the effect on the running of the fine-structure constant Δα. Furthermore, their intermediate resonance structure is studied, yielding among other results the previously unmeasured branching fractions for the processes J/ψ → π+ π- 2π0 and J/ψ → π+ π- 3π0.