Gutenberg Open Science

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  • Item type: Item , DissertationAccess status: Open Access ,
    Study of Neutral-Pion Pair Production in Single-Tagged Two-Photon Collisions at BESIII
    (2026) Lellmann, Max; Denig, Achim
    The anomalous magnetic moment of the muon, $a_\mu=(g-2)_\mu/2$, is one of the most precisely measured quantities in particle physics and a key test of the Standard Model. The current theoretical uncertainty is dominated by hadronic effects. The calculation of the hadronic Light-by-Light scattering contribution -- the coupling of four photons to hadronic states within the muon's interaction with an external magnetic field -- is limited by the knowledge of hadronic two-photon couplings, especially of tensor mesons, at arbitrary photon virtualities, typically described by electromagnetic transition form factors. The hadronic Light-by-Light scattering contribution is the second largest source of uncertainty of the Standard Model prediction of $a_\mu$. In this work, the singly virtual electromagnetic transition form factors of the scalar $f_0(500)$ and $f_0(980)$ as well as the tensor $f_2(1270)$ are measured using the BESIII detector setup. Especially, he $f_0(500)$ transition form factor is measured for the first time and the $f_0(500)$ and $f_2(1270)$ form factors are determined for the first time in the range $Q^2=(0.1-3.0)\,$GeV$^2$, most relevant for the Standard Model prediction of the muon's anomalous magnetic moment. The analysis is based on single-tagged $e^+e^-\to e^+e^-\pi^0\pi^0$ events, where the pions originate from the fusion of two virtual photons, and one lepton remains undetected, leaving one photon quasi on-shell. To make use of the full potential of the data, new Monte Carlo generator for hadron production in two-photon scattering, HadroTOPS, has been developed to model the process and perform efficiency corrections. The generator itself is modular and intended to be used in a number of analyses of two-photon reactions. From the selected events, the effective $\gamma\gamma^\ast\to\pi^0\pi^0$ cross section is measured over the full helicity angle range and the mass range $W=(0.3-2.0)\,$GeV, followed by a partial-wave analysis to extract the transition form factors. Using the same selection, the doubly virtual $\gamma^\ast\gamma^\ast\to\pi^0\pi^0$ cross section in the $f_2(1270)$ region is measured for the first time. The momentum transfer of the second lepton is reconstructed from energy-momentum conservation via a kinematic fit, enabling the determination of the second virtuality. The reduced virtuality of the second photon compared to a standard double-tagged measurement increases the cross section, making this measurement possible in the first place. Together, these analyses provide essential new input for improving the Standard Model prediction of the hadronic Light-by-Light contribution to $a_\mu$.
  • Item type: Item , DissertationAccess status: Open Access ,
  • Item type: Item , DissertationAccess status: Open Access ,