Loop-induced form factors in light-meson physics
Loop-induced form factors in light-meson physics
View/Type of Scholarly Publication
DissertationDate of Exam
27.11.2025Date of Publication
06.02.2026Advisor
Kubis, BastianCo-Referee
Hanhart, ChristophMetadata
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Abstract
The hadronic sector is a part of the Standard Model (SM) of particle physics that is not yet fully understood. Specifically, the energy dependence of the coupling of hadrons to photons, described by form factors, is an active field of research. We investigate two processes where form factors play a crucial role. In the first project, we calculate decay widths and branching ratios of rare semileptonic decays of the η and η' mesons into π0/η and two leptons. These decays proceed via a two-photon intermediate state, which couples to the hadrons via transition form factors (TFFs). Different parameterisations of these TFFs are implemented and their effect on the results is discussed. Thus, we obtain a prediction for the SM contribution with some control over these systematic effects, which can be tested against future experimental results in order to investigate the possibility of beyond-the-SM (BSM) effects. In the second project, we construct a framework to dynamically generate doubly-virtual TFFs of the a1 and a2 mesons, for which not much experimental data exists, from a ρπ intermediate state. For this, we describe the ρπ → γ*γ* system in a gauge-invariant way and, making use of unitarity, reconstruct it from left-hand cuts, taking care to remove kinematic singularities. We connect this system to the TFFs via the imaginary part of a loop diagram, reconstructing the real part dispersively; the convergence of said loop diagram limits our framework. These TFFs are relevant for a precise determination of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon.
Subjects
Hadronenphysik, starke Wechselwirkung, leichte Mesonen, Übergangsformfaktoren, semileptonische Zerfälle, Schleifenberechnung, anomales magnetisches Moment, hadronische Licht-an-Licht-Streuung, seltene Zerfälle, Bardeen-Tung-Tarrach-Prozedere, Dispersionsrelationen, hadron physics, strong interaction, light mesons, transition form factors, semileptonic decays, loop calculation, anomalous magnetic moment, hadronic light-by-light scattering, rare decays, Bardeen-Tung-Tarrach procedure, dispersion relations
Classification (DDC)
530 PhysikRelated Publications
https://pos.sissa.it/479/035https://doi.org/10.1103/PhysRevD.108.074025
Schäfer, Hannah: Loop-induced form factors in light-meson physics. - Bonn, 2026. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-87867
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5-87867
@phdthesis{handle:20.500.11811/13877,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-87867,
doi: https://doi.org/10.48565/bonndoc-779,
author = {{Hannah Schäfer}},
title = {Loop-induced form factors in light-meson physics},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = feb,
note = {The hadronic sector is a part of the Standard Model (SM) of particle physics that is not yet fully understood. Specifically, the energy dependence of the coupling of hadrons to photons, described by form factors, is an active field of research. We investigate two processes where form factors play a crucial role. In the first project, we calculate decay widths and branching ratios of rare semileptonic decays of the η and η' mesons into π0/η and two leptons. These decays proceed via a two-photon intermediate state, which couples to the hadrons via transition form factors (TFFs). Different parameterisations of these TFFs are implemented and their effect on the results is discussed. Thus, we obtain a prediction for the SM contribution with some control over these systematic effects, which can be tested against future experimental results in order to investigate the possibility of beyond-the-SM (BSM) effects. In the second project, we construct a framework to dynamically generate doubly-virtual TFFs of the a1 and a2 mesons, for which not much experimental data exists, from a ρπ intermediate state. For this, we describe the ρπ → γ*γ* system in a gauge-invariant way and, making use of unitarity, reconstruct it from left-hand cuts, taking care to remove kinematic singularities. We connect this system to the TFFs via the imaginary part of a loop diagram, reconstructing the real part dispersively; the convergence of said loop diagram limits our framework. These TFFs are relevant for a precise determination of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon.},
url = {https://hdl.handle.net/20.500.11811/13877}
}
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5-87867,
doi: https://doi.org/10.48565/bonndoc-779,
author = {{Hannah Schäfer}},
title = {Loop-induced form factors in light-meson physics},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2026,
month = feb,
note = {The hadronic sector is a part of the Standard Model (SM) of particle physics that is not yet fully understood. Specifically, the energy dependence of the coupling of hadrons to photons, described by form factors, is an active field of research. We investigate two processes where form factors play a crucial role. In the first project, we calculate decay widths and branching ratios of rare semileptonic decays of the η and η' mesons into π0/η and two leptons. These decays proceed via a two-photon intermediate state, which couples to the hadrons via transition form factors (TFFs). Different parameterisations of these TFFs are implemented and their effect on the results is discussed. Thus, we obtain a prediction for the SM contribution with some control over these systematic effects, which can be tested against future experimental results in order to investigate the possibility of beyond-the-SM (BSM) effects. In the second project, we construct a framework to dynamically generate doubly-virtual TFFs of the a1 and a2 mesons, for which not much experimental data exists, from a ρπ intermediate state. For this, we describe the ρπ → γ*γ* system in a gauge-invariant way and, making use of unitarity, reconstruct it from left-hand cuts, taking care to remove kinematic singularities. We connect this system to the TFFs via the imaginary part of a loop diagram, reconstructing the real part dispersively; the convergence of said loop diagram limits our framework. These TFFs are relevant for a precise determination of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon.},
url = {https://hdl.handle.net/20.500.11811/13877}
}
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