Meson Transition Form Factors (Project TFF)

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The interaction of two real or virtual photons with hadronic matter is described by transition form factors (TFFs). These do not only provide information on the internal structure of the involved hadrons, but are also necessary input to further constrain the uncertainties of the hadronic Light-by-Light contribution to the anomalous magnetic moment of the muon. The aim of Project TFF is to provide the information exploiting synergies of experimental and theoretical efforts. High-precision measurements at the A2/MAMI experiment will provide world-leading information on time-like TFFs from meson Dalitz decays. Complementary information on space-like TFFs will be obtained at the BESIII experiment in two-photon fusion reactions producing single mesons and multi-meson systems. Making use of the information, data-driven phenomenological approaches, which have already been developed based on dispersive analysis for real photons are extended to virtual photons and higher meson multiplicities in order to provide access to the relevant scalar, tensor and axial mesons. In addition, ab-initio calculations on the lattice will provide the information on double-virtual TFFs of single mesons, which is challenging to obtain from experiments. Eventually, the data obtained and tools developed in project TFF can be used to access hadron structure from a different angle by studying pion polarizabilities.