# M1: γγ physics and meson structure

### Goals

Project M1 consists in studying meson structure using  two-photon processes, with two main goals. On the one hand, these studies provide the necessary input to constrain the hadronic light-by-light (HLbL) contribution to the anomalous magnetic moment of the muon $$a_\mu$$. On the other hand, they provide one of the cleanest probes of the structure of mesons, allowing to study the structure of pseudo-scalar, scalar, axial-vector, and tensor mesons through the measurement of their transition form factors (TFFs), the underlying meson distribution amplitudes, the interplay of symmetry breaking mechanisms in the $$\eta,\eta^\prime$$ meson system, or the pion polarizabillities. To access these quantities requires a strong interplay between theoretical and experimental efforts, which are both represented in this project. The experimental part will focus in detail on:

• A major new program of  measurements of photon-photon fusion processesat BESIII, with one spacelike virtual photon, through single tagged events.
• An exploratory study of the doubly virtual TFF $$\gamma^\ast \gamma^\ast \to \pi^0$$ BESIII.
• The measurement of timelike meson FFs in $$e^+e^-$$ annihilation at BESIII.
• The measurement of timelike meson FFs through Dalitz decays of $$\pi^0$$, $$\eta$$ and $$\eta^\prime$$ mesons measured at A2/MAMI.

The experimental results  to be expected from this project will provide the first precision measurements of TFFs below a momentum transfer of approximately 2 GeV$$^2$$. In the case of the multi-body final states, and in the case of doubly virtual TFFs, no previous measurements were existing at all.

The theoretical part will focus on sum rules and dispersion techniques. These techniques are a key ingredient in interpreting meson TFFs and the production of multi-meson states from photon-photon fusion processes or meson Dalitz decays. They will allow to estimate the HLbL contribution to $$a_\mu$$ within a data-driven approach in order to reduce its model dependence. The theoretical part will focus in detail on:

• Dispersive analyses of $$\gamma^\ast \gamma^\ast \to \pi^+ \pi^-, \pi^0 \pi^0$$ and explorative theoretical studies of $$\gamma^\ast \gamma \to \pi^0 \eta, \eta \eta, K^+ K^-, K^0 \bar K^0$$ processes, study of pion generalized polarizability.
• Dispersive analyses of HLBL contributions to $$\mathbf{a_\mu}$$.
• Doubly virtual meson TFFs and the $$\mathbf{P \to e^+e^-}$$ decay.
• Light-by-light sum rules and its application to charmonium states.