# M2: Meson dynamics

### Goals

Project M2 deals with the structure and dynamics of mesons. The interaction of light mesons such as the pseudoscalar and the vector mesons is constrained by spontaneous chiral symmetry breaking in QCD, the explicit symmetry breaking by the quark masses, and the chiral anomaly. On the other hand, study of
charmonium decays and charmonium-like systems gives access to structure and dynamics beyond conventional quark-antiquark systems.

In this project we will, in particular, focus on:

1. Consistent quantum field-theoretical treatment of the $$VP\gamma^{(\ast)}$$ interaction
We have achieved a phenomenological description of the 12 physical $$VP\gamma$$ transitions $$(\rho^\pm\to\pi^\pm\gamma, \ldots$$) in the so-called vector formulation of the theory. Our final goal is a combined $$1/N_c$$ and quark-mass expansion including the calculation of loop corrections.
2. Analysis of $$\omega$$ decays using photoproduction data from A2@MAMI
As experimental input to the study of $$VP\gamma^{(\ast)}$$, we will measure the transition form factor $$\omega \to \pi^0 e^+ e^-$$ using A2 data. We will finalize the analysis of A2 end-point tagger data taken in 2014 and publish results on $$\eta' \to \omega \gamma$$.
3. Study of charmonium decays to three pseudoscalars
The huge difference in branching fraction and decay dynamics between the processes $$J/\psi \rightarrow \pi^+\pi^-\pi^0$$ and $$\psi^\prime \rightarrow \pi^+\pi^-\pi^0$$ is known as the $$\rho-\pi$$-puzzle. We will study these decay channels and the related decays to $$\pi^+\pi^-\eta$$ in BESIII data using Veneziano fits.
4. "Exotic" charmonium states
After finishing the search for isospin singlet partners of the established exotic isospin triplets in the
decays $$\eta_c \eta$$ and $$J/\psi\ \eta$$, we start to search for a resonance decaying to $$h_c \eta$$. This corresponds to the isospin triplet decaying to $$h_c\pi$$. The analysis and interpretation of the reaction channels $$e^+e^{-} \rightarrow (D\bar{D}^*)^+ \pi^{-}$$ and $$(D \bar{D}^*)^0\pi^{0}$$ will be continued by using the method of partial wave analysis. The goal of this sub-project is to perform a simultaneous fit for both reaction channels separately across the five large statistics data sets available at BESIII. The ultimate goal of this effort (within the whole community) is to perform a global fit of these two reaction channels together with the two channels $$e^+e^- \to J/\psi \pi\pi$$.