# M2: Meson dynamics

### Goals

The strong-interaction part of the Standard Model is described by an SU(3) gauge theory - Quantum Chromodynamics (QCD) - in terms of quarks and gluons as the fundamental dynamical degrees of freedom. However, experimentally only colour-neutral combinations, namely, mesons and baryons, are observed as the asymptotic states of the theory. Unraveling the structure and dynamics of mesons is still one of the fascinating challenges of the strong interactions. In the low-energy regime, the interaction of pseudoscalar mesons (π, K, η) among themselves, with vector mesons, photons, and baryons is constrained by spontaneous chiral symmetry breaking in QCD in combination with explicit symmetry breaking by the quark masses and their electric charges. On the other hand, the study of "exotic" heavy states in charmonium and charm mesons allows one to address structure and dynamics beyond the interpretation as conventional quark-antiquark systems. In this project we will, in particular, focus on:

• Hadronic η, ω and η' decays
The neutral decays η' → π0π0η  and η' → π0π0π0 will be measured with the Crystal Ball detector using η' mesons photoproduced at the new endpoint tagging facility at MAMI. These decays will provide strong experimental constraints on the influence of quark-mass differences and rescattering effects in the decay amplitudes. We will also study $\omega$ decays to final states including real or virtual photons.
• Anomalous processes in chiral effective field theory including vector mesons
At leading order in chiral effective field theory, anomalous processes such as η → γγ, η → π+π-γ are driven by the Wess-Zumino-Witten effective action. We will study the influence of vector-meson degrees of freedom on higher-order corrections. To that end, we will combine a Dirac constraint analysis with a renormalizability analysis to constrain the possible vector-meson interactions. We will investigate various vector-meson approaches with respect to their VPγ interactions. We plan to develop a power-counting scheme which allows for the implementation of unstable particles (complex-mass scheme).
• Light hadron dynamics in charmonium decays
A simultaneous amplitude analysis of the decays J/ψ and ψ' → π+π-π0 will provide insights into the source of the large and unexpected differences in their branching fractions and decay dynamics and allows the study of rescattering effects.
• "Exotic" charmonium states
A long standing question in hadron spectroscopy is experimental evidence for mesons which fall outside of the conventional quark-antiquark system. The recent discoveries of charmonium-like states which cannot be explained by potential models and which often have exotic quantum numbers (`XYZ states') have been interpreted as hybrid mesons, meson molecules (loosely bound states), and four-quark states. Combined fits of the large data samples available at BESIII as well as searches for isospin singlet states will help to shed light on these questions.
• "Exotic" charm states
We plan to search for new states containing a charm quark in the recoil spectra of charmed mesons with BESIII data in order to investigate whether there are states analogous to the XYZ particles. The first surprising discovery was the Ds0(2317)+ (seen in 2003 by BABAR), which is lighter and more narrow than expected from a quark model calculation. LHCb has recently also observed higher excited states with open charm.