Studies of unconventional baryon structure in the light quark sector with the BGOOD photoproduction experiment

Abstract

The discoveries of the pentaquark states and $XYZ$ mesons in the charm quark sector initiated a new epoch in hadron physics, where the existence of exotic multi-quark states beyond conventional valence three quark and quark–antiquark systems has been unambiguously confirmed. Such states could manifest as single colour bound objects, or evolve from meson–baryon and meson–meson interactions, creating molecular like systems and re-scattering effects near production thresholds. Molecular-like structures may be apparent over the full quark flavour and mass range, with equivalent states evidenced in the light, $uds$ quark sector. This is the focus of the BGOOD photoproduction experiment at the ELSA electron accelerator at the University of Bonn. The combination of a central electromagnetic calorimeter and forward charged particle spectrometer permits access to low momentum exchange kinematics and corresponding forward meson production angles, which is crucial to study spatially extended, molecular-like structure which may manifest in reaction mechanisms.

The reviewed publications span two areas of research connected via the kinematics associated with molecular-like hadron structure. The first is in the strangeness sector where meson–baryon dynamics may play prominent roles. Forward angle differential cross section measurements from threshold for $K^{+}\Lambda$, $K^{+}{\Sigma }^0$, $K^{+}\Sigma {\left(1385\right)}^0$, $K^{+}\Lambda \left(1405\right)$ and $K^{+}\Lambda \left(1520\right)$ indicate an equivalence to the $P_C$ states observed at the $D{\Sigma }_C$, $D{\Sigma }_C^{\ast }$ and $D^{\ast }{\Sigma }_C$ thresholds. The second area of research is in the non-strange baryon–baryon sector, where coherent meson photoproduction off the deuteron enables access to proposed dibaryon states, including the recently discovered $d^{\ast }\left(2380\right)$. The forward angle acceptance of deuterons enables differential cross section measurements that challenge conventional descriptions of coherent photoproduction, which should be suppressed due to the large momentum transfer.