Research Topics
We investigate different reactions in meson photoproduction at the BGOOD experiment, especially strangeness photoproduction and coherent production on the deuteron. We focus on search and investigation of unconventional baryon and meson states, unconventional here means a quark configuration of more than 3 quarks in case of baryons and more than a quark-antiquark pair in case of mesons. Such states have been observed in c- and b-quark sectors, but there are interesting issues and similarities to the s-quark sector and possible relations to results in coherent production on the deuteron (di-baryon).
A brief overview on our research is given below. More details can be found on the respective group webpages.
AG Hartmut Schmieden, University of Bonn, Physikalisches Institut
Strangeness Photoproduction
Several states have been discovered in c- and b-quark sectors, which are assigned unconventional structure, among them the tetraquark X(3872) and possible pentaquark Pc(4380/4450) states. While the existence of these states is undisputed, their internal strucutre remains unclear. A possible explaination would be a molecular-like structure of two meson (DD*) in case of X(3872) and a meson and a baryon (D*Σc/DΛ*c) in case of Pc(4380/4450).
If the emergence of such hadronic structures where a common feature of QCD, similar states may be expected in the strange quark sector as well. Respective states would then be KK* tetraquark and K*Σ/KΛ* pentaquark states.
We investigate these unconventional states in the s-quark sector, results show many unexpected issues, which might support existance of unconventional states also in the lighter s-quark sector.
Coherent photoproduction off the deuteron
Until recently, the deuteron was considered the only bound dibaryon system. However, as early as 1964 Dyson and Xuong predicted a sextet of dibaryon states from SU(6) symmetry for baryons. In addition to two deuteron like ground states, four further non-strange states were predicted, the masses of which were determined from the deuteron mass and nucleon scattering data. These calculations led to a plethora of searches for dibaryon states throughout the 1960s and 70s, focussing mainly on isovector dibaryon candidates with mixed interpretations.
The presumed discovery of the d*(2380) hexaquark, identified in the deuteron fusion reaction, has sparked renewed interest in dibaryon searches in the non-strange, isoscalar sector. The photoproduction of the d*(2380) (and other dibaryon candidates) enables us to determine physical parameters. The existence of such hexaquark configurations may have profound implications for our understanding of astrophysical phenomena. It has been suggested for example, that a significant fraction of baryons within neutron star interiors may exist as d*(2380), which would affect the neutron star equation of state and cooling mechanisms.
Coherent photoproduction off the deuteron enables access to dibaryon candidates, with a judicious selection of final state mesons acting as an isospin filter. BGOOD is an ideal experiment for such measurements: the Forward Spectrometer enables a clean selection of deuterons and the BGO Rugby Ball identifies neutral mesons via their electromagnetic decay.