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).
This project is funded by Deutsche Forschungsgemeinschaft (project numbers 388979758 and 405882627) and by the European Union's Horizon research and innovation programme (824093).
Several unconventional baryon and meson states are know in c- and b-quark sector, 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).
Such structures are expandable to the s-quark sector by simply replacing the c-quarks by s-quarks. Respective states would then be KK* tetraquark and K*Σ/KΛ* pentaquark states.
We investigate these unconventional states in the s-quark sector, first results support their existance also in the lighter s-quark sector.
K⁺Λ(1405) photoproduction is investigated at the BGOOD experiment via the neutral decay of the Λ(1405) to π⁰Σ⁰. The unique setup of the experiment allows the measurement of the cross section as well as the invariant mass distribution (line shape) over a large polar angle range of the K⁺ and extends the measurement to extreme forward K⁺ angles.
Evidence is provided for the significant contribution of a triangle singularity, driven by the N*(2030) resonance, to the photoproduction of the reaction
The differential cross section of the quasi-free photoproduction of K⁰Σ⁰ is measured at the BGOOD experiment from threshold to a center-of-mass energy of 2400 MeV and provides the first measurement of this reaction over the K* threshold. In K⁰ forward direction an increase of the cross section is visible above 2000 MeV.
A conclusive description is difficult within the available statistics, however, the structure is consistent with predictions of a model including a dynamically generated vector meson-baryon resonance. An equivalent model predicted the Pc pentaquark states at LHCb, an observation of the predicted structure in K⁰Σ⁰ photoproduction would be a strong indication for parallels between strange and charm quark sector.
The differential cross section and recoil polarisation data of the reaction γp→K⁺Λ can be measured at the BGOOD experiment using the very good angular and momentum resolution in the Forward Spectrometer.
The high acceptance and resolution in forward direction allows the measurement of the cross section in regions of minimal momentum transfer.
The differential cross section of K⁺Σ⁰ photoproduction was measured at the BGOOD experiment at extreme forward K⁺ angles. Above a center-of-mass energy of 1900 MeV the cross section shows a rapid drop and a strong dependence on the K⁺ polar angle, which becomes visible thanks to the high angular resolution of the forward spectrometer.
The observed structure is in direct proximity to several open and hidden strangeness thresholds, and is consistent to meson-baryon threshold effects that may contribute to the reaction mechanism.
Publications
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. With the addition of the ground state deuteron, four 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 discovery of the d*(2380) hexaquark, first identified in the deuteron fusion reaction have 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, such as the magnetic moment and size. This may have profound effects on 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 effect 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.
The cross section of the coherent reaction γd→π⁰π⁰d was measured at the BGOOD experiment from threshold to a center-of-mass energy of 2850 MeV. The unique setup of the experiment allows the complete reconstruction of the reaction by detecting the deuteron in the forward spectrometer and the pions in the central calorimeter.
The measured cross section is significantly larger then expected from coherent photoproduction, but supports other observations of isoscalar di-baryon candidates.
Publications
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