The BGOOD experiment at ELSA Exotic structures in the strange quark sector?

. The BGOOD photoproduction experiment accesses forward meson angles and low momentum exchange kinematics in the uds sector, which may be sensitive to molecular-like hadronic structure. Recent highlights are summarised in these proceedings

Abstract. The BGOOD photoproduction experiment accesses forward meson angles and low momentum exchange kinematics in the uds sector, which may be sensitive to molecular-like hadronic structure. Recent highlights are summarised in these proceedings.
The recent discoveries of the pentaquark, P C , states [1,2] and XYZ mesons in the charmed quark sector [3,4] have initiated a new epoch in hadron physics, where the existence of exotic multi-quark states beyond the conventional three and two quark systems has been realised. Such states could manifest as colour-singlet objects, or evolve from meson-baryon and meson-meson interactions, creating molecular like systems and re-scattering effects near production thresholds. Equivalent structures may be evidenced in the light, uds sector. This includes, for example the Λ(1405) which is now considered the archetypal molecular state in the strangeness sector [5]. A cusp-like structure was also recently observed in K 0 Σ + photoproduction at the K * Σ threshold [6] which was suggested to derive from a vector mesonbaryon dynamically generated state, the N * (2030) [7]. This was considered as the strangeness counterpart to the P C states, which were predicted by an equivalent model [8].
To experimentally study any potentially loosely bound molecular hadronic structure, access to a low momentum exchange and forward meson production region is mandatory. The BGOOD experiment [9] at the ELSA electron accelerator facility [10] is ideally suited for such photoproduction measurements. A 3 GeV electron beam impinges upon a thin radiator to produce an energy tagged bremsstrahlung photon beam which is subsequently incident  Figure 1. Differential cross section of γn → K 0 Σ 0 as a function of W for two intervals in cos θ K CM and two different fitting methods (red triangles, and black circles). Data from the A2 Collaboration and calculations from KMAID and BnGa are labelled in the legend. The predicted total cross section from Ramos and Oset [7] is included in the angular bin 0.2 <cos θ K CM < 0.5 (red solid line) at an arbitrary scale denoted by the red axis on the right. Figure (adapted) and references in Ref. [11].
upon the target at the centre of the BGOOD experiment. BGOOD is comprised of two main parts, a central calorimeter region ideal for the reconstruction of neutral mesons via their decays, and a forward spectrometer used for charged particle identification and momentum reconstruction at forward angles.
BGOOD has an extensive strangeness photoproduction physics programme, recent of results of which are highlighted below. Due to space constraints, previous data or calculations that are not cited above are referenced in the corresponding BGOOD publication.
Measurement of the γn → K 0 Σ 0 differential cross section over the K * threshold [11] The calculation of Ramos and Oset, which suggested the dynamically generated N * (2030) amplified the cusp in the K 0 Σ + channel, predicted constructive interference in K 0 Σ 0 photoproduction resulting in a peak [7]. Observing this experimentally would therefore be direct evidence of such a molecular state in the uds sector.
Using a deuterium target, the reaction γn(p) → K 0 Σ 0 was identified via the decays K 0 π 0 π 0 and Σ 0 → γ(Λ → pπ − ). After numerous selection criteria, the π 0 π 0 invariant mass was fitted to separate signal and background events from other channels, and reactions off the proton were subtracted by an equivalent analysis using a hydrogen target.
Two intervals in the cosine centre-of-mass polar angle of the kaon, cos θ K CM are shown in fig. 1. The data are in reasonable agreement with the previous data from the A2 collaboration, calculations from the KaonMAID isobar model and the Bonn Gatchina partial wave analysis (BnGa). In the more forward interval, a peak is observed which is consistent with the predicted peak from the model of Ramos and Oset. Further data has now been taken to improve the statistical precision and enable a firm interpretation. Photoproduction of K + Λ(1405) → K + π 0 Σ 0 extending to forward angles and low momentum transfer [12] The BGOOD setup enables a clean identification of the decay Λ(1405) → π 0 Σ 0 over a cos θ K CM range enabling access to previously unattainable low momentum transfer regions. The reaction channel was identified by identifying all final state particles and using a kinematic fit. Subsequent fitting to two-dimensional Σ 0 π 0 and γΛ invariant mass spectra enabled a clean separation of signal to background. Fig. 2 shows the cross section versus photon beam energy, E γ , integrated over all cos θ K CM , with good agreement to previous CLAS data however much improved E γ resolution. The purple line is a calculation by Wang et al., of a triangle singularity being driven by the N * (2030) resonance. The excellent agreement supports the N * (2030) as being a molecular K * Σ system. K + Λ and K + Σ 0 photoproduction at forward angles and low momentum transfer [13,14] Forward K + Λ photoproduction provides low-t data which is an important constraint on the electroproduction of hypernuclei, where the Q 2 is required to be small. It is also particularly sensitive to intermediate high spin N * . Fig. 3 shows the differential cross section data for cos θ K CM > 0.9. The statistical precision is markedly improved and enables a discrimination between previous conflicting datasets. The K + Σ 0 differential cross section for cos θ K CM > 0.9 is shown in fig. 4. Similarly for K + Λ, the statistical precision of the data is able to discriminate between differences in previous datasets. A cusp-like structure is resolved close to the pK + K − threshold at W ∼ 1900 MeV. Fig. 5 shows the data extrapolated to minimum momentum transfer, t min and cos θ K CM = 1, where the cross section drops by 75 %. No firm interpretations have been made, but it is interesting to note the proximity of multiple thresholds and predicted bound states  Figure 4. γp → K + Σ 0 differential cross section for cos θ K CM > 0.90 (black circles). The systematic uncertainties on the abscissa are in three components, where the grey bars are the total. Previous data (only including statistical errors) is indicated in the legend. The CLAS data are at the more backward angle of 0.85 < cos θ K CM < 0.95. The Bonn-Gatchina PWA solutions with and without the inclusion of the new data are the solid magenta and dashed cyan lines respectively. The pK + K − threshold is indicated by the dashed black line. Figure and references in Ref. [14].