K⁺Λ photoproduction at forward angle

see also: K⁺Λ photoproduction at forward angle, S. Alef et al.

K⁺Λ photoproduction has played a central role in the quest for solving the missing resonance problem. This is partly due to the self analysing weak decay of the Λ, which allows relatively easy access to recoil baryon single and double polarisation observables. A judicious choice of polarisation observables then enables a complete measurement with respect to unambiguously extracting reaction amplitudes as a basis for partial wave analyses. Despite a wealth of high statistics measurements at several laboratories worldwide [1-7], a consistent description by theory and phenomenological models has not been achieved. On the experimental side the reason is twofold. Firstly, there is a dearth of data at extreme, in particular very forward kaon angles. This kinematic regime is important to determine for known strong t-channel and also potential high-spin s-channel contributions. Secondly, particularly at forward angles, the different data sets exhibit inconsistencies beyond their statistical accuracy.

The BGOOD experiment at ELSA is ideally suited to pursue such measurements.
Setup, performance and basic analysis procedures are described in detail in ref. [8]. It is basically comprised of a combination of forward magnetic spectrometer and central electromagnetic calorimeter complemented by detectors for charged particle tracking and identification. The experiment uses a tagged bremsstrahlung photon beam of energies up to 3.2 GeV.
Forward particles are identified in the spectrometer from momentum and β. Fig. 1 (left panel) shows the reconstructed mass distribution m = p/(γβ) in the range of the K⁺ mass for two example momentum intervals. The low energy shoulder in the real data is from misidentified π⁺, and positrons from pair production in the beam. The K⁺ missing mass obtained after K⁺ identification is shown in Fig. 1 (right panel). The
K⁺Λ channel is enhanced compared to K⁺Σ⁰ via the identification of the decay Λ →π⁰n. The π⁺ and e⁺ background is well described by an equivalent analysis for negatively charged particles, where π⁻ and e⁻ have the same distribution.

Fig. 1: Left part: Mass reconstruction for K⁺ candidates in the forward spectrometer for real and simulated data (red and blue lines, respectively).
Right part: Missing mass from forward K⁺ candidates for four photon beam energies labelled inset.
Our data is shown by black points, with fitted spectra from simulated K⁺Λ and K+Σ⁰, and e⁺/π⁺background (red, green and cyan lines, respectively)

From the fitted K⁺Λ yields, the differential cross sections are determined after appropriate absolute normalisations. The result for forward going K⁺ with cos θ^K_cm > 0.9 is shown in Fig. 4 (left) in comparison to previous measurements. Our data represents the highest statistical accuracy to date and resolves the inconsistency between the previous data sets. Our excellent statistics and angular resolution allow the splitting of the forward region into five bins as shown in Fig. 4 (right). Additionally, the Λ recoil polarisation has been determined for the first time in this most forward angle interval (not shown here).

Fig. 1: Left panel: γp →K⁺Λ differential cross section for cos θ^K_cm > 0.9 (black points). Error bars are purely statistical. Systematic uncertainties are indicated on the abscissa: scaling (shaded blue), point-to-point (red) and total (grey). Previous data shown are from CLAS (McCracken et al. [4], blue squares, and Bradford et al. [3], red triangles) and SAPHIR (Glander et al. [1], green circles). The lines represent the Bonn-Gatchina PWA solutions with and without the inclusion of our new data (magnenta and cyan), the Regge-plus-resonance model [9] (red), and isobar models BS1 (green) and BS3 (blue) of Bydzovsky and Skoupil [10,11].

Right panel: Cross section split into intervals of 0.02 in cos θ^K_cm . Curves as in left panel.

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[9] P. Bydzovsky and D. Skoupil, Phys. Rev. C 100 (2019) 035202
[10] P. Bydzovsky and D. Skoupil, Phys. Rev. C 93 (2016) 025204
[11] P. Bydzovsky and D. Skoupil, Phys. Rev. C 97 (2018) 025202