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K+ photoproduction at SPring-8/LEPS

A series of experiments have been carried out by using a linearly polarized photon beam at the SPring-8/LEPS facility from December 2000 to June 2001. The photon beam asymmetries and differential cross sections of the p(gamma,K+)Lambda and p(gamma,K+)Sigma^0 reactions have been measured in the photon energy range from 1.5 GeV to 2.4 GeV at forward angles, 0 deg.<Theta_cm<60 deg.. We report preliminary results of the photon beam asymmetries.


INTRODUCTION
Experimental information on nucleon resonances, N * and ∆ * , has mainly come from studies of the πN and N(γ,π) reactions. However, considerably large number of nucleon resonances predicted by quark models have not been observed in the pionic reactions. Quark model studies suggest that a part of these missing resonances may couple to strangeness channels, such as KΛ and KΣ channels [1]. The p(γ,K + )Λ and p(γ,K + )Σ 0 reactions give good means to search for such nucleon resonances. In fact, indications of new resonance contributions were seen around E γ = 1.5 GeV (W = 1.9 GeV in totall energy) in the cross section data of the p(γ,K + )Λ reaction measured by the SAPHIR [2] and CLAS [3] collaborations. To reproduce the experimental data, missing resonances like D 13 (1900), were included in several theoretical calculations [4,5]. However, there still remains a controversy in the theoretical description of the cross sections [5,6], because there are ambiguities of meson-hadron couplings and form factors at hadronic vertices together with the ambiguity of nucleon resonance amplitudes. Therefore, additional observables are necessary to examine the theoretical models. The photon beam asymmetry is one of good candidates for the studies because the observable is quite sensitive to model differences.
The contribution of the t-channel meson exchange is expected to become large at forward angles above the resonance region (E γ > 2 GeV). Mesons exchanged in the kaon photoproduction are K, K * , K 1 , etc. The dominance of the unnatural parity exchange (K and K 1 ) leads to the photon beam asymmetry equal to −1 while the natural parity exchange (K * ) leads to the photon beam asymmetry equal to +1 [7,8]. Therefore, we may provide information on the t-channel meson exchange by measuring the photon beam asymmetries. Since the LEPS spectrometer covers forward angles, the experimental data in the kinematical regions which cannot be accessed at other facilities can be studied with good accuracy and statistics.

EXPERIMENTAL PROCEDURE
At the SPring-8/LEPS facility, linearly polarized photons are produced by the backward-Compton-scattering process of laser photons on the 8-GeV circulating electrons in the storage ring [9]. In the present measurement, the maximum energy of the produced photons was 2.4 GeV since an Ar laser (351 nm) was used. The photon energy ranged from 1.5 GeV to 2.4 GeV was measured by the tagging counter with the energy resolution of 15 MeV (RMS). The polarization of the laser photons was typically 98%, and the polarization of the produced photons was about 92% at the maximum photon energy. A half of data was taken by the vertically polarized photons and another half was taken by the horizontally polarized photons to measure the photon beam asymmetries. The target was liquid hydrogen with a thickness of 5 cm. The LEPS magnetic spectrometer has been used to identify charged particles by the momentum and time-of-flight measurement. Figure 1 shows the mass spectrum measured by the LEPS spectrometer with the liquid hydrogen target. Protons, kaons and pions are identified. A missing mass was calculated after selecting the K + events. Figure 2 shows the missing mass spectrum of the p(γ,K + )X reaction. One can see clearly separated Λ(1116) and Σ 0 (1193) samples, and hyperon resonances, Σ(1385)/Λ(1405) and Λ(1520). About 75,000 K + Λ and 50,000 K + Σ 0 events were detected in the photon energy range 1.5 GeV < E γ < 2.4 GeV and the angular range, 0 • < Θ K + cm < 60 • .

RESULTS OF PHOTON BEAM ASYMMETRIES
By making the vertically and horizontally polarized photons, two sets of data were accumulated to measure the photon beam asymmetries. The relation between the production yields in the two data sets and the photon beam asymmetry (Σ) is given as follows: where N v (N h ) is the K + photoproduction yield with the vertically (horizontally) polarized photons, φ K + is the K + azimuthal angle (angle between the reaction plane and the horizontal plane) and P is the polarization of the photons. The φ K + dependence of the ratio was fitted by the function cos(2φ K + ). The amplitude ΣP was estimated by the fitting, and the degree of polarization P was obtained by using the photon energy E γ . Figures 3 and 4 show preliminary results of the photon beam asymmetries as a function of cosΘ K + cm for the p( γ,K + )Λ and p( γ,K + )Σ 0 reactions. Only statistical errors are included in the error bars in the data plots. The signs of the beam asymmetries of both reactions are positive in the measured kinematical region. The values of our preliminary asymmetry data for the p( γ,K + )Λ reaction increase as the increase of the photon energy. The photon asymmetry slightly drops at larger angles above E γ = 1.7 GeV. The photon asymmetry of the p( γ,K + )Σ 0 reaction also increases as the photon energy increases. The angular distributions are flat over all the photon energy range.

SUMMARY
In the previous experiments, the cross section data of the p(γ,K + )Λ reaction have been discussed in connection with missing resonances, but the experimental data are not conclusive to confirm the resonances. We measured the photon beam asymmetries for the p( γ,K + )Λ and p( γ,K + )Σ 0 reactions in the photon energy range 1.5 GeV < E γ < 2.4 GeV and the angular range 0 • < Θ K + cm < 60 • . Theoretical models can be examined and improved by including our photon beam asymmetry data. Current data will extend our knowledge of the K + photoproduction mechanism including the effect of nucleon resonances.