Measurement of differential cross sections and Cx and Cz for γp→ K+Λ and γp →K+Σ° using CLAS at Jefferson Lab

Abstract

This work presents several observables for the reactions γ p → K+Λ and γ p → K+Σ°. In addition to measuring differential cross sections, we have made first measurements of the double polarization observables Cx and Cz. Cx and C z characterize the transfer of polarization from the incident photon to the produced hyperons. Data were obtained at Jefferson Lab using a circularly polarized photon beam at endpoint energies of 2.4, 2.9, and 3.1 GeV. Events were detected with the CLAS spectrometer. In the Λ channel, the cross sections support the recent observation of new resonant structure at W = 1900 MeV. Studies of the invariant cross section, dsdd show scaling behavior suggesting that the production mechanism becomes t-channel dominated near threshold at forward kaon angles. The double polarization observables show that the recoiling Λ is almost maximally polarized along the direction of the incident photon from mid to forward kaon angles. While Σo differential cross sections are of the same magnitude as the Λ differential cross sections, there is evidence of different physics dominating the production mechanism. The Σ° invariant cross sections do not show the same t-scaling behavior present in the Λ results. The double polarization observables indicate that the Σ° is not polarized as strongly as the Λ. They also fail to identify one preferred polarization axis. Complete interpretation of these results will rely on model calculations. Currently available isobar models obtain varying degrees of success while attempting to predict the double polarization observables. While the models are in better agreement with the differential cross sections, discrepancies with our cross sections indicate that they must be re-optimized in light of the new data. While the data suggest that a Regge model would provide a good description of K+Λ over most of the range in W, the available Regge calculation overstates the strength of the cross section and does not predict Cx and Cz correctly.