(Juengst)

Strangeness production has been studied in the reactions gp ® KY to the hyperon ground states, L(1116) and S(1193), for almost 40 years. In some theoretical models [Ade90] the exchange of a virtual hyperon resonance in the u-channel has been included in order to reproduce the measured cross sections and polarizations for the production of the hyperon ground states. Various hyperon resonances, with their corresponding coupling constants, have been considered as candidates for the propagator in the u-channel for hyperon ground-state production. However, the actual contribution of the hyperon resonance in the u-channel is only poorly known [Jue95], and some models do not include any hyperon resonance [Lev73] [Mar00].

In our approach, actual photoproduction data for hyperon resonances on hydrogen is used to determine the contributions of the hyperon resonances in the u-channel to gp ® KY by simultaneously fitting the data for gp ® KY and gp ® KY^*. The inclusion of the measured hyperon-resonance data constrains the coupling constants for the u-channel in the reaction gp ® KY. In particular, we analyze data from the CLAS for photoproduction on hydrogen with an emphasis on the extraction of hyperon-resonance data, gp ® KY^*, at photon energies between the respective thresholds and 3.1 GeV. Our goal is to obtain as complete a set of hyperon resonance data as possible for use in a new theoretical analysis of the mechanism for photoproduction of hyperons and hyperon resonances on the proton.

The cross sections and polarizations for the reactions gp ® KY can be compared with the predictions from isobar models [Ade90]. The s-, t-, and u-channel Feynman diagrams can be grouped into Born and resonance-exchange poles. Every vertex within a diagram is parametrized by a coupling constant. The kaon-hyperon-nucleon coupling constants g_KYN and g_KY*_N are of particular interest. Typically they are obtained from a fit to the cross section and polarizations of gp ® KY and the resulting c² is used to select the best-fitting hyperon resonance candidate. By using the data from the reaction gp ® KY^* as a constraint, we hope to improve the significance of the fit.

Figure 12 shows the missing-mass spectrum for a measured K⁺. In addition to the L(1116) and S(1193), the unresolved resonances S^o(1385) and L(1405) are seen, as well as the L(1520). It is clear that the signal-to-background ratio needs to be improved. In addition, a method to separate the S^o(1385) and L(1405) resonances has to be developed. The shaded areas in this and the following figures indicate the energy regions corresponding to the full width at half maximum of the resonances according to the compilation of the Particle Data Group [Gro00].

Figure 12. Missing-mass spectrum m_X for gp ® K⁺X.

We next require that we detect a proton in addition to the K⁺. This allows us to focus on the reaction gp ® K⁺L(1520) because the L(1520) can decay into pK^- (branching fraction 45%). The corresponding spectrum is shown in Fig. 13 (left). The K^- can be treated as a missing particle or, if available, can be included in the analysis. Figure 13 (center) shows the spectrum obtained by requiring the detection of a K⁺ and the K^- and proton in the decay L(1520) ® pK^-. The signal-to-background ratio and the resolution in the invariant-mass spectrum are significantly improved in comparison with the missing-mass spectrum. Figure 13 (right) shows the result for the decay channel L(1520) ® nK^o, followed by K^o ® p⁺p^-. The K^o is identified by cutting on the invariant mass of the decay pions in the K^o mass region. The detection of pions from the decay of the neutral kaon serves as an additional criterion for the event identification.

Shown in Fig. 14 are the spectra that were extracted with the requirement of the detection of the decay Y^* ® S⁺p^- followed by S⁺ ® np⁺ (left) and the detection of the decay Y^* ® S^-p⁺ followed by S^- ® np^- (right). The S± decay neutron was missing, while the 4-momentum of the decay p± was measured. The events in these spectra are primarily from the photoproduction of the Y^* resonances L(1520), S(1385), and L(1405) in the gp ® Y^*K⁺ reaction.

Figure 13. Left: missing-mass spectrum m_X for gp ® K⁺X with identified proton and missing K^- from L(1520) ® pK^-. Center: invariant-mass spectrum m_(p,K-₎ for gp ® K⁺L(1520), detecting p, K⁺, and K^-. Right: missing-mass spectrum m_X for gp ® K⁺X, with identified K^o and missing neutron from L(1520) ®  nK^o, with the K^o identified via a cut on the invariant mass of decay pions (K^o ® p⁺p^-).

Figure 14. Missing-mass spectra m_X for gp ® K⁺Y^* with detection of the decay Y^{* ®} S⁺p^- followed by S⁺ ® np⁺ decay (left) and with detection of the decay Y^* ® S^-p⁺ followed by S^- ® np^- (right).

The decay channel S^o(1385) ® Lp^o is important for the separation of the reactions gp ®  K⁺S^o(1385) and gp ® K⁺L(1405) (see Fig. 15). The decay channel Lp^o is prohibited for L(1405) because of isospin conservation. We will use this decay channel to determine the cross sections for gp ® K⁺S^o(1385) first and then subtract them from the combined cross sections for the decay channel S^o(1385)/L(1405) ® S^op^o in order to extract the cross sections for gp ®  K⁺L(1405). These results have been presented at the 2001 International Nuclear Physics Conference [Jue01].

Figure 15. Missing-mass spectrum m_X for gp ® K⁺S^o(1385) with S^o(1385) ® Lp^o.

We also searched for an exotic state called Z⁺⁺ (or Q⁺⁺), which is supposed to be a bound state of a proton and a K⁺. In the reaction gp ® pK⁺K^- we found a peak at 1.58 GeV for the invariant mass of the proton and K⁺ as well as for the missing mass of the K^-. However, after removing all possible reflections from the reactions gp ® pf (f ® K⁺K^-) and gp ® K⁺L(1520) (L(1520) ® pK^-), there was no signal of the Z⁺⁺ remaining.

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Last modified: 03/31/05