(Benmouna, Niculescu, Berman)

As enumerated in Section A6 above, the quasi-free kaon-photoproduction reaction in nuclei is governed by three main mechanisms:

• The elementary amplitudes of the six kaon production reactions possible on the nucleon (gp ® K⁺L, gp ® K⁺S^o, gp ® K^oS⁺, gn ® K⁺S^-, gn ® K^oL, gn ® K^oS^o);
• The Fermi motion of the proton and neutron inside the nucleus;
• The interaction between the final-state hadrons.

Experimental information exists for the first three exclusive kaon-photoproduction channels [Tra98, Ben98], together with our recent results for the gn ® K⁺S^- reaction on the deuteron [Nic01]. Until recently, information on the hyperon-nucleon interaction was available only through hadronic (K,p) reactions. Electromagnetic production of strangeness is complementary to hadron-induced reactions. (g,K⁺) reactions allow the study of spin observables, as they tend to favor the excitation of spin-flip states [Dov91]. Natural- as well as unnatural-parity states can be probed with photons.

In a quasi-free reaction, the interaction is considered to take place on a single nucleon, thus making this type of reaction a good tool to study the propagation of hadrons in the nuclear medium. The photon can penetrate deep inside the nucleus, producing only minimal distortions to the initial state. K⁺ and K⁰ photoproduction data on nuclear targets thus can be used to probe the exclusive N(g,K)L, S vertex functions in the nuclear medium [Hyd91].

The data on deuterium from the g2 run period (see Section A6) were used to extract cross sections for the S^- photoproduction on the neutron. In this analysis, the reaction gn ®  K⁺S^- was selected by detecting the positive kaon and the decay products of the S^-, namely, the neutron and the negative pion. This analysis is approaching completion, currently going through the review process of the CLAS Collaboration. A similar technique can be used to extract the S^- contribution for heavier targets. The CLAS capability of detecting multiparticle final states thus presents an advantage over similar experiments that detect only the outgoing kaon [Zei01]. The availability of circularly polarized photons also enables us to access polarization observables.

The main challenges of measuring hyperon electromagnetic production cross sections are the detection of the kaon and the identification of the reaction channel. Figure 15 shows the technique used in our ³He analysis. The top left panel shows the time-of-flight mass of the kaon candidates, before and after tightening the PID cuts (see [Nic01] for details). The top right panel shows the invariant mass of the pp^- system detected in coincidence with the emerging kaon. One sees a narrow peak corresponding to the mass of the L hyperon. The bottom left panel shows the invariant mass of the np^- system; the sharp structure seen corresponds to the S^- events produced from the neutron. The bottom right panel shows the missing mass recoiling against the K⁺ meson, only for the events in which a proton and a p^- were also detected. The two vertical lines correspond to the L and S^o thresholds.

Figure 15. Illustration of the major analysis steps of the g³He ®  K⁺Y photoproduction data acquired with CLAS. The top right panel shows the invariant mass of the pp^- system detected in coinci-dence with the emitted kaon. The bottom left panel shows the invariant mass of the np^- system. The bottom right panel shows the missing mass recoiling against the K⁺ meson, only for the events in which a proton and a p^- were also detected. The two vertical lines correspond to the L and S^o thresholds.

The analysis of the g3 data on ³He is underway. This analysis will be extended to ⁴He as well. Combined with our earlier work on ²H (g2 data), we will be able to obtain information on the exclusive N(g,K)L,S vertex functions as a function of nuclear density.

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Last modified: 03/31/05