Favorable Conditions of Θ+Formation in γDReaction

The first evidence for the pentaquark hadron Θ+, discovered by the LEPS collaboration at SPring-8,1) was subsequently confirmed in some other experiments. However, many other experiments failed to find the Θ+ signal. For surveys see Ref. 2). The situation became particularly dramatic after the recent publication of the high statistics results of the CLAS collaboration.3),4) Thus, the first experiment is designed to search for the Θ+ signal in γD → pK−nK+ in direct γn interactions at relatively low photon energy, Eγ = 1.7 − 3.5 GeV. Within the experimental significance, no Θ+ signal was observed. The CLAS null result for a given finite Θ+ decay width means large off-shell suppression in Θ+NK vertices of the amplitudes. The K∗-exchange amplitude may be additionally suppressed by the small value of gΘNK∗ coupling constant because it is not related directly the Θ+ decay width and therefore remains unconstraint. Therefore, the best way to check whether the Θ+ exists or not is to study the KN → Θ+ fusion reaction with a quasi-free kaon and a nucleon in the initial state. In this case the gΘNK coupling is fixed (for given ΓΘ), and there is no ambiguity with the off-shell form factor because all hadrons are almost on the mass shell. This situation may be realized in the reaction γD → Λ∗Θ+ → pK−nK+ (Λ∗ ≡ Λ(1520)) with the Θ+ showing up as a peak in the nK+ invariant mass distribution. There are several conditions which can enhance this effect.5) First, the pK− invariant mass must be close to the mass of Λ∗. In this case, the total amplitude is the coherent sum of two amplitudes with charged and neutral kaon exchange shown in Fig. 1. The dominance of the K∗ meson exchange in Λ∗ photoproduction6) results