Phi-meson photo-production to study multi-gluon exchange process

由于$phi$介子几乎是纯s s_bar态，因此在Regge理论和矢量优势模型的框架下，在较宽的能量范围内，质子产生Ф介子的衍射光可以很好地描述为波美子交换（多胶子交换）过程；当介子交换被OZI规则抑制时，高能光子转化为Ф介子，然后通过介子交换从质子散射出去。然而，在低能量下，介子（π, η）交换、标量（0^<++>胶球）交换和ss_bar敲除也可能产生其他贡献。这些贡献随着入射光子能量的增加而迅速下降，并且只能在接近产生阈值的低能区域进行研究。线偏振光子是分解这些贡献的理想探针。对于pomeron和0^<++>胶球交换等自然宇称交换，K^+K^-$的衰减面集中在光子偏振矢量上。对于π和η交换过程等非自然宇称交换过程，它垂直于偏振矢量。对Φphoto-productions正向角的KK衰变不对称性的初步分析表明，自然宇称交换的贡献仍然占主导地位。横截面在2gev附近呈现凸起结构，不能仅用波美龙交换贡献来解释。因此，我们得出结论，必须有额外的自然宇称交换贡献，这可能归因于与0^<++>胶球相关的新Regge轨迹。作为本研究的副产物，我们发现了中子产生光的窄S=+1重子共振的证据，这可能被解释为五夸克态Θ^+。

Since a $phi$ meson is almost pure s s_bar state, diffractive photo-production for a Ф meson off a proton in a wide energy range is well described as a pomeron-exchange (multi gluon-exchange) process in the framework of the Regge theory and of the Vector Dominance Model ; a high energy photon converts into a Ф meson and then it is scattered from a proton by an exchange of the pomeron while the meson-exchange is suppressed by the OZI rule. However, at low energies other contributions arising from meson (π, η)-exchange, a scaler (0^<++>glueball)-exchange, and ss_bar knock-out are possible. These contributions fall off rapidly as the incident photon energy increases, and can be studied only in the low energy region near the production threshold. Linearly polarized photons are an ideal probe to decompose these contributions. For natural-parity exchange such as pomeron and 0^<++>glueball exchanges, the decay plane of $K^+K^-$ is concentrated in the photon polarization vector. For unnatural-parity exchange processes like π and ηexchange processes, it is perpendicular to the polarization vector. A preliminary analysis of the KK decay asymmetry for Φphoto-productions in the forward angles showed the natural-parity exchange contributions are still dominant. The cross-section shows a bump structure near 2 GeV which cannot be explained by Pomeron exchange contribution only. Thus we conclude there must be additional natural-parity exchange contribution, which may be attributed to a new Regge trajectory associated with 0^<++> glueball. As a bi-product of this study, we found evidence for a narrow S=+1 baryon resonance in photo-production from a neutron, which may be interpreted as a Pentaquark state Θ^+.