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

由于φ介子几乎是纯的s_s_bar态，在很宽的能量范围内，质子衍射光生φ介子可以很好地描述为坡密子交换在Regge理论和矢量优势模型的框架下研究了多胶子交换过程;高能光子转换成一个介子，然后它被散射从质子的交换pomeron，而介子-交换被OZI规则抑制。然而，在低能量下，介子（π，η）-交换，标度（0^<++>胶球）-交换和ss_bar敲除产生的其他贡献是可能的。随着入射光子能量的增加，这些贡献迅速下降，并且只能在产生阈值附近的低能量区域中进行研究。线偏振光是分解这些贡献的理想探针。对于自然宇称交换，如坡密子和0^<++>胶球交换，$K^+K^-$的衰变平面集中在光子偏振矢量中。对于非自然宇称交换过程，如π和η交换过程，它垂直于极化矢量。对前向角Φ光生的KK衰减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。截面图显示了在2GeV附近的凸点结构，这不能仅用Pomeron交换贡献来解释。因此，我们得出结论，必须有额外的自然平价交换的贡献，这可能是由于一个新的Regge轨迹与0^<++>胶球。作为这项研究的一个副产品，我们发现了中子光生过程中存在S=+1重子共振的证据，这可以解释为五夸克态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^+.