利用横向电荷密度实现矢量介子优势
Realizing vector meson dominance with transverse charge densities
摘要：
快速运动核子中的横向电荷密度表示为狄拉克形状因子虚部在类时区域的色散积分（谱函数）。在给定的横向距离b处，积分有效地在sqrt{t} ~< 1/b的范围内的能量上延伸，具有较大值的指数抑制。在周边距离的横向电荷密度，从而作为一个低通滤波器的光谱功能，并允许一个选择的能量区域占主导地位的特定的t-沟道状态，对应于明确的交换机制中的类空形状因子。我们发现，距离b ~ 0.5 - 1.5 fm的等矢量密度是最敏感的ρ介子区域，只有10%的贡献，从更高的质量状态。由手征动力学控制的软π介子交换只有在更大的距离上才有意义。在等密度宇宙中，ω以外的高质量态相对更重要。色散的方法表明，在b ~ 1 fm的中子中的正横向电荷密度，发现以前在类空形状因子数据的傅立叶分析，可以作为一个敏感的测试的isoscillation强度在~1 GeV的质量区。在部分子结构方面，矢量介子区b ~ 1fm的横向密度支持核子中价夸克运动的近似平均场图象。
英文摘要：
The transverse charge density in a fast-moving nucleon is represented as a dispersion integral of the imaginary part of the Dirac form factor in the timelike region (spectral function). At a given transverse distance b the integration effectively extends over energies in a range sqrt{t} ~< 1/b, with exponential suppression of larger values. The transverse charge density at peripheral distances thus acts as a low-pass filter for the spectral function and allows one to select energy regions dominated by specific t-channel states, corresponding to definite exchange mechanisms in the spacelike form factor. We show that distances b ~ 0.5 - 1.5 fm in the isovector density are maximally sensitive to the rho meson region, with only a ~10% contribution from higher-mass states. Soft-pion exchange governed by chiral dynamics becomes relevant only at larger distances. In the isoscalar density higher-mass states beyond the omega are comparatively more important. The dispersion approach suggests that the positive transverse charge density in the neutron at b ~ 1 fm, found previously in a Fourier analysis of spacelike form factor data, could serve as a sensitive test of the the isoscalar strength in the ~1 GeV mass region. In terms of partonic structure, the transverse densities in the vector meson region b ~ 1 fm support an approximate mean-field picture of the motion of valence quarks in the nucleon.
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