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arXiv 0908.1535nucl-thhep-phnucl-ex
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从强子到核的电磁形状因子和电荷密度

Electromagnetic Form Factors and Charge Densities From Hadrons to Nuclei

Gerald A. Miller

备注：
18 pages, 11 figures
发表信息：
Phys.Rev.C80:045210,2009
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摘要：

使用一个简单的精确协变模型来阐明电磁形状因子的相对论方面，其中将标量粒子建模为两个不同粒子的束缚态。模型的形状因子的计算使用一个精确的协变计算的最低阶三角图，并显示为使用光阵面技术获得的相同。使用坐标空间变量解释横向密度的含义，使我们能够识别与形状因子直接相关的真实均方横向尺寸。我们表明，休息帧的电荷分布一般是不可观察的，因为未能坚持目前的守恒。中性系统的两个带电的成分服从的知识，较重的一个通常是更接近横向原点比轻的一个。有人认为，负的中心荷密度的中子出现，在介子云模型，从介子的高纵向动量。精确地定义了非相对论极限，并证明了相对论效应的影响取决于最轻组分的结合能与质量之比。对于非常有限的参数范围，形状因子的精确相对论公式简化为熟悉的波函数的平方的三维傅里叶变换。对于模拟核子的夸克-双夸克模型的质量，我们发现对任何$Q^2$值都有相当大的相对论修正。利用原子核最低s态的示意模型发现，相对论效应使轻核的形状因子减小，而使重核的形状因子增大。此外，这些状态受到相对论的强烈影响。

英文摘要：

A simple exact covariant model in which a scalar particle is modeled as a bound state of two different particles is used to elucidate relativistic aspects of electromagnetic form factors. The model form factor is computed using an exact covariant calculation of the lowest-order triangle diagram and shown to be the same as that obtained using light-front techniques. The meaning of transverse density is explained using coordinate space variables, allowing us to identify a true mean-square transverse size directly related to the form factor. We show that the rest-frame charge distribution is generally not observable because of the failure to uphold current conservation. Neutral systems of two charged constituents are shown to obey the lore that the heavier one is generally closer to the transverse origin than the lighter one. It is argued that the negative central charge density of the neutron arises, in pion-cloud models, from pions of high longitudinal momentum. The non-relativistic limit is defined precisely and the ratio of the binding energy to that of the mass of the lightest constituent is shown to govern the influence of relativistic effects. The exact relativistic formula for the form factor reduces to the familiar one of the three-dimensional Fourier transform of a square of a wave function for a very limited range of parameters. For masses that mimic the quark-di-quark model of the nucleon we find substantial relativistic corrections for any value of $Q^2$. A schematic model of the lowest s-states of nuclei is used to find that relativistic effects decrease the form factor for light nuclei but increase the form factor for heavy nuclei. Furthermore, these states are strongly influenced by relativity.

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