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arXiv 0810.4126hep-phnucl-exnucl-th
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电磁N-δ跃迁中的D态效应

D-state effects in the electromagnetic N-Delta transition

G. Ramalho, M. T. Pena, Franz Gross

摘要:

我们考虑一个明显协变的核子和Delta的夸克模型,其中一个夸克是离壳的,另外两个夸克形成在壳的双夸克对。利用这个模型,我们已经证明了核子形状因子和γ N → δ跃迁的主导形状因子(磁偶极子(M1)形状因子)可以用只含S态分量的核子和δ波函数很好地描述。本文证明了在δ价夸克波函数中加入D态分量,可以得到小电(E2)和库仑(C2)四极形状因子的非零结果。我们提出了这些组件的协变定义,并计算它们的贡献的形状因子。我们发现,这些组件本身不能描述数据。还必须加上明确的π介子云贡献,这些贡献支配着E2和C2的形状因子。通过用中子电形状因子来参数化π云对跃迁电和库仑形状因子的贡献,我们估计与夸克核自旋为3/2耦合的δ D态的贡献约为1%,与夸克核自旋为1/2耦合的δ D态的贡献约为4%.

英文摘要:

We consider here a manifestly covariant quark model of the nucleon and the Delta, where one quark is off-shell and the other two quarks form an on-shell diquark pair. Using this model, we have shown previously that the nucleon form factors and the dominant form factor for the gamma N -> Delta transition (the magnetic dipole (M1) form factor) can be well described by nucleon and Delta wave functions with S-state components only. In this paper we show that non-vanishing results for the small electric (E2) and Coulomb (C2) quadrupole form factors can be obtained if D-state components are added to the Delta valence quark wave function. We present a covariant definition of these components and compute their contributions to the form factors. We find that these components cannot, by themselves, describe the data. Explicit pion cloud contributions must also be added and these contributions dominate both the E2 and the C2 form factors. By parametrizing the pion cloud contribution for the transition electric and Coulomb form factors in terms of the neutron electric form factor, we estimate that the contributions of the Delta D-state coupled to quark core spin of 3/2 is of the order of 1%, and the contributions of the Delta D-state coupled to quark core spin 1/2 is of the order of 4%.

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