$γ^\ast N \to Δ(1232)$四极形状因子的参数化和Siegert定理
Parametrizations of the $γ^\ast N \to Δ(1232)$ quadrupole form factors and Siegert's theorem
摘要：
大的$N_c$极限提供的关系，可用于计算的$γ^\ast N \to Δ(1232)$四极形状因子在低和中等$Q^2$的假设下，π云的优势。这些参数化有两个局限性。首先，库仑四极形状因子的参数化低估了低$Q^2$数据。第二，当外推到类时区域时，形状因子违反Siegert定理的阶数为$1/N_c^2$。我们在这里提出的电四极形状因子的参数化，违反Siegert的定理，仅由条款的顺序$1/N_c^4$的更正。在四极跃迁形状因子的协变夸克模型的基础上，将改进的大$N_c$介子云参数化与价夸克贡献相结合，得到了符合Siegert定理的类时区外推，并对数据进行了很好的描述.
英文摘要：
The large $N_c$ limit provides relations that can be used to calculate the $γ^\ast N \to Δ(1232)$ quadrupole form factors at low and intermediate $Q^2$ under the assumption of the pion cloud dominance. There are two limitations in those parametrizations. First, the parametrization of the Coulomb quadrupole form factor underestimate the low $Q^2$ data. Second, when extrapolated for the timelike region, the form factors violate Siegert's theorem by terms of the order $1/N_c^2$. We propose here corrections to the parametrization of the electric quadrupole form factor, which violate Siegert's theorem only by terms of the order $1/N_c^4$. Combining the improved large $N_c$ pion cloud parametrizations with the valence quark contributions based on a covariant quark model for the quadrupole transition form factors, we obtain an extrapolation to the timelike region consistent with Siegert's theorem, and accomplish also a very good description of the data.
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