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%.
Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory
本文用相对论手征有效场论计算了Delta(1232)共振区核子($e^- N \to e^- N π$)外的π介子电生。在固定三个低能常数后,对应于磁(M1),电(E2)和库仑(C2)$γNΔ$耦合,我们的计算提供了一个预测的动量转移和π介子质量依赖的$γNΔ$形状因子。对π介子质量依赖性的预测解决了最近格点量子色动力学结果与低$Q^2$下“C2/M1比”实验值之间的差异。
A model for the Delta(1600) resonance and gamma N -> Delta(1600) transition
用协变旁观者组分夸克模型研究了γ N → Delta(1600)跃迁。在跃迁过程中有两个重要的过程:一个光子耦合到Delta(1600)核的单个夸克(夸克核),一个光子耦合到中间的π介子-重子态(π介子云)。假设Delta(1600)是Delta(1232)的第一个径向激发,估算夸克核的贡献,而估算π介子云的贡献是基于与γ N -> Delta(1232)跃迁的类比。为了估计π云在γ N -> Delta(1600)跃迁中的贡献,我们包括了相关的中间态,π-N,π-Delta,π-N(1440)和π-Delta(1600)。预测了磁偶极跃迁形状因子GM*(Q2)以及螺旋度振幅A_1/2(Q2)和A_3/2(Q2)对四动量转移平方Q2的依赖性。将Q2=0时的结果与现有数据进行比较。
γ* N → Δ(1232)跃迁是研究强子形变、微扰QCD在中等动量转移下的适用性以及非微扰现象对强子观测量影响的一个窗口。我们解释说,阿什公约的磁跃迁形状因子必须下降快于中子的磁形状因子和非零值的相关四极比揭示了夸克轨道角动量的核子和Δ(1232)的影响,并表明,这些四极比确实接近其预测的渐近极限,虽然缓慢。
Valence quark contribution for the gamma N -> Delta quadrupole transition extracted from lattice QCD
我们从物理π质量区核子(N)和δ的旁观夸克模型出发,将反应γ N -> δ的预言推广到格点量子色动力学体系。夸克模型在夸克-双夸克波函数中包含S波和D波。在这个框架内,它是D-波部分的德尔塔波函数,产生非零价的贡献的四极形状因子的过渡。然而,这些贡献不足以解释物理数据,因为π介子云的贡献占主导地位。为了分离这两种效应,我们将模型应用到晶格区中的一个区域,在该区域中,π介子云效应可以忽略不计,并直接调整D-状态参数的晶格数据。这个过程使我们能够更好地确定D态的贡献。最后,通过添加一个简单的参数化的介子云,我们建立了实验数据和晶格数据之间的联系。
We investigate the axial-vector nucleon-to-delta transition form factors in the framework of relativistic baryon chiral perturbation theory at the one-loop order using the complex-mass renormalization scheme. We determine the available six free parameters by fitting to an empirical parametrization of the form factors obtained from the BNL neutrino bubble chamber experiments. A unique feature of our calculation is the prediction of a non-vanishing form factor $C_3^A(Q^2)$. Moreover, our results show a surprising sensitivity to the coupling constant $\texttt{g}_1$ of the leading-order Lagrangian…
Light Cone Sum Rules for gamma* N -> Delta Transition Form Factors
本文提出了一种利用光锥求和规则计算γ * N -> Delta跃迁形状因子的理论框架。导出了首阶求和规则,并与已有的实验数据进行了比较。我们发现,在几个GeV的区域的过渡形状因子的“软”的贡献,可以被认为是重叠积分的强子波函数的价分量占主导地位。夸克场的“负”分量对结果有重要贡献,这可以重新解释为夸克轨道角动量的大贡献
Three-Neutrino Mixing and Combined Vacuum Oscillations and MSW Transitions of Solar Neutrinos
Assuming three flavour neutrino mixing takes place in vacuum, we investigate the possibility that the solar nu_e take part in MSW transitions in the Sun due to Delta m^2_{31} from 10^{-7} eV^2 to 10^{-4} eV^2, followed by long wave length vacuum oscillations on the way to the Earth, triggered by Delta m^2_{21} (or Delta m^2_{32}) from 10^{-12} eV^2 to 10^{-10} eV^2, Delta m^2_{31} and Delta m^2_{21} (Delta m^2_{32}) being the corresponding neutrino mass squared differences. The solar nu_e survival probability is shown to be described in this case by a simple analytic expression. Depending on w…