标题： 从QCD求和规则计算$<p|\overline{u}u-\overline{d}d|p>$以及中子-质子质量差 显示英文标题

标题： Calculation of $<p|\overline{u}u-\overline{d}d|p>$ from QCD sum rule and the neutron-proton mass difference

摘要： 质子矩阵元的等矢标密度，$<p|\overline{u}u-\overline{d}d|p>/2M_p$，通过在外部等矢标场中计算核子电流关联函数，使用QCD求和法则方法进行计算。 除了QCD真空的通常手征和胶子凝聚外，手征凝聚对外部等矢标场的响应也进入计算。 后者由两种独立的方法确定。 一种方法将其与上夸克和下夸克手征凝聚之间的差异相关联，另一种方法使用手征微扰理论。 在夸克质量差$\delta m=m_d-m_u$的一阶近似下，中子-质子质量差的非电磁部分由乘积$\delta m <p|\overline{u}u-\overline{d}d|p>/2M_p$给出；所得值与实验值有合理的吻合。 显示英文摘要

摘要： The proton matrix element of the isovector-scalar density, $<p|\overline{u}u-\overline{d}d|p>/2M_p$, is calculated by evaluating the nucleon current correlation function in an external isovector-scalar field using the QCD sum-rule method. In addition to the usual chiral and gluon condensates of the QCD vacuum, the response of the chiral condensates to the external isovector field enters the calculation. The latter is determined by two independent methods. One relates it to the difference between the up and down quark chiral condensates and the other uses the chiral perturbation theory. To first order in the quark mass difference $\delta m=m_d-m_u$, the non-electromagnetic part of the neutron-proton mass difference is given by the product $\delta m <p|\overline{u}u-\overline{d}d|p>/2M_p$; the resulting value is in reasonable agreement with the experimental value.