JLAB-THY-18-2675 Accurate nucleon electromagnetic form factors from dispersively improved chiral effective field theory
J. M. Alarcón C. Weiss Theory Center, Jefferson Lab, Newport News, VA 23606, USA
Abstract
We present a theoretical parametrization of the nucleon electromagnetic form factors (FFs) based on a combination of chiral effective field theory and dispersion analysis. The isovector spectral functions on the two-pion cut are computed using elastic unitarity, chiral pion-nucleon amplitudes, and timelike pion FF data. Higher-mass isovector and isoscalar $t$ -channel states are described by effective poles, whose strength is fixed by sum rules (charges, radii). Excellent agreement with the spacelike proton and neutron FF data is achieved up to $Q^{2}\sim 1$ GeV2. Our parametrization provides proper analyticity and theoretical uncertainty estimates and can be used for low- $Q^{2}$ FF studies and proton radius extraction.
中文速览
质子和中子并非点状粒子，其内部结构由电磁形状因子（electromagnetic form factors）来描述，而精确测定这些形状因子对于提取质子电荷半径等基本常数至关重要。研究者将手征有效场论（chiral effective field theory）与色散分析（dispersion analysis）相结合，先用弹性幺正性条件从第一性原理计算双π介子对形状因子谱函数的贡献，再用少量有效极点参数化更高质量的同位旋标量和矢量激发态，其强度则通过电荷、磁矩及半径等求和规则唯一确定，全程无需拟合任何空间类形状因子实验数据。最终在动量传递 Q² ≲ 1 GeV²（乃至约 2 GeV²）范围内，质子和中子的四个形状因子均与最新实验参数化高度吻合，并给出了可靠的理论不确定度估计。这一参数化方案既具备严格的解析结构，又能将核子半径作为自由参数直接输入或从数据中提取，为质子电荷半径的精密测量及标准模型检验提供了坚实的理论工具。
原文 arXiv:1803.09748;中英对照 + 大白话阅读 https://aha.fim.ai/paper/1803.09748v1