The three-dimensional structure of nucleons (protons and neutrons) is embedded in so-called generalized parton distributions, which are accessible from deeply virtual Compton scattering. In this process, a high-energy electron is scattered off a nucleon by exchanging a virtual photon. Then, a highly energetic real photon is emitted from one of the quarks inside the nucleon, which carries information on the quark’s transverse position and longitudinal momentum. By measuring the cross-section of deeply virtual Compton scattering, Compton form factors related to the generalized parton distributions can be extracted. Here, we report the observation of unpolarized deeply virtual Compton scattering off a deuterium target. From the measured photon-electroproduction cross-sections, we have extracted the cross-section of a quasifree neutron and a coherent deuteron. Due to the approximate isospin symmetry of quantum chromodynamics, we can determine the contributions from the different quark flavours to the helicity-conserved Compton form factors by combining our measurements with previous ones probing the proton’s internal structure. These results advance our understanding of the description of the nucleon structure, which is important to solve the proton spin puzzle.

核子（质子和中子）的三维结构嵌入在所谓的广义部分子分布中，可以从深度虚拟康普顿散射中获得。在这个过程中，一个高能电子通过交换一个虚光子从一个核子上散射出去。然后，一个高能实光子从核子内部的一个夸克中发射出来，它携带有关夸克横向位置和纵向动量的信息。通过测量深度虚拟康普顿散射的横截面，可以提取与广义部分子分布相关的康普顿形状因子。在这里，我们报告了对氘目标散射的非极化深度虚拟康普顿的观察。从测量的光子电产生横截面，我们提取了准自由中子和相干氘核的截面。由于量子色动力学的近似同位旋对称性，我们可以通过将我们的测量与之前探测质子内部结构的测量相结合来确定不同夸克风味对螺旋度守恒康普顿形状因子的贡献。这些结果促进了我们对核子结构描述的理解，这对于解决质子自旋难题很重要。