Physics Division, Argonne National Laboratory, Lemont, IL, USA.
Department of Physics, Temple University, Philadelphia, PA, USA.
Nature. 2023 Mar;615(7954):813-816. doi: 10.1038/s41586-023-05730-4. Epub 2023 Mar 29.
The proton is one of the main building blocks of all visible matter in the Universe. Among its intrinsic properties are its electric charge, mass and spin. These properties emerge from the complex dynamics of its fundamental constituents-quarks and gluons-described by the theory of quantum chromodynamics. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering. An example is the highly precise measurement of the electric charge radius of the proton. By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/ψ particle. We determined the gluonic gravitational form factors of the proton from our measurement. We used a variety of models and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics. This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.
质子是宇宙中所有可见物质的主要组成部分之一。它的固有属性包括电荷、质量和自旋。这些性质源于其基本组成部分——夸克和胶子的复杂动力学，由量子色动力学理论描述。质子的电荷和自旋是由夸克共享的，之前已经使用电子散射进行了研究。一个例子是对质子电荷半径的高度精确测量。相比之下，质子内部质量密度知之甚少，它主要由胶子携带的能量决定。由于胶子不带电磁电荷，因此很难使用电子散射来探测它们。在这里，我们使用小的颜色偶极子，通过 J/ψ 粒子的阈光产生，研究了质子中胶子的引力密度。我们从测量结果中确定了质子的胶子引力形状因子。我们使用了多种模型，并在所有情况下都确定了一个质量半径，明显小于电荷半径。在某些情况下（但不是所有情况下），具体取决于模型，所确定的半径与格点量子色动力学的第一性原理预测吻合得很好。这项工作为深入了解胶子在为可见物质提供引力质量方面的突出作用铺平了道路。
