The fundamental building blocks of the proton—quarks and gluons—have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin¹. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force². An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter–antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter–antimatter quark pair annihilates. In this picture of quark–antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton³. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results⁴, and point to future measurements that can distinguish between these mechanisms.

几十年来，质子的基本组成部分——夸克和胶子——已经为人所知。然而，对于这些粒子及其动力学如何产生质子的量子束缚态及其物理性质（例如其自旋¹ ），我们仍然有不完整的理论和实验理解。在最简单的图片中，构成质子的两个上夸克和一个下夸克仅占质子质量的百分之几，其中大部分以夸克动能和势能以及来自强力的胶子能量的形式存在². 正如量子色动力学所描述的，这种力的一个基本特征是它能够在质子内部产生仅存在很短时间的物质-反物质夸克对。它们短暂的存在使得质子中的反物质夸克难以研究，但在物质-反物质夸克对湮灭的反应中，它们的存在是显而易见的。在这张由强力产生的夸克-反夸克的图片中，上下反物质夸克存在的作为动量函数的概率分布应该几乎相同，因为它们的质量与夸克的质量非常相似并且很小。质子³. 在这里，我们提供了来自 μ 子对产生测量的证据，这些分布有很大不同，在很宽的动量范围内，下反物质夸克比上反物质夸克更丰富。预计这些结果将重新引起人们对质子中这种反物质不对称起源的几种提议机制的兴趣，这些机制已被先前的结果⁴所反对，并指出未来的测量可以区分这些机制。