The use of accelerated beams of electrons, protons or ions has furthered the development of nearly every scientific discipline. However, high-energy muon beams of equivalent quality have not yet been delivered. Muon beams can be created through the decay of pions produced by the interaction of a proton beam with a target. Such ‘tertiary’ beams have much lower brightness than those created by accelerating electrons, protons or ions. High-brightness muon beams comparable to those produced by state-of-the-art electron, proton and ion accelerators could facilitate the study of lepton–antilepton collisions at extremely high energies and provide well characterized neutrino beams^1,2,3,4,5,6. Such muon beams could be realized using ionization cooling, which has been proposed to increase muon-beam brightness^7,8. Here we report the realization of ionization cooling, which was confirmed by the observation of an increased number of low-amplitude muons after passage of the muon beam through an absorber, as well as an increase in the corresponding phase-space density. The simulated performance of the ionization cooling system is consistent with the measured data, validating designs of the ionization cooling channel in which the cooling process is repeated to produce a substantial cooling effect^9,10,11. The results presented here are an important step towards achieving the muon-beam quality required to search for phenomena at energy scales beyond the reach of the Large Hadron Collider at a facility of equivalent or reduced footprint⁶.

电子、质子或离子加速束的使用促进了几乎所有科学学科的发展。然而，尚未交付同等质量的高能μ子束。μ子束可以通过质子束与目标相互作用产生的介子衰变产生。这种“三次”光束的亮度远低于加速电子、质子或离子产生的亮度。与最先进的电子、质子和离子加速器产生的相媲美的高亮度 μ 子束可以促进在极高能量下轻子-反轻子碰撞的研究，并提供特征良好的中微子束^{1,2,3,4, 5,6}。这种μ子束可以使用电离冷却来实现，已提出提高μ子束亮度^7,8。在这里，我们报告了电离冷却的实现，这通过观察到 μ 子束通过吸收体后低幅度 μ 子数量的增加以及相应相空间密度的增加而得到证实。电离冷却系统的模拟性能与测量数据一致，验证了电离冷却通道的设计，其中重复冷却过程以产生显着的冷却效果^9,10,11。^{这里介绍的结果是朝着实现在等效或减少占地面积6}的设施中寻找大型强子对撞机无法达到的能量尺度现象所需的 μ 子束质量迈出的重要一步。