Nature. 2022 May;605(7910):440-446. doi: 10.1038/s41586-022-04572-w. Epub 2022 May 18.
In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD). These partons subsequently emit further partons in a process that can be described as a parton shower, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m and energy E, within a cone of angular size m/E around the emitter. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.
在粒子对撞机实验中，具有大动量转移的基本粒子相互作用产生夸克和胶子（称为部分子），它们的演化由强相互作用所支配，由量子色动力学（QCD）理论描述。这些部分子随后进一步发射出更多的部分子，这一过程可以被描述为部分子喷注，最终形成可探测的强子。研究部分子喷注的模式是测试 QCD 的关键实验工具之一。这种模式预计取决于起始部分子的质量，这是一种称为死锥效应的现象，它预测在发射器周围以角度大小 m/E 的圆锥内，质量为 m 和能量为 E 的重夸克发射的胶子谱会受到抑制。以前，由于从实验上可探测的强子中重建级联夸克和胶子的挑战，QCD 中的死锥效应的直接观察一直是不可能的。我们通过使用新的迭代去簇技术来重建魅夸克的部分子喷注，直接观察到 QCD 中的死锥。这一结果证实了 QCD 的一个基本特征。此外，死锥角的测量构成了对魅夸克非零质量的直接实验观测，魅夸克质量是粒子物理标准模型中的一个基本常数。
