Entanglement is an intrinsic property of quantum mechanics and is predicted to be exhibited in the particles produced at the Large Hadron Collider. A measurement of the extent of entanglement in top quark-antiquark ( tt¯) events produced in proton–proton collisions at a center-of-mass energy of 13 TeV is performed with the data recorded by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 36.3 fb⁻¹. The events are selected based on the presence of two leptons with opposite charges and high transverse momentum. An entanglement-sensitive observable D is derived from the top quark spin-dependent parts of the tt¯ production density matrix and measured in the region of the tt¯ production threshold. Values of D<−1/3 are evidence of entanglement and D is observed (expected) to be −0.480−0.029+0.026 ( −0.467−0.029+0.026) at the parton level. With an observed significance of 5.1 standard deviations with respect to the non-entangled hypothesis, this provides observation of quantum mechanical entanglement within tt¯ pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced.

中文翻译：

---

在 s=13 TeV 时质子-质子碰撞中顶夸克对产生的量子纠缠的观察


纠缠是量子力学的固有特性，预计会在大型强子对撞机产生的粒子中表现出来。使用 2016 年欧洲核子研究组织 LHC 的 CMS 实验记录的数据，测量了质子-质子碰撞中产生的顶夸克-反夸克 （tt ̄） 事件中的纠缠程度，对应于 36.3 ^fb-1 的积分光度。这些事件是根据两个具有相反电荷和高横向动量的轻子的存在来选择的。纠缠敏感的可观察对象 D 源自 tt ̄ 产生密度矩阵的顶部夸克自旋依赖部分，并在 tt ̄ 产生阈值的区域中进行测量。D<-1/3 的值是纠缠的证据，在 parton 水平观察到（预期）D 为 -0.480-0.029+0.026 （-0.467-0.029+0.026）。相对于非纠缠假设，观察到的显著性为 5.1 个标准差，这提供了对该相空间中 tt ̄ 对内的量子力学纠缠的观察。这项测量为有史以来最高能量的量子力学提供了新的探针。