Observation of quantum entanglement in top quark pair production in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper reports the first observation of quantum entanglement in top quark pairs produced at the LHC, using a novel entanglement-sensitive observable and data from the CMS experiment at 13 TeV.

Contribution

It introduces a new measurement of entanglement in top quark pairs at high energies, providing experimental evidence of quantum entanglement in particle physics.

Findings

Observed entanglement-sensitive observable D = -0.480 at parton level

Achieved 5.1 sigma significance for entanglement detection

Provides new experimental probe of quantum mechanics at high energies

Abstract

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 ($\mathrm{t\bar{t}}$) 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 $\mathrm{t\bar{t}}$ production density matrix and measured in the region of the $\mathrm{t\bar{t}}$ production threshold. Values of $D$ $\lt$ $-$1/3 are evidence of entanglement and $D$ is observed (expected) to be $-$0.480 $^{+0.026}_{-0.029}$ $(-$0.467 $^{+0.026}_{-0.029})$ 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 $\mathrm{t\bar{t}}$ pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced.