Measurement of spin correlation and entanglement in ATLAS and CMS

TL;DR

The paper surveys recent ATLAS and CMS results on top-quark pair spin correlations and quantum entanglement at the LHC, using dilepton and lepton+jets final states. It frames the ttbar spin state with a spin-density-matrix formalism and extracts 15 coefficients from angular distributions, testing SM predictions and enabling the construction of entanglement witnesses such as D. CMS and ATLAS report entanglement signals exceeding $5\\sigma$ near the production threshold, with CMS extending measurements to the boosted regime; findings reveal regions where modelling of threshold dynamics remains challenging, motivating refined theory and future quantum-information studies of top-quark systems. These measurements push the experimental frontier of quantum effects in high-energy quark systems and provide stringent tests of SM spin structure in ttbar production.

Abstract

The Large Hadron Collider has delivered exceptionally large proton-proton collision datasets at centre-of-mass energies of up to 13.6 TeV. These datasets, collected by the ATLAS and CMS detectors, enable precision tests of theoretical predictions using extensive samples of top-quark events. A key example is the study of top-quark pair spin correlations, which can be accessed through the angular distributions of the top-quark decay products owing to the unique property that the top quark decays before it can hadronise. This wealth of data has also enabled new measurements of top-quark pair properties, particularly those with enhanced sensitivity in the threshold region, such as quantum entanglement, that were previously out of reach. In this contribution, the latest highlights in this area from the ATLAS and CMS experiments, are presented.

Figures (6)

• Figure 1: Measured spin correlation coefficients (circles) together with the nominal prediction using the Monte Carlo (MC) event generators POWHEG2 (triangles), an alternative sample from MADGRAPH5_aMC@NLO (inverted triangles), and the fixed-order NLO calculation of Ref. Bernreuther_2015 (squares). The inner vertical bars represent the statistical uncertainty and the outer bars the total uncertainty PhysRevD.100.072002.
• Figure 2: Measured values of $f_{\mathrm{SM}}$, the strength of the observed spin correlations relative to the SM prediction. The inner vertical bars indicate the statistical uncertainty, the middle bars the total experimental uncertainty, and the outer bars the total uncertainty PhysRevD.100.072002.
• Figure 3: The particle-level $D$ results in the signal and validation regions are compared with various MC models. The entanglement limit, converted from the parton-level value $D=-1/3$ to particle level, is shown Entanglement_2024.
• Figure 4: Summary of the measured entanglement proxy $D$ (black points) compared with MC predictions with (solid) and without (dashed) the $\eta_t$ state. Inner (outer) error bars show the statistical (total) uncertainty in data. Light (dark) shaded regions indicate the MC statistical (total) uncertainty. The entanglement limit $D=-1/3$ is shown as a shaded band CMS_Collaboration_2024.
• Figure 5: Results of the inclusive full matrix measurement. The measurements (markers) are shown with statistical (inner) and total (outer) uncertainties and compared to predictions from POWHEG+PYTHIA, POWHEG+Herwig, MadGraph5_aMC@NLO+PYTHIA, and MiNNLO+PYTHIA. Right panels show results with the POWHEG+PYTHIA prediction subtracted. The corresponding $\Delta_E$ values are indicated PhysRevD.110.112016.