Quantumness of top quark pairs produced at LHC within SMEFT framework

Abstract

Top and anti-top quark pair production at LHC provides a unique setting to probe non-classical correlations at the TeV scale. We study quantum information (QI) properties of the $t\bar{t}$ spin state in $pp$ collisions at $\sqrt{s}=13$ TeV within the Standard Model Effective Field Theory (SMEFT), focusing on dimension-6 operators that induce anomalous chromo- and weak dipole moments of the top quark within their current experimental bounds. The $t\bar{t}$ spin density matrix is reconstructed from the joint angular distribution of the final state charged leptons in the $k$-$r$-$n$ helicity basis. We analyze three complementary QI quantities: concurrence-based quantum entanglement (QE), geometric quantum discord (GQD), and the Bell parameter,across four $t\bar{t}$ invariant-mass bins. Within the Standard Model (SM), non-vanishing QE appears only near threshold ($m_{t\bar{t}}\lesssim 400$ GeV), while GQD remains nonzero across the full phase space, indicating persistent non-classical correlations even for separable states. Anomalous chromo-dipole interactions modify these observables primarily near threshold: $\hat{\mu}_t$ induces asymmetric shifts, whereas $\hat{d}_t$ produces a mild symmetric response without Bell inequality violation. Among weak dipole operators, the CP-even coupling $C_2^V$ generates the largest deformation of the QI observables, while $\Delta C_1^{A,V}$ leave them unchanged. These results demonstrate that QI observables derived from the $t\bar{t}$ spin density matrix provide a complementary probe of anomalous top-quark interactions with distinct sensitivity to CP-even and CP-odd operator structures.