Experimental probe of quantum coherence in top-quark pair production

Abstract

We investigate quantum coherence in top--antitop spin states produced at the LHC using the $l_1$-norm of coherence applied to the reconstructed spin density matrix. Combining Standard Model predictions with recent CMS measurements of spin-correlation coefficients, we study the dependence of coherence on the invariant mass $M_{t\bar t}$ and the scattering angle. We find that coherence is large both near the production threshold and in boosted central events, whereas an intermediate-mass region exhibits reduced interference strength and enhanced sensitivity to radiative effects. This non-monotonic kinematic behavior originates from the helicity-interference structure of the underlying QCD production amplitudes. Recasting the CMS measurements in terms of quantum coherence yields values that are broadly consistent with Standard Model expectations. Our results establish quantum coherence as an experimentally accessible probe of spin dynamics in top-quark pair production and demonstrate its potential as a precision observable for studies of the top-quark spin-density matrix at hadron colliders.