Evidence for spin correlation in ttbar production

TL;DR

This paper measures the spin correlation in top-antitop quark pairs using data from the Fermilab Tevatron, providing evidence for spin correlation consistent with the standard model at over three sigma significance.

Contribution

It introduces a matrix-element-based method to measure the spin correlation ratio in single-lepton ttbar events and combines it with dilepton results for enhanced evidence.

Findings

Measured the spin correlation ratio f consistent with the standard model

Combined results show more than three sigma evidence for spin correlation

Supports the standard model prediction of spin correlations in ttbar production

Abstract

We present a measurement of the ratio of events with correlated t and tbar spins to the total number of ttbar events. This ratio f is evaluated using a matrix-element-based approach in events with a single lepton (electron or muon) and at least four jets. We analyze ppbar collisions data corresponding to an integrated luminosity of 5.3 fb^-1 collected with the D0 detector at the Fermilab Tevatron collider operating at a center of mass energy $\sqrt{s}=1.96$ TeV. Combining this result with a recent measurement of f in dilepton final states, we find f in agreement with the standard model. In addition, the combination provides evidence for the presence of spin correlation in ttbar events with a significance of more than three standard deviations.