Observation of spin correlation in ttbar events from pp collisions at sqrt(s) = 7 TeV using the ATLAS detector

TL;DR

This paper reports the first measurement of spin correlation in top-antitop quark pairs at the LHC, confirming Standard Model predictions and excluding zero correlation with high significance.

Contribution

It presents the first measurement of spin correlation in ttbar events at 7 TeV using ATLAS data, aligning with Standard Model predictions.

Findings

Measured spin correlation degree of 0.40 +0.09 -0.08

Zero spin correlation hypothesis excluded at 5.1 sigma

Results agree with next-to-leading-order Standard Model predictions

Abstract

A measurement of spin correlation in ttbar production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2.1 fb^-1. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. The difference in azimuthal angle between the two charged leptons in the laboratory frame is used to extract the correlation between the top and antitop quark spins. In the helicity basis the measured degree of correlation corresponds to 0.40 +0.09 -0.08, in agreement with the next-to-leading-order Standard Model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations.