Abstract

A measurement of spin correlation in tt¯ 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 Ahelicity=0.40−0.08+0.09, in agreement with the next-to-leading-order standard model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations.Received 19 March 2012DOI:https://doi.org/10.1103/PhysRevLett.108.212001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2012 CERN, for the ATLAS Collaboration

Highlights

  • A measurement of spin correlation in tt" production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2:1 fbÀ1

  • Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets

  • The lifetime of the top quark is at least an order of magnitude shorter than the time scale for strong interactions, implying that the top quark decays before hadronization [3,4,5,6,7]

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Summary

Introduction

A measurement of spin correlation in tt" production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2:1 fbÀ1. This analysis uses a fit to templates constructed from simulated event samples to determine the amount of spin correlation from the Á distribution. These events are difficult to properly model in simulations due to uncertainties on the non-Gaussian tails of the EmT iss distribution, on the cross section for Z-boson production with multiple jets, and on the lepton energy resolution.

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