Spin correlation in top-quark pairs. Almost 200 times heavier than the proton, the top quark is by far the most massive of the six quark species that, together with their antiquarks, are the building blocks of the hadrons. But because it’s so massive, a top quark (or its antiparticle) decays within 10–24 seconds of its creation—too fast to be affected by the processes that ordinarily clothe a quark in hadronic garb. So top–antitop pairs produced in CERN’s Large Hadron Collider (LHC) provide a unique opportunity for studying the production and decay of quarks without the obscuring hadronic complications. Now the team that runs the LHC’s gargantuan ATLAS detector has reported an analysis of correlation between the spin orientations of the top and antitop quarks in some 4000 identifiable top–antitop pairs produced in 1014 collisions between 3.5-TeV protons. The spin correlation is deduced from the directional correlations of high-energy...

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