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 (see the drawing) 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 decay leptons. The standard model (SM) of particle theory predicts that the top and antitop spins prefer to emerge with the same rather than opposite “helicities” (spin projections along the momentum direction). The team measured the correlation parameter A (same minus opposite fraction of all pairs) to be (40 ± 8)%,  in reasonable agreement with the SM prediction. If, as some theories suggest, nonstandard Higgs bosons are involved in the production or decay processes, A could differ from the SM prediction. (G. Aad et al., ATLAS collaboration, Phys. Rev. Lett. 108, 212001, 2012.)—Bertram Schwarzschild