Non-factorizable effects in top quark production

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The production of top-antitop pairs at the Fermilab Tevatron shows a forward-backward asymmetry in which the top quark tends to follow the proton direction, while the antitop tends to follow the antiproton direction. The effect grows with increasing effective mass mt t of the top-antitop pair, and with increasing rapidity difference between the top and antitop. The observed effect is about three times as large as predicted by next-to-leading-order QCD, but with the same sign. An estimate of non-factorizable effects based on a QCD string picture finds they are negligible, but that small distortions of the mₜ spectrum are possible. Tests for such effects, both at and above the level of this estimate, are suggested.