Event shape criteria for single-lepton top-quark signals

Single-lepton plus jets signals from [ital t] [bar t] production at hadron colliders generally give more spherically symmetrical events than the principal backgrounds from [ital W] production. We show that sphericity and aplanarity criteria, applied to lepton plus neutrino plus four-jet final states at the Fermilab Tevatron [ital p] [bar p] collider, help to discriminate; they can be used either to validate an eventual top-quark signal or simply to reduce background. An alternative circularity criterion in the transverse plane is less successful.