Event shape criteria for single - lepton top signals

Single-lepton plus jets signals from t ¯ t production at hadron colliders generally give more spherically symmetrical events than the principal backgrounds from W production. We show that sphericity and aplanarity criteria, applied to lepton plus neutrino plus four jet final states at the Fermilab Tevatron p¯ p collider, help to discriminate; they can be used either to validate an eventual top signal or simply to reduce background. An alternative circularity criterion in the transverse plane is less successful. The predicted top quark, an essential component of the Standard Model (SM), is apparently too heavy to have been discovered yet at high energy colliders; the present experimental limit is m t > 113 GeV from the CDF detector [1] and independently m t > 103 GeV from the D0 detector [2]. The consistency of radiative corrections to all electroweak data however indicates that m t = 150 +19 −24 +15 −20 GeV [3], and hence that the top quark can probably be discovered at the Fermilab Tevatron p¯ p collider sooner or later [4]. Here the SM predicts mainly t ¯ t pair production via QCD with t → bW decays; the smallest and cleanest signals are expected in two-lepton channels [5, 6], but larger single-lepton [7, 8] and all-jet [9] signals can also in principle be separated from backgrounds. Since the top signals will initially (and perhaps for a considerable time) be based on a small number of events, it will be important to suppress the backgrounds as far as possible in ways that do not seriously reduce the signals. In this note we propose the use of event-shape criteria to distinguish the top signal and reduce backgrounds in the relatively copious single-lepton-plus-4 jet channel. The underlying idea is that Tevatron t ¯ t production will be mainly near threshold in the CM frame of the lowest-order QCD subprocesses q ¯ q → t ¯ t or gg → t ¯ t, and hence will mostly lead to rather spherical event shapes in this frame. Phase-space factors favor spherical configurations; the t ¯ t spin correlations [10] and decay matrix elements introduce substructure but do not drastically change this spherical tendency, which can help to distinguish the signal from backgrounds. This idea is very familiar in the context of heavy quark searches at e + e − colliders and has recently been applied to 6-jet top signals at the …