Angular correlations in single-top-quark and Wjj production at next-to-leading order

TL;DR

This work investigates whether spin-induced angular correlations in single-top-quark production persist under next-to-leading order QCD corrections and can be used to discriminate against the dominant Wjj background. By analyzing fully correlated angular distributions in the top-quark rest frame and comparing LO with NLO predictions, the study demonstrates that these correlations are largely robust, with LO shapes accurately describing NLO results up to calculable K-factors. The author proposes a concrete set of angular cuts, and an invariant-mass discriminant, that together can improve the discovery significance by about 25% and enhance the signal-to-background ratio by up to a factor of three, with relatively small theoretical uncertainty. The results support using LO spin-dependent matrix elements, embedded in shower MCs, to model these correlations for experimental analyses, while noting that t-channel production benefits from NLO-matched samples.

Abstract

I demonstrate that the correlated angular distributions of final-state particles in both single-top-quark production and the dominant Wjj backgrounds can be reliably predicted. Using these fully-correlated angular distributions, I propose a set of cuts that can improve the single-top-quark discovery significance by 25%, and the signal to background ratio by a factor of 3 with very little theoretical uncertainty. Up to a subtlety in t-channel single-top-quark production, leading-order matrix elements are shown to be sufficient to reproduce the next-to-leading order correlated distributions.

Figures (23)

• Figure 1: Representative leading-order Feynman diagrams for (a) $t$-channel, and (b) $s$-channel production of a single top quark.
• Figure 2: Decay products of the top quark, and the angle $\theta^t_{\hat{s}\,e^+}$ between the charged lepton $e^+$ and the spin $\hat{s}_t$ of the top quark in the top-quark rest frame. It is convenient to choose the spin to be projected in the direction of the down-type quark $d$ in the event.
• Figure 3: Cosine of angle ($\cos \theta^t_{ej_1}$) between the charged lepton and highest-$E_T$ light-quark jet in the top-quark rest frame of $t$-channel single-top-quark production at LO and NLO. The lepton isolation cut suppresses events at large $\cos \theta^t_{ej_1}$. A nearly angle-independent underlying contribution comes from misidentification of which jet contains the down-type quark.
• Figure 4: Cosine of angle ($\cos \theta_{ej_1}$) between the charged lepton and highest-$E_T$ light-quark jet $j_1$ in the lab frame and top-quark rest frame of single-top-quark production at NLO.
• Figure 5: Cosine of the angle ($\cos \theta^t_{e\bar{p}}$) between the charged lepton and antiproton in the top-quark rest frame of $s$-channel single-top-quark production at LO at the Fermilab Tevatron (a $1.96$ TeV $p\bar{p}$ collider). The solid line corresponds to reconstructing the top-quark frame using the exact neutrino and $b$-jet from the top-quark decay. The dashed line uses the missing transverse energy and $W$ mass to fit a putative neutrino momentum. The dotted line adds the effect of using a randomly chosen $b$-jet. The rescaled NLO cross section with a randomly chosen $b$ jet is indicated by open circles.