Single-top-quark production at hadron colliders

TL;DR

The paper presents a detailed study of single-top-quark production at the Tevatron and LHC, focusing on the $W$-gluon fusion (t-channel) and $s$-channel processes as probes of the charged-current weak interaction and direct measurements of $V_{tb}$. It develops an acceptance calculation that properly treats the collinear $b\bar{b}$ region, uses a subtraction method to obtain the observable cross section for low $p_T$ of the $ar{b}$, and evaluates backgrounds with realistic detector effects and tagging efficiencies. The authors quantify theoretical uncertainties and demonstrate that the $W$-gluon fusion signal can be observed and used to extract $V_{tb}$ with meaningful precision, while the $s$-channel provides a complementary, theoretically cleaner channel, particularly at the Tevatron. They also show that the top quark polarization in single-top production is observable through angular distributions of decay leptons, offering an additional handle on the weak interaction, and they discuss forward-jet tagging as a diagnostic feature. Overall, the work provides a framework for measuring $V_{tb}$ and probing new physics via single-top processes, while highlighting the need to reduce theoretical uncertainties to fully exploit the LHC's statistical power.

Abstract

Single-top-quark production probes the charged-current weak interaction of the top quark, and provides a direct measurement of the CKM matrix element V_{tb}. We perform two independent analyses to quantify the accuracy with which the W-gluon fusion (gq -> t\bar{b}q) and (q\bar{q} -> t\bar{b}) signals can be extracted from the backgrounds at both the Tevatron and the LHC. Although perturbation theory breaks down at low transverse momentum for the W-gluon fusion \bar{b} differential cross section, we show how to obtain a reliable cross section integrated over low \bar{b} transverse momenta up to a cutoff. We estimate the accuracy with which V_{tb} can be measured in both analyses, including theoretical and statistical uncertainties. We also show that the polarization of the top quark in W-gluon fusion can be detected at the Fermilab Tevatron and the CERN LHC.

Figures (8)

• Figure 1: Representative Feynman diagrams for single-top-quark production at hadron colliders: (a) $s$-channel production, (b) $t$-channel production ($W$-gluon fusion), and (c) associated production with a $W$ boson.
• Figure 2: Transverse momentum distributions of the $\bar{b}$ antiquark (solid line), the $b$ quark from top decay (dashed line), and the light-quark jet (dotted line), in single-top-quark production via $W$-gluon fusion ($gq \to t\bar{b} q$) at the Tevatron ($\sqrt S=2$ TeV).
• Figure 3: The $b\ell\nu$ invariant-mass ($M$) distribution for single-top-quark production and backgrounds at the Tevatron ($\sqrt S=2$ TeV) with single $b$ tagging. The $W$-gluon-fusion signal is denoted by $t\bar{b} j$, and the $s$-channel process by $t\bar{b}$ (the $Wt$ process is negligibly small). The $Wjj$ background includes $Wb\bar{b}$, $Wc\bar{c}$, $Wcj$, and $Wjj$. The $t\bar{t}$ background is shown separately.
• Figure 4: The $b\ell\nu$ invariant-mass ($M$) distribution for single-top-quark production and backgrounds at the LHC with single $b$ tagging. The $W$-gluon-fusion signal is denoted by $t\bar{b} j$, and the $Wt$ process is also shown (the $s$-channel process is negligibly small). The $Wjj$ background includes $Wb\bar{b}$, $Wc\bar{c}$, $Wcj$, and $Wjj$. The $t\bar{t}$ background is shown separately.
• Figure 5: Significance of the single-top-quark signal in Run II at the Tevatron ($\sqrt S=2$ TeV, 2 fb$^{-1}$) versus the minimum rapidity of the non-$b$-tagged jet in the signal. Curves are shown with and without the jet veto imposed.