Single top production in association with a Z boson at the LHC

TL;DR

This paper computes the production of a Z boson in association with a single top quark at NLO in QCD, including leptonic decays of both the top and Z. The authors demonstrate that the tZ and tZbar processes yield a trilepton signature with cross sections competitive with ttZ, making the channel observable at LHC 8 TeV data and offering a probe of the top–Z coupling. They provide a detailed LO and NLO treatment in a five-flavor framework, discuss scale and PDF uncertainties, and analyze jet-bin distributions to explore distinguishing tZ from ttZ, while also highlighting tZ as a potentially dominant irreducible background in FCNC top-decay searches. The work also discusses the related tH process and provides code in MCFM v6.6, underscoring the phenomenological relevance for precision top-quark studies and new-physics constraints.

Abstract

We present results for the production of a Z boson in association with single top at next-to-leading order (NLO), including the decay of the top quark and the Z boson. This electroweak process gives rise to the trilepton signature l+l-l'+- + jets + missing energy. We present results for this signature and show that the rate is competitive with the contribution of the mixed strong and electroweak production process, ttZ. As such it should be observable in the full data sample from LHC running at $\sqrt{s}=8$ TeV. The single top + Z process is a hitherto unconsidered irreducible background in searches for flavour changing neutral current decays of the top quark in tt production. For a selection of cuts used at the LHC involving a b-tag it is the dominant background. In an appendix we also briefly discuss the impact of NLO corrections on the related tH process.

Figures (7)

• Figure 1: Feynman graphs to calculate the lowest order amplitudes. The wavy line denotes a $W$ or $Z/\gamma^*$ boson.
• Figure 2: NLO inclusive cross sections for single and top quark pair production with and without an accompanying Z boson. The NLO $t \bar{t}Z$ cross section is estimated from the lowest order result using a $K$-factor of 1.39 and renormalization and factorization scales $\mu = m_t + m_Z/2$Kardos:2011na.
• Figure 3: Cross sections for $tZ$ and $\bar{t} Z$ production as a function of $\sqrt s$. The leading order predictions are shown as dashed lines and the next-to-leading order solid lines.
• Figure 4: Scale dependence of single top + $Z$ cross sections at $8$ TeV. The renormalization and factorization scales are varied in opposite directions according to $\mu_R = r \, m_t$, $\mu_F = m_t/r$.
• Figure 5: Comparison of jet-binned cross-sections calculated at LO at $\sqrt{s}=14$ TeV. The left-hand plots show $tZ$ production and $t\bar{t}Z$ production with the subsequent semi-leptonic decay of the top, resulting in a final state of $\mu^-\mu^+e^+$. The right-hand plots show $\bar{t}Z$ and $t\bar{t}Z$ production with the subsequent decay of the $\bar{t}$, with a final state of $\mu^-\mu^+e^-$. The first row corresponds to the standard cuts described in the text, the second row uses the $|\eta_j| < 2.0$ cuts, and the final row has the $\Delta R=0.7$ setup. The scale $\mu=m_t$ is used for $tZ$ and $\bar{t}Z$, and $\mu=m_t+m_Z/2$ is used for $t\bar{t}Z$.