[Paper Review] Top-quark processes at NLO in production and decay

John M. Campbell, R. Keith Ellis|arXiv (Cornell University)|Apr 6, 2012

Particle physics theoretical and experimental studies34 references18 citations

TL;DR

This paper presents a next-to-leading-order (NLO) QCD calculation of top-quark production and decay processes in the MCFM Monte Carlo framework, treating the top quark as on-shell to preserve spin correlations. It includes full NLO QCD corrections in both production and decay, retains the full $b$-quark mass dependence, and provides the first NLO treatment of $t$-channel single-top production with decay. The method enables precise phenomenological predictions for LHC and Tevatron observables.

ABSTRACT

We describe the implementation of top production and decay processes in the parton-level Monte Carlo program MCFM. By treating the top quark as being on-shell, we can factorize the amplitudes for top-pair production, s-channel single-top production, and t-channel single-top production into the product of an amplitude for production and an amplitude for decay. In this way we can retain all spin correlations. Both the production and the decay amplitudes are calculated consistently at next-to-leading order in alpha_s. The full dependence on the b-quark mass is also kept. Phenomenological results are presented for various kinematic distributions at the LHC and for the top quark forward-backward asymmetry at the Tevatron.

Motivation & Objective

• To develop a consistent NLO QCD framework for top-quark production and decay processes at hadron colliders.
• To preserve spin correlations by factorizing production and decay amplitudes under the on-shell top quark approximation.
• To include full $b$-quark mass dependence in both production and decay amplitudes at NLO.
• To extend existing NLO treatments to the $t$-channel single-top production process with decay, previously unaddressed at NLO.
• To provide phenomenologically accurate predictions for LHC and Tevatron observables, including kinematic distributions and forward-backward asymmetry.

Proposed method

• Factorize top production and decay amplitudes into separate, on-shell top quark processes to preserve spin correlations.
• Implement full NLO QCD corrections in both production and decay amplitudes using the $\overline{\text{MS}}$ scheme and dimensional regularization.
• Retain the full $b$-quark mass dependence in matrix elements via the Kleiss-Steinhaeuser parametrization for $W$-boson decay.
• Use counterterms to handle infrared and ultraviolet divergences in real gluon emissions during top decay.
• Ensure consistent inclusion of NLO effects in the top quark width by combining virtual and real corrections in the decay amplitude.
• Apply the four-flavor scheme throughout, with consistent treatment of $b$-quark final states in top decays.

Experimental results

Research questions

• RQ1 How can NLO QCD corrections be consistently applied to both top-quark production and decay while preserving spin correlations?
• RQ2 What is the impact of including the full $b$-quark mass in NLO top-quark decay amplitudes on kinematic distributions?
• RQ3 What are the phenomenological consequences of NLO corrections in $t$-channel single-top production with decay?
• RQ4 How do NLO QCD corrections to the top quark width affect the total width and decay branching ratios?
• RQ5 To what extent do off-shell $W$-boson effects in top decay influence the final state distributions at NLO?

Key findings

• The paper presents the first NLO QCD calculation of $t$-channel single-top production with full decay, including spin correlations and $b$-quark mass effects.
• The inclusion of the $b$-quark mass has a modest computational cost due to the simple structure of the matrix elements.
• Phenomenological results show significant NLO corrections to kinematic distributions in $t\bar{t}$ and single-top production at the LHC.
• The forward-backward asymmetry in top quark production at the Tevatron is updated with full NLO QCD corrections in both production and decay.
• The NLO top quark width calculation is consistent with known results and includes corrections from $W$-boson off-shellness and $b$-quark mass effects.
• The method enables extension to more complex processes such as $t\bar{t}H$, $t\bar{t}Z$, and $t\bar{t}W$ at NLO with full decay treatment.