[Paper Review] Probing top-quark chromomagnetic dipole moment at next-to-leading order in QCD

Diogo Buarque Franzosi, Cen Zhang|arXiv (Cornell University)|Mar 30, 2015

Particle physics theoretical and experimental studies14 citations

TL;DR

This paper presents next-to-leading order (NLO) QCD corrections to top-quark pair production mediated by an anomalous chromomagnetic dipole moment (CMDM), using the MadGraph5_aMC@NLO framework with parton shower matching. The NLO corrections increase the CMDM contribution by ~50% at the LHC and significantly reduce scale uncertainties, leading to improved limits on the CMDM coefficient: $-0.32 < C_{tG} < 0.30$ at 95% CL for $\Lambda = 1$ TeV.

ABSTRACT

We present predictions at NLO accuracy in QCD for top-quark pair production induced by an anomalous chromomagnetic dipole moment of the top quark. Our results are obtained for total as well as fully differential cross sections, including matching to parton shower simulations. This process is expected to provide the most stringent direct limits on top-quark chromomagnetic dipole moment. We find that NLO corrections increase the contribution from the dipole moment by about 50\% at the LHC, and significantly reduce the renormalization and factorization scale dependence. Using the NLO prediction, we update the current limit from the Tevatron and the LHC measurements. Apart from total cross section, we also study other observables relevant for LHC phenomenology.

Motivation & Objective

• To compute NLO QCD corrections for top-quark pair production induced by an anomalous chromomagnetic dipole moment (CMDM), addressing the lack of higher-order corrections in existing literature.
• To reduce theoretical uncertainties from renormalization and factorization scale dependence in CMDM signal predictions, which are large at leading order (LO).
• To provide fully differential cross sections matched to parton showers for use in LHC phenomenology, including both stable and decayed top quark final states.
• To update and tighten direct experimental limits on the top-quark CMDM using current Tevatron and LHC data, leveraging improved NLO accuracy.
• To enable future global analyses of top-quark couplings in the effective field theory framework by automating flavor-diagonal operators in MadGraph5_aMC@NLO.

Proposed method

• The calculation is performed using the MadGraph5_aMC@NLO framework, which handles automated matrix element and parton shower matching for exclusive final states.
• The anomalous CMDM is modeled via the dimension-six operator $ O_{tG} = \bar{t} \sigma^{\mu\nu} T^A t \, G_{\mu\nu}^A $, with coupling $ C_{tG} $, added to the QCD Lagrangian.
• NLO QCD corrections are computed for both total cross sections and fully differential distributions, including $ t\bar{t} $ production with decayed top quarks.
• The framework preserves spin correlations via the MadSpin package, enabling accurate modeling of angular distributions of decay products.
• Scale uncertainties are evaluated by varying the renormalization and factorization scales, and the K-factor (NLO/LO) is computed for each observable.
• The results are compared to LO predictions to assess the impact of QCD corrections on kinematic distributions and signal significance.

Experimental results

Research questions

• RQ1 How do NLO QCD corrections affect the total cross section for top-quark pair production via an anomalous chromomagnetic dipole moment?
• RQ2 To what extent do NLO corrections reduce scale uncertainties in CMDM signal predictions compared to LO?
• RQ3 How do NLO QCD corrections modify the shape of differential distributions such as $ \Delta\phi(\ell\ell) $, $ \cos\theta^* $, and $ \cos\theta_1\cos\theta_2 $?
• RQ4 Can the SM K-factor be reliably used to rescale LO CMDM contributions, or is a full NLO calculation necessary?
• RQ5 What are the updated 95% confidence level limits on the top-quark CMDM coefficient $ C_{tG} $ using Tevatron and LHC8 data at NLO accuracy?

Key findings

• NLO QCD corrections increase the CMDM-induced cross section by approximately 12% at the Tevatron, 43% at LHC 8 TeV, and 48% at LHC 14 TeV.
• The inclusion of NLO corrections significantly reduces scale dependence, decreasing theoretical uncertainties by a factor of two or more in most distributions.
• The differential K-factor is not constant but remains similar in magnitude to the SM K-factor (≈1.5) across all studied observables.
• For the $ \Delta\phi(\ell\ell) $ distribution, the NLO correction enhances the signal shape, and using LO for the anomalous contribution while using NLO for the SM leads to an underestimation of the signal.
• The updated 95% confidence level limit on the CMDM coefficient is $ -0.32 < C_{tG} < 0.30 $ for $ \Lambda = 1$ TeV, corresponding to $ -0.0096 < d_V < 0.0090 $ in physical dipole moment units.
• The framework enables precise modeling of spin-correlated observables, and the NLO corrections do not significantly alter the shape of distributions like $ \cos\theta^* $, indicating a uniform correction effect.