Single-top t-channel hadroproduction in the four-flavour scheme with POWHEG and aMC@NLO (1207.5391v2)

Abstract: We present results for the QCD next-to-leading order (NLO) calculation of single-top t-channel production in the 4-flavour scheme, interfaced to Parton Shower (PS) Monte Carlo programs according to the POWHEG and MC@NLO methods. Comparisons between the two methods, as well as with the corresponding process in the 5-flavour scheme are presented. For the first time results for typical kinematic distributions of the spectator-b jet are presented in an NLO+PS approach.

• The paper introduces an NLO+PS analysis of t-channel single-top production in the 4F scheme using POWHEG and aMC@NLO.
• It compares the 4F approach, where b-quarks emerge via gluon splitting, with the 5F scheme, revealing notable differences in kinematic distributions.
• The study validates advanced Monte Carlo techniques and offers insights to improve top-quark decay simulations and new physics searches at colliders.

Single-Top $t$-Channel Hadroproduction in the Four-Flavour Scheme Using POWHEG and aMC@NLO

The paper of single-top (or antitop) production, specifically in the $t$-channel, provides crucial insights into the electroweak interactions of the top quark and potential signatures of Beyond Standard Model (BSM) physics. This paper introduces a comprehensive numerical analysis of single-top $t$-channel hadroproduction in the 4-flavour scheme using two distinct methods: POWHEG and aMC@NLO, interfaced with Parton Shower (PS) Monte Carlo programs. The analysis also compares these methods with calculations in the 5-flavour scheme, highlighting differences and similarities significant to kinematic properties, including spectator-$b$ jet observables.

Methodology and Theoretical Framework

The paper addresses a comparison between simulations achieved within the 4-flavour and 5-flavour schemes. In the 4-flavour scheme, the $b$ quarks are not part of the initial-state parton distribution functions (PDFs); they emerge through gluon splitting, which allows direct implementation of $b$-quark mass in matrix elements. Conversely, the 5-flavour scheme considers $b$ quarks in the PDFs, contributing to the resummation of large logarithms, though it neglects explicit mass considerations due to factorisation needs.

By employing advanced Monte Carlo event generators, this research leverages the POWHEG and aMC@NLO frameworks—each known for its competent handling of higher-order corrections and parton shower interface. These methods have previously shown robustness in simulating single-top processes and are thus crucial for this NLO+PS analysis.

Results and Observations

The numerical results, implemented at both the Tevatron and LHC, demonstrate good agreement between POWHEG and aMC@NLO for inclusive observable and kinematic parameters of the top quark and the hardest light jet. However, there are observable differences in the modeling of the kinematic distributions of the second-hardest $b$ jet, which is instrumental in distinguishing single-top events from background processes like $t\bar{t}$ production. This discrepancy is attributed to the method through which POWHEG and aMC@NLO implement higher-order corrections and parton showers.

Moreover, in the 5-flavour scheme's context, the paper highlights potential mismodeling in the low transverse momentum region due to the intrinsic shower description, as exhibited by Pythia's unphysical predictions. This is notably absent in the 4-flavour scheme's predictions, providing a compelling argument for the distinct treatment of $b$ jets offered by the 4-flavour approach.

Implications and Future Directions

This investigation strengthens the existing theoretical infrastructure for predicting single-top production processes at hadron colliders. The findings underscore the utility of integrating accurate NLO corrections with parton showers to enhance the fidelity of simulated particle interactions, particularly in discriminating new physics models from Standard Model backgrounds.

Looking forward, researchers can build on these results by incorporating complete top-quark decay channels within the matrix-element framework to further refine predictions at the NLO+PS level. The paper's methodologies also serve as a benchmark for upcoming developments in multi-flavour scheme analyses, as explorations in this field move towards achieving even higher precision for future collider experiments.

The implementation code and results, being publicly available, facilitate transparency and reproducibility, encouraging further engagement and cross-validation within the high-energy physics research community. The continued advancement of these computational techniques will undoubtedly contribute to the elucidation of complex single-top phenomena and beyond.