Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

Abstract: We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

• The paper presents a comparative analysis of merging algorithms (CKKW, MLM, dipole cascade) that integrate parton showers with matrix elements.
• It shows that variations in merging scales cause up to 40% differences in jet spectra, highlighting key systematic uncertainties.
• The study confirms that systematic effects observed at the Tevatron extend to LHC energies, supporting the robustness of these merging strategies.

Merging Algorithms for Parton Showers and Matrix Elements in Hadronic Collisions

The paper "Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions" presents an in-depth analysis of different schemes for combining fixed-order tree-level matrix element generators with parton showers. The context of this research is situated within the domain of high-energy particle physics, focusing on the prediction of multi-jet events in hadronic collisions as observed at experiments such as those conducted at the Tevatron and the LHC.

Overview

The authors investigate three prominent merging schemes: CKKW, MLM, and a variation of the CKKW using the dipole cascade. Each scheme represents a strategy to manage the inherent complexity of separating hard matrix elements and softer parton shower components in multi-jet processes, particularly when multiple hard scales are involved. These merging schemes are evaluated with respect to their implementation in various event generators, including ALPGEN, SHERPA, ARIADNE, HELAC, and MadGraph, each coupled with different showering mechanisms.

Key Findings

1. Consistency and Differences: The study reveals that while all the investigated approaches aim to achieve consistent descriptions of multi-jet events, there are notable differences in their predictions. This becomes particularly evident in comparisons of absolute rates and the shapes of kinematic distributions. Such differences are attributed to the unique handling of Sudakov reweighting and jet vetoing across the algorithms.
2. Systematic Variations: Across implementations, systematic uncertainties arise from variations in the merging scales and renormalization factors. Notably, the scale changes yield sizeable impacts on observables such as jet spectra, which can shift by up to 40%, illustrating the sensitivity of these predictions to initial conditions and the level of approximation inherent in the merging procedure.
3. Performance Across Energies: The paper demonstrates that the systematic uncertainties for these schemes at the Tevatron are mirrored at the LHC, suggesting that they are robust across different energy scales. This consistency supports their use in extrapolating predictions from Tevatron to LHC conditions, with potential adjustments based on experimental data.

Implications

The choice of merging strategy significantly affects the precision of multi-jet predictions, a critical aspect for background characterizations in new physics searches and Standard Model analyses at hadron colliders. The detailed comparison presented in the paper is crucial for the experimental collaborations, aiding in tuning various event generators to match measured data.

Future Directions

This study lays the groundwork for further refinement of merging algorithms. Moving forward, a logical step would be to integrate these predictions with next-to-leading-order (NLO) corrections, which could potentially mitigate some of the observed discrepancies at large transverse momenta. Moreover, integrating these methodologies with more sophisticated frameworks that account for soft gluon resummation could enhance their predictive power.

In conclusion, the comparative evaluation performed in this paper provides valuable insights into the operational intricacies and performance metrics of different merging schemes. It underscores the necessity for systematic approaches in assessing the uncertainties inherent in theoretical predictions for complex hadronic events at colliders like the LHC. As the field progresses, the established benchmark will undoubtedly facilitate ongoing efforts toward precision phenomenology in particle physics.