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Matrix Elements and Parton Showers in Hadronic Interactions

F. Krauss

TL;DR

The paper addresses merging tree-level matrix elements with parton showers for multi-jet final states in hadronic collisions. It extends a previously developed ME+PS framework from e+e- to hadron collisions by partitioning radiation with a k_T measure, reweighting MEs with PS histories, and applying vetoes to avoid double counting, aiming to preserve NLL jet rates. The method yields a process-independent algorithm that reproduces the full tree-level ME plus all leading and next-to-leading log contributions, with initial-state backward evolution and PDF handling. While promising and implemented in related generators, formal proofs and full implementation in a dedicated event generator are still in progress, with potential impact on more reliable multi-jet predictions and background estimates.

Abstract

A method is suggested to combine tree level QCD matrix for the production of multi jet final states and the parton shower in hadronic interactions. The method follows closely an algorithm developed recently for the case of $e^+e^-$ annihilations.

Matrix Elements and Parton Showers in Hadronic Interactions

TL;DR

The paper addresses merging tree-level matrix elements with parton showers for multi-jet final states in hadronic collisions. It extends a previously developed ME+PS framework from e+e- to hadron collisions by partitioning radiation with a k_T measure, reweighting MEs with PS histories, and applying vetoes to avoid double counting, aiming to preserve NLL jet rates. The method yields a process-independent algorithm that reproduces the full tree-level ME plus all leading and next-to-leading log contributions, with initial-state backward evolution and PDF handling. While promising and implemented in related generators, formal proofs and full implementation in a dedicated event generator are still in progress, with potential impact on more reliable multi-jet predictions and background estimates.

Abstract

A method is suggested to combine tree level QCD matrix for the production of multi jet final states and the parton shower in hadronic interactions. The method follows closely an algorithm developed recently for the case of annihilations.

Paper Structure

This paper contains 11 sections, 22 equations, 1 figure.

Table of Contents

  1. Introduction: ME vs. PS
  2. ME+PS in $e^+e^-$ annihilations
  3. Jet rates at NLL accuracy
  4. Differential jet rates and the ME weight
  5. Vetoing the PS
  6. ME+PS for hadron collisions
  7. Some brief reminders
  8. Proposed algorithm
  9. Constructing the ME weight
  10. Vetoed showers
  11. Summary and outlook

Figures (1)

  • Figure 1: Pictorial representation of contributions to single gluon emission in $e^+e^-\to q\bar{q}g$ as given by the ME and the PS. As indicated, the PS does not take into account the interference contributions present in the ME. This leads to the ratio $ME/PS$ as depicted in the contour plot. At the soft and collinear boundaries of the phase space in the $x_1$-$x_2$ plane (where $x_i$ corresponds to the energy fraction of the quark and antiquark) the PS correctly reproduces the ME, whereas in the region of hard gluon emission the PS omits the (destructive) interference contribution.