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A proposal for a standard interface between Monte Carlo tools and one-loop programs

T. Binoth, F. Boudjema, G. Dissertori, A. Lazopoulos, A. Denner, S. Dittmaier, R. Frederix, N. Greiner, S. Hoche, W. Giele, P. Skands, J. Winter, T. Gleisberg, J. Archibald, G. Heinrich, F. Krauss, D. Maitre, M. Huber, J. Huston, N. Kauer, F. Maltoni, C. Oleari, G. Passarino, R. Pittau, S. Pozzorini, T. Reiter, S. Schumann, G. Zanderighi

TL;DR

The paper argues for a standardized interface between Monte Carlo tree-level tools and one-loop amplitude programs to enable efficient, automated NLO predictions for complex multi-leg processes. It proposes a two-phase MC/OLP model (initialisation and run-time) using Les Houches–style order and contract files, plus a runtime API (OLP_EvalSubProcess) to pass kinematics and retrieve the Laurent coefficients and Born terms. The authors detail QCD and EW considerations, including how to handle IR/UV divergences, regularisation schemes, and resonance treatments, with explicit guidance for both the QCD-dominated and electroweak extensions. The outline envisions modular, interoperable components that can speed up NLO computations and encourages community feedback and public implementations, while noting future steps like interfacing with parton showers.

Abstract

Many highly developed Monte Carlo tools for the evaluation of cross sections based on tree matrix elements exist and are used by experimental collaborations in high energy physics. As the evaluation of one-loop matrix elements has recently been undergoing enormous progress, the combination of one-loop matrix elements with existing Monte Carlo tools is on the horizon. This would lead to phenomenological predictions at the next-to-leading order level. This note summarises the discussion of the next-to-leading order multi-leg (NLM) working group on this issue which has been taking place during the workshop on Physics at TeV colliders at Les Houches, France, in June 2009. The result is a proposal for a standard interface between Monte Carlo tools and one-loop matrix element programs.

A proposal for a standard interface between Monte Carlo tools and one-loop programs

TL;DR

The paper argues for a standardized interface between Monte Carlo tree-level tools and one-loop amplitude programs to enable efficient, automated NLO predictions for complex multi-leg processes. It proposes a two-phase MC/OLP model (initialisation and run-time) using Les Houches–style order and contract files, plus a runtime API (OLP_EvalSubProcess) to pass kinematics and retrieve the Laurent coefficients and Born terms. The authors detail QCD and EW considerations, including how to handle IR/UV divergences, regularisation schemes, and resonance treatments, with explicit guidance for both the QCD-dominated and electroweak extensions. The outline envisions modular, interoperable components that can speed up NLO computations and encourages community feedback and public implementations, while noting future steps like interfacing with parton showers.

Abstract

Many highly developed Monte Carlo tools for the evaluation of cross sections based on tree matrix elements exist and are used by experimental collaborations in high energy physics. As the evaluation of one-loop matrix elements has recently been undergoing enormous progress, the combination of one-loop matrix elements with existing Monte Carlo tools is on the horizon. This would lead to phenomenological predictions at the next-to-leading order level. This note summarises the discussion of the next-to-leading order multi-leg (NLM) working group on this issue which has been taking place during the workshop on Physics at TeV colliders at Les Houches, France, in June 2009. The result is a proposal for a standard interface between Monte Carlo tools and one-loop matrix element programs.

Paper Structure

This paper contains 14 sections, 7 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Modular structure of one-loop computations
  3. A computational model for an interface
  4. Proposal for the implementation of the MC/OLP interface
  5. The initialisation phase
  6. The order file
  7. The contract file
  8. The run-time phase
  9. Extending the interface to include EW OLP
  10. Treatment of resonances
  11. Electroweak regularisation scheme
  12. Electroweak renormalisation scheme
  13. The run-time phase
  14. Summary and outlook

Figures (1)

  • Figure 1: Modular structure of next-to-leading order computations for partonic processes. All structures related to tree amplitudes can be evaluated using LO MC tools. The one-loop module contains the UV renormalised interference term. The treatment of IR subtraction should be kept separate to allow for flexibility. The IR modules contain subtraction terms, ${\cal S}_j$, for the real-emission part and their integrated variants which compensate IR divergences in the One-Loop Module. In case of collinear initial-state divergences, collinear subtraction terms, ${\cal C}$, have to be provided too. Subsequently, the contributions in each horizontal line are independently finite after summation.