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Update of the Binoth Les Houches Accord for a standard interface between Monte Carlo tools and one-loop programs

S. Alioli, S. Badger, J. Bellm, B. Biedermann, F. Boudjema, G. Cullen, A. Denner, H. van Deurzen, S. Dittmaier, R. Frederix, S. Frixione, M. V. Garzelli, S. Gieseke, E. W. N. Glover, N. Greiner, G. Heinrich, V. Hirschi, S. Hoeche, J. Huston, H. Ita, N. Kauer, F. Krauss, G. Luisoni, D. Maitre, F. Maltoni, P. Nason, C. Oleari, R. Pittau, S. Plaetzer, S. Pozzorini, L. Reina, C. Reuschle, T. Robens, J. Schlenk, M. Schoenherr, F. Siegert, J. F. von Soden-Fraunhofen, F. Tackmann, F. Tramontano, P. Uwer, G. Salam, P. Skands, S. Weinzierl, J. Winter, V. Yundin, G. Zanderighi, M. Zaro

TL;DR

The paper addresses the need for a standardized interface between Monte Carlo tools and one-loop amplitude providers to enable automated NLO calculations. It introduces BLHA2, expanding the original BLHA1 with capabilities to pass dynamic parameters, define models (including UFO), handle unstable phase-space points, accommodate mixed coupling powers and jet-multiplicity merging, and exchange color/spin-correlated amplitudes. It provides concrete mechanisms such as OLP_SetParameter, OLP_EvalSubProcess2, OLP_Polvec, and new workflow primitives, along with example order files to illustrate usage. The proposed update aims to increase interoperability and flexibility across MCs and OLPs, facilitating broader, more reliable NLO predictions for LHC phenomenology and beyond.

Abstract

We present an update of the Binoth Les Houches Accord (BLHA) to standardise the interface between Monte Carlo programs and codes providing one-loop matrix elements.

Update of the Binoth Les Houches Accord for a standard interface between Monte Carlo tools and one-loop programs

TL;DR

The paper addresses the need for a standardized interface between Monte Carlo tools and one-loop amplitude providers to enable automated NLO calculations. It introduces BLHA2, expanding the original BLHA1 with capabilities to pass dynamic parameters, define models (including UFO), handle unstable phase-space points, accommodate mixed coupling powers and jet-multiplicity merging, and exchange color/spin-correlated amplitudes. It provides concrete mechanisms such as OLP_SetParameter, OLP_EvalSubProcess2, OLP_Polvec, and new workflow primitives, along with example order files to illustrate usage. The proposed update aims to increase interoperability and flexibility across MCs and OLPs, facilitating broader, more reliable NLO predictions for LHC phenomenology and beyond.

Abstract

We present an update of the Binoth Les Houches Accord (BLHA) to standardise the interface between Monte Carlo programs and codes providing one-loop matrix elements.

Paper Structure

This paper contains 18 sections, 5 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Existing features of the interface
  3. New features of the interface
  4. Passing parameters
  5. Defining the model
  6. Treatment of unstable phase-space points
  7. Different powers of coupling constants, merging different jet multiplicities
  8. Extras
  9. Electroweak corrections
  10. An example order file
  11. Colour- and spin correlated tree amplitudes
  12. Restrictions such as diagram filters, exploitation of special symmetries, etc.
  13. Conclusions
  14. List of valid keywords for order/contract file
  15. Required keywords
  16. ...and 3 more sections

Figures (6)

  • Figure 1: Illustration of an interplay between Monte Carlo program and One Loop Provider (OLP). In the pre-runtime phase of the interface, the OLP receives an order file placed by the MC and checks availability of the contents. Then it returns a contract file to the MC where the contents are confirmed if available. At runtime, the Monte Carlo program provides Born, real radiation part minus infrared subtraction terms and integrated subtraction terms. The OLP provides the virtual amplitude for each phase space point. The phase space integration is done by the Monte Carlo program.
  • Figure 2: Example of a BLHA1 order file for the process $Z+$jet, created by Sherpa.
  • Figure 3: Example of a BLHA1 contract file for the process $Z+$jet, created by GoSam. As the original interface did not define a standard way how to pass parameters, the definition of masses and widths is marked as " Ignored by OLP" in the contract file, while the parameters are passed in a non-standardised way.
  • Figure 4: List of keywords to define parameters to be passed by the function OLP_SetParameter.
  • Figure 5: Example of the part of an order file containing different settings for different sets of subprocesses.
  • ...and 1 more figures