HELAC-NLO

Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program that is capable of numerically evaluating QCD virtual corrections to scattering amplitudes. A detailed presentation of the algorithm is given, along with instructions to run the code and benchmark results. The program is part of the HELAC-NLO framework that allows for a complete evaluation of QCD NLO corrections.Program summaryProgram title:HELAC-1LOOPCatalogue identifier: AEOC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOC_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 290945No. of bytes in distributed program, including test data, etc.: 3013326Distribution format: tar.gzProgramming language: Fortran (gfortran(http://gcc.gnu.org/fortran/), lahey95 (http://www.lahey.com), ifort3(http://software.intel.com)).Computer: Any.Operating system: Linux, Unix, Mac OS.Classification: 11.1.Nature of problem:The evaluation of virtual one-loop amplitudes for multi-particle scattering is a long-standing problem [1]. In recent years the OPP reduction technique [2] opened the road for a fully numerical approach based on the evaluation of the one-loop amplitude for well-defined values of the loop momentum.Solution method:By using HELAC [3-5] and CutTools [6], HELAC-1LOOP is capable of evaluating QCD virtual corrections [7]. The one-loop n-particle amplitudes are constructed as part of the n+2 tree-order ones, by using the basic recursive algorithm used in HELAC. A Les Houches Event (LHE) file is produced, combining the complete information from tree-order and virtual one-loop contributions. In conjunction with real corrections, obtained with the use of HELAC-DIPOLES [8], the full NLO corrections can be computed. The program has been successfully used in many applications.Running time:Depending on the number of particles and generated events from seconds to days.References:[1]R.K. Ellis, Z. Kunszt, K. Melnikov and G. Zanderighi, arXiv:1105.4319[hepph].[2]G. Ossola, C. G. Papadopoulos and R. Pittau, Nucl. Phys. B 763 (2007) 147 [arXiv:hep-ph/0609007].[3]A. Kanaki and C. G. Papadopoulos, Comput. Phys. Commun. 132 (2000) 306 [arXiv:hep-ph/0002082].[4]C. G. Papadopoulos, Comput. Phys. Commun. 137 (2001) 247 [arXiv:hepph/ 0007335].[5]A. Cafarella, C. G. Papadopoulos and M. Worek, Comput. Phys. Commun. 180 (2009) 1941 [arXiv:0710.2427 [hep-ph]].[6]G. Ossola, C. G. Papadopoulos and R. Pittau, JHEP 0803 (2008) 042 [arXiv:0711.3596 [hep-ph]].[7]A. van Hameren, C. G. Papadopoulos and R. Pittau, JHEP 0909, 106 (2009) [arXiv:0903.4665 [hep-ph]].[8]M. Czakon, C. G. Papadopoulos and M. Worek, JHEP 0908, 085 (2009) [arXiv:0905.0883 [hep-ph]].