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Manuscript Title: Numerical evaluation of multi-gluon amplitudes for High Energy Factorization
Authors: M. Bury, A. van Hameren
Program title: AMP4HEF
Catalogue identifier: AEXF_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 196(2015)592
Programming language: Fortran 2003, implemented at least as far as in gfortran- 4.6.
Computer: Any computer with the required Fortran compiler.
Operating system: Any operating system with the required Fortran compiler.
RAM: Negligible
Keywords: QCD, amplitudes, Off-shell.
Classification: 11.1.

Nature of problem:
The numerical evaluation of matrix elements applicable in factorization prescriptions for the cross section calculation of hadron scattering processes that require off-shell partonic initial states.

Solution method:
The generalization of the recursiveprocessrequest.pl method of Britto, Cachazo, Feng and Witten [1] to amplitudes with off-shell gluons [2] is applied, both to obtain expressions which are hard-coded in the program, and numerically.

Matrix element summed over color and spin can be evaluated for up to 6 external gluons with up to 2 of them off-shell. Color-ordered amplitudes can be evaluated essentially for an arbitrary number of external gluons, with up to 2 of them off-shell.

Running time:
The evaluation of the matrix element for a single phase space point summed over colors and helicities takes 0.005ms for 4 gluons with 2 of them off-shell, 0.1ms for 5 gluons with 2 of them off-shell, and 2.7ms for 6 gluons with 2 of them off-shell. This is on a single 2.30 GHz Intel Core i7, without any compiler optimization.

[1] R. Britto, F. Cachazo, B. Feng, and E. Witten, Direct proof of tree-level recursion relation in Yang-Mills theory, Phys.Rev.Lett. 94 (2005) 181602, [hep-th/0501052].
[2] A. van Hameren, BCFW recursion for off-shell gluons, JHEP 1407 (2014) 138, [1404.7818].