Abstract
We present the program package GoSam which is designed for the automated calculation of one-loop amplitudes for multi-particle processes in renormalisable quantum field theories. The amplitudes, which are generated in terms of Feynman diagrams, can be reduced using either D-dimensional integrand-level decomposition or tensor reduction. GoSam can be used to calculate one-loop QCD and/or electroweak corrections to Standard Model processes and offers the flexibility to link model files for theories Beyond the Standard Model. A standard interface to programs calculating real radiation is also implemented. We demonstrate the flexibility of the program by presenting examples of processes with up to six external legs attached to the loop.
Highlights
The Standard Model is currently being re-discovered at the LHC, and new exclusion limits on Beyond the Standard Model particles—and on the Higgs mass—are being delivered by the experimental collaborations with an impressive speed
Already existing excellent public tools, each containing a collection of hard-coded individual processes, like e.g. MCFM [27, 28], VBFNLO [29, 30], Monte Carlo (MC)@next-to-leading order (NLO) [31, 32], POWHEG-Box [33, 34], POWHEL [35,36,37], can be flanked by flexible automated tools such that basically any process which may turn out to be important for the comparison of LHC findings to theory can be evaluated at NLO accuracy
In order to import the model into GOSAM one needs to set the model variable in the input card to specify the keyword FeynRules in front of the directory name, where we assume that the model description is in the directory $HOME/models/MSSM_UFO
Summary
The Standard Model is currently being re-discovered at the LHC, and new exclusion limits on Beyond the Standard Model particles—and on the Higgs mass—are being delivered by the experimental collaborations with an impressive speed. As a consequence, decomposing one-loop amplitudes in terms of basic integrals becomes equivalent to reconstructing the polynomial forms of the residues to all multi-particle cuts Within this algorithm, the integrand of a given scattering amplitude, carrying complete and explicit information on the chosen dimensionalregularisation scheme, is the only input required to accomplish the task of its evaluation. The computing algorithm can proceed either diagram-by-diagram or by grouping diagrams that share a common set of denominators (suitable for a unitarity-based reduction), and it can deal with the evaluation of the rational terms either on the same footing as the rest of the amplitude, or through an independent routine which evaluates them analytically These options and the other features of GOSAM will be discussed in detail in the following. The release of GOSAM is accompanied by the generated code for some example processes, listed in Appendix A
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