Calculation of HELAS amplitudes for QCD processes using graphics processing unit (GPU)

We use a graphics processing unit (GPU) for fast calculations of helicity amplitudes of quark and gluon scattering processes in massless QCD. New HEGET (HELAS Evaluation with GPU Enhanced Technology) codes for gluon self-interactions are introduced, and a C++ program to convert the MadGraph generated FORTRAN codes into HEGET codes in CUDA (a C-platform for general purpose computing on GPU) is created. Because of the proliferation of the number of Feynman diagrams and the number of independent color amplitudes, the maximum number of final state jets we can evaluate on a GPU is limited to 4 for pure gluon processes (gg→4g), or 5 for processes with one or more quark lines such as $q\overline{q}\rightarrow 5g$ and qq→qq+3g. Compared with the usual CPU-based programs, we obtain 60–100 times better performance on the GPU, except for 5-jet production processes and the gg→4g processes for which the GPU gain over the CPU is about 20.