# Process computation¶

Functions for process computation:

`compute_process_rcl` (npr,p,order) |
Computes the structure-dressed helicity amplitudes and the summed squared amplitude (unless polarizations specified, see `define_process_rcl()` ) for the process with process number `npr` . |

`compute_colour_correlation_rcl` (…) |
Computes the colour-correlated summed squared amplitude, between particle with leg number `i1` and particle with leg number `i2` , for the process with process number `npr` . |

`compute_spin_correlation_rcl` (npr,p,j,v,order=) |
Computes the spin-correlated summed squared amplitude, where the polarization vector of particle `j` , necessarily being a gluon or photon, is is substituted for the user-defined polarization vector `v` . |

`compute_spin_colour_correlation_rcl` (…) |
Computes the spin-colour-correlated summed squared amplitude, between particle with leg number `i1` and particle with leg number `i2` , for the process with process number `npr` . |

`rescale_process_rcl` (npr,order) |
Adjusts the results calculated by `compute_process_rcl()` for a new value of \(\alpha_\mathrm{s}\), rescaling the structure-dressed helicity amplitudes and recomputing the summed squared amplitude for the process with process number `npr` . |

`get_amplitude_rcl` (…) |
Extracts a specific contribution to the amplitude of the process with process number `npr` , according to the values of `pow` , `order` , `colour` and `hel` . |

`get_squared_amplitude_rcl` (…) |
Extracts the computed value of the summed squared amplitude with `pow` being order in the fundamental couplings at loop-order `order` for the process with process number `npr` . |

## Conventions for momenta¶

The momenta passed to Recola are standard lorentz vectors. For a \(2\to n\) process the first two momenta are considered incoming and need to have positive energy while the other \(n\) are outgoing and, thus, have to fulfill the four momentum conservation:

\[p_1^\mu + p_2^\mu = \sum_{k=3}^{n+2} p_k^\mu\]

For decay processes \(1\to n\) the four-momentum conservaion reads:

\[p_1^\mu = \sum_{k=2}^{n+1} p_k^\mu\]

Recola2 provides a RAMBO module to generate random PSP for any beam initial and final state:

`set_outgoing_momenta_rcl` (npr,pIn) |
Random phase space generation given incoming momenta. |