Description of the method

The method consist to use a sample of events (weighted or unweighted events) and associate to those events a new weight corresponding to a new theoretical hypothesis. This weight can be propagate through the all simulation chain in order to avoid to have to perform the full-simulation on a huge number of sample. This methods works only if the original hypothesis and the new one are both significant in the same part of the phase-space. the new weight is given by

$$W_{new} = |M_{new}|^{2 /|M_{old}|}2 * W_{old} $$

Technical details.

1. The two theoretical hypothesis should differ ONLY by difference in the model parameters. So this should be the same process, the same model and the same pdf.
2. The scale choice for the reweighting is done events by events and is given by the value in the LHE file. The same scale is used for both Hypothesis
3. In presence of decay chain the order of the particle in the events file is important. This is important if you want to use this tools with LHE events not produced by MadGraph/MadEvent.