Weights

Care has been taken to ensure that all events generated by VINCIA by default have weights which are positive and equal to unity. Indeed, this property is central both to the shower and to the matching scheme. Ordinary runs should therefore produce a simple set of unweighted events, equivalent to normal PYTHIA 8 output.

However, Vincia does allow for weighted events to be generated, in some circumstances. The simplest case is when it is used to shower a set of externally generated weighted events, in which case VINCIA normally just passes the weights through, without modification. However, in a few cases VINCIA may actively modify the input event weight, as, e.g., in the automatic evaluation of uncertainty bands.

In rare cases, VINCIA's matching scheme may cause slight violations of the Monte Carlo overestimates used to drive the evolution. In such cases, warnings are normally printed to the output, giving the magnitude of the violation as well as some minimal information about what caused the violation. Such occurrences should normally be infrequent and can then safely be ignored. Alternatively, you can force VINCIA to change the weight of the generated event in such cases, using the following flag.

flag  Vincia:allowReweighting   (default = Off)
Flag to allow VINCIA to modify event weights in case violations of its Monte Carlo overestimates occur during the event evolution.

If changes to the event weight are made, these are bookkept and the final weight is accessible through the weight() method in the Vincia plugin. For instance, if the VinciaPlugin object is called vincia the generated event weight can be accessed by

vincia.weight();

A convenient yes/no shorthand for checking whether any event weights were modified during the current run is provided by the method

bool vincia.reweightingOccurred();

Again, note that VINCIA's weights fully include any non-unity input weights associated with the event from PYTHIA (normally, PYTHIA's weights are unity, but, as mentioned, when using external events, e.g., from LHEF files, these can in principle come with non-unity and/or even negative weights). The weight VINCIA started from is not modified and remains accessible through PYTHIA's own info.weight() method.