[Rivet] Born and dressed level in Rivet

[David Grellscheid]

Hi Tim,

> However the issue here is that we have many measurement which did use
> only identified FSR photons in their dressing cones. 

If there is a procedure by which you can identify FSR photons in
experimental data, it can be used in Rivet. That operational definition
of FSR would then differ from truth-FSR in the same way that the
operational definition of a gluon jet is not the same thing as a jet
coming from an MC-truth gluon.

The usual classification that MC generators make into ISR and FSR is a
purely technical convenience in the codes, with no physical reality, as
Jon was saying. This makes it very dependent on the actual generator
used, and impossible to implement sensibly in Rivet.

  David