This will eventually need modifications to support newer Sherpa releases, eventually by adding #ifdef conditions.

We rely here on the fact that PHOTONS++ adds the new FSR photons at the end of the blob. We assume this wont change in the future. We need to check also how to obtain the info from l+l- produced by virtual-photon splitting in future Sherpa releases.

Based on the review by Marek, the order of particles in the Blob and the "S" flag will most likely remain as they are. As for the l+l- from virtual photons, we will have to adapt the code once Sherpa-3.0.0 is out.

I chatted with @jback about this. I think it would be good to enable the user to change the generator settings via decay file. But this is to be added in a subsequent merge request.

Can we make a comment about what the integers 0 and 1 mean here?

Provide a comment about what the -1 integer is for.

Provide a comment to say this sets the state as "initial"?

Comment to say this is a "final" particle?

Do we need "eventually" here?

If we want to do this now, then perhaps something like the following could work (but I have not tested this):

#include "EvtGenBase/EvtSymTable.hh"

else if ( fsrGenerator == "Sherpa" ) {

   // Define Sherpa integer flags (say what they are)
   // Integer flag ierr set when retrieving decay file parameters
   int S1{1}, S2{0}, ierr{0};
   // Retrieve parameters from decay file.
   const std::string S1Name = EvtSymTable::get( "SherpaFSR1", ierr );
   const std::string S2Name = EvtSymTable::get( "SherpaFSR2", ierr );  

   // Update S1 and S2 parameters if they have been defined (may need to use string compare function here...)
   if ( S1Name != "SherpaFSR1" ) {
      S1 = atoi( S1Name.c_str() );
   }
   if ( S2Name != "SherpaFSR2" ) {
      S2 = atoi( S2Name.c_str() );
   }
   // Setup the Sherpa Photons model
   radCorrEngine = genList.getSherpaPhotonsModel( 1e-7, S1, S2 );  
}

Could also add the cut-off as a parameter.

The ierr flag is a dummy control integer used by the EvtSymTable class:

https://gitlab.cern.ch/evtgen/evtgen/-/blob/master/src/EvtGenBase/EvtSymTable.cpp#L55

It is always set to 0 and is not updated to be 1 (or -1) for "failed" when the decay parameter string is not found. When it fails, it just returns the parameter word and not the number. For the above code, if "Define SherpaFSR1 2" is set in the decay file, then the returned string will be "2", which is then converted to an integer. If this is not found, then the returned string will be "SherpaFSR1", which is not the integer string, and so the integer we declared earlier is not modified.

This is from the original code that Marek pointed us to. I am leaving it as it is.

In principle a good idea and I gave it a try, but it doesn't work yet because we need to first create the EvtGen object and then pass the decay file via readUDecay to have the EvtSymTable::get function retrieving the value. However, we need to setup the radCorrEngine to create the EvtGen object. So it is a sort of chicken and egg problem. So I think we need to leave this to a future differential.