Merge pull request GENIE-MC#452 from GENIE-MC/bugfix/res_stored-phspvar

correctly record the phase space used in RES calculator

Author: idkakorin

src/Physics/Resonance/EventGen/RESKinematicsGenerator.cxx

@@ -167,7 +167,6 @@ void RESKinematicsGenerator::ProcessEventRecord(GHepRecord * evrec) const
 
      //-- Computing cross section for the current kinematics
      xsec = fXSecModel->XSec(interaction, kPSWQD2fE);
-
      //-- Decide whether to accept the current kinematics
      if(!fGenerateUniformly) 
      {
@@ -195,8 +194,13 @@ void RESKinematicsGenerator::ProcessEventRecord(GHepRecord * evrec) const
         double M = init_state.Tgt().HitNucP4().M();
         kinematics::WQ2toXY(E,M,gW,gQ2,gx,gy);
 
-        // set the cross section for the selected kinematics
-        evrec->SetDiffXSec(xsec,kPSWQ2fE);
+        // set the cross section for the selected kinematics.
+        // we're converting here to the more familiar "W*Q2" space
+        // rather than the "W*Q2D" (precomputed dipole) space that's used above
+        // for generation efficiency in the accept-reject loop
+        double J = kinematics::Jacobian(interaction, kPSWQD2fE, kPSWQ2fE);
+        xsec *= J;
+        evrec->SetDiffXSec(xsec, kPSWQ2fE);
 
         // for uniform kinematics, compute an event weight as
         // wght = (phase space volume)*(differential xsec)/(event total xsec)