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Rivet analyses reference

MC_XS

MC analysis for process total cross section
Experiment: ()
Status: VALIDATED
Authors:
  • Marek Schoenherr
  • Christian Gutschow
No references listed
Beams: * *
Beam energies: ANY
Run details:
  • Suitable for any process.

Analysis for bookkeeping of the total cross section, number of generated events and the ratio of events with positive and negative weights. This analysis will not produce sensible distributions in case of correlated NLO sub-events.

Source code: MC_XS.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/AnalysisHandler.hh"

#ifndef RIVET_ENABLE_HEPMC_3
#include "HepMC/HepMCDefs.h"
#endif

namespace Rivet {

  /// @brief Analysis for the generated cross section
  class MC_XS : public Analysis {
  public:

    /// @name Constructors etc.
    //@{

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(MC_XS);

    //@}


  public:

    /// @name Analysis methods
    //@{

    /// Book histograms and initialise projections before the run
    void init() {
      /// @todo Convert to Scatter1D or Counter
      book(_h_XS, "XS");
      book(_h_N, "N", 1, 0.0, 1.0);
      book(_h_pmXS, "pmXS", 2, -1.0, 1.0);
      book(_h_pmN, "pmN", 2, -1.0, 1.0);

      _mc_xs = _mc_error = 0.;
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {

      #if defined RIVET_ENABLE_HEPMC_3
      //@todo HepMC3::GenCrossSection methods aren't const accessible :(
      RivetHepMC::GenCrossSection gcs = *(event.genEvent()->cross_section());
      _mc_xs    = gcs.xsec();
      _mc_error = gcs.xsec_err();
      #elif defined HEPMC_HAS_CROSS_SECTION
      _mc_xs    = event.genEvent()->cross_section()->cross_section();
      _mc_error = event.genEvent()->cross_section()->cross_section_error();
      #endif // VERSION_CODE >= 3000000

      assert(event.weights().size() >= handler().numWeights());
      for (size_t m = 0; m < handler().numWeights(); ++m) {
        const double weight = event.weights()[m];
        _h_pmXS.get()->_getPersistent(m)->fill(0.5*(weight > 0 ? 1. : -1), abs(weight));
        _h_pmN.get()->_getPersistent(m)->fill(0.5*(weight > 0 ? 1. : -1), 1.);
        _h_N.get()->_getPersistent(m)->fill(0.5, 1.0);
      }
      
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h_pmXS, crossSection()/sumOfWeights());
      #ifndef HEPMC_HAS_CROSS_SECTION
      _mc_xs = crossSection();
      _mc_error = 0.0;
      #endif
      _h_XS->addPoint(0, _mc_xs, 0.5, _mc_error);
    }

    //@}


  private:

    /// @name Histograms
    //@{
    Scatter2DPtr _h_XS;
    Histo1DPtr _h_pmXS, _h_pmN, _h_N;
    double _mc_xs, _mc_error;
    //@}

  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(MC_XS);

}