Rivet analyses

Cross section for e+e− → ηπ+π− between 1.22 and 2 GeV

Experiment: SND (VEPP-2M)

Inspire ID: 1332929

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D91 (2015) 052013, 2015

Beams: e+ e-

Beam energies: (0.6, 0.6); (0.6, 0.6); (0.6, 0.6); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.7, 0.7); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.8, 0.8); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (0.9, 0.9); (1.0, 1.0); (1.0, 1.0); (1.0, 1.0); (1.0, 1.0); (1.0, 1.0)GeV

Run details: - e+e- to hadrons

Measurement of the cross section for e+e− → ηπ+π− at energies between 1.22 and 2 GeV.

Source code:SND_2015_I1332929.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief e+e- > eta pi+pi-
  class SND_2015_I1332929 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(SND_2015_I1332929);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_numEtaPiPi, 1, 1, 1);
      for (const string& en : _numEtaPiPi.binning().edges<0>()) {
        double eval = stod(en) * GeV;
        if (isCompatibleWithSqrtS(eval)) {
          _sqs = en;
          break;
        }
      }
      raiseBeamErrorIf(_sqs.empty());
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) const {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          --nRes[child.pid()];
          --ncount;
        }
        else
          findChildren(child, nRes, ncount);
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const FinalState& fs = apply<FinalState>(event, "FS");

      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      for (const Particle& p : ufs.particles()) {
        if (p.children().empty()) continue;
        // find the eta
        if (p.pid() == 221) {
          map<long, int> nRes = nCount;
          int ncount = ntotal;
          findChildren(p, nRes, ncount);
          // eta pi+pi-
          if (ncount != 2) continue;
          bool matched = true;
          for (const auto& val : nRes) {
            if (abs(val.first) == 211) {
              if (val.second != 1) {
                matched = false;
                break;
              }
            }
            else if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) _numEtaPiPi->fill(_sqs);
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_numEtaPiPi, crossSection() / sumOfWeights() / nanobarn);
    }

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _numEtaPiPi;
    string _sqs = "";
    /// @}
  };


  RIVET_DECLARE_PLUGIN(SND_2015_I1332929);
}