Rivet analyses

Cross-section for e+e− → π0π0ψ(2S) for energies between 4.009 to 4.600 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1633425

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D97 (2018) no.5, 052001

Beams: e+ e-

Beam energies: (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.3, 2.3)GeV

Run details: - e+ e- to hadrons

Cross section for e+e− → π+π−hc for energies between 3.89 and 4.6 GeV measured by the BESIII collaboration.

Source code:BESIII_2018_I1633425.cc

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

namespace Rivet {


  /// @brief Cross-section for e+e- -> pi0pi0psi(2S) for energies between 4.009 to 4.600 GeV
  class BESIII_2018_I1633425 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1633425);


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

    /// Book histograms and initialise projections before the run
    void init() {
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_nPsi2, 1, 1, 1);
      for (const string& en : _nPsi2.binning().edges<0>()) {
        const 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 psi(2S)
        if (p.pid()==100443) {
          map<long,int> nRes = nCount;
          int ncount = ntotal;
          findChildren(p,nRes,ncount);
          // omega pi+pi-
          if (ncount!=2) continue;
          bool matched = true;
          for (const auto& val : nRes) {
            if (abs(val.first)==111) {
              if (val.second !=2) {
                matched = false;
                break;
              }
            }
            else if (val.second!=0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            _nPsi2->fill(_sqs);
            break;
          }
        }
      }
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_nPsi2, crossSection()/ sumOfWeights() /picobarn);
    }
    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _nPsi2;
    string _sqs = "";
    /// @}

  };


  RIVET_DECLARE_PLUGIN(BESIII_2018_I1633425);

}