Rivet analyses

Cross section for e+e− → π0π0hc for $\sqrt{s}=4.23$, 4.26 and 4.30 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1318650

Status: VALIDATED NOHEPDATA SINGLEWEIGHT

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 113 (2014) 21, 212002

Beams: e+ e-

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

Run details: - e+ e- to hadrons, pi0 set stable

Measurement of the cross section for e+e− → π0π0hc for $\sqrt{s}=4.23$, 4.26 and 4.30 GeV. The cross section for e+e− → Zxc(4020)0π0 → π0π0hc is also measured. As there is no PDG code for the Zc(4020)0 we take it to be the first unused exotic cc̄ value 9030443, although this can be changed using the PID option.

Source code:BESIII_2014_I1318650.cc

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

namespace Rivet {


  /// @brief e+ e- > pi0 pi0 hc
  class BESIII_2014_I1318650 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2014_I1318650);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // set the PDG code
      _pid = getOption<double>("PID", 9030443);
      // projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // histograms
      for (size_t ix=0; ix<2; ++ix) {
        book(_sigma[ix], 1, 1, 1+ix);
      }

      for (const string& en : _sigma[0].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) {
      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 UnstableParticles & ufs = apply<UnstableParticles>(event, "UFS");
      // loop over any h_c
      for (const Particle& hc : ufs.particles(Cuts::pid==10443)) {
        if (hc.children().empty()) continue;
        // find the h_c
        map<long,int> nRes = nCount;
        int ncount = ntotal;
        findChildren(hc,nRes,ncount);
        // h_c pi0 pi0
        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) {
          _sigma[0]->fill(_sqs);
          if (hc.parents().empty()) break;
          Particle Zc = hc.parents()[0];
          if (Zc.pid()==_pid && Zc.children().size()==2 &&
             (Zc.children()[0].pid()==PID::PI0 ||
              Zc.children()[1].pid()==PID::PI0)) _sigma[1]->fill(_sqs);
          break;
        }
      }
    }


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

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _sigma[2];
    string _sqs = "";
    int _pid;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2014_I1318650);

}