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BESIII_2020_I1793431

$e^+e^-\to \pi^0\pi^0 J/\psi$ for $\sqrt{s}=3.808$ to $4.6$ GeV
Experiment: BESIII (BEPC)
Inspire ID: 1793431
Status: VALIDATED NOHEPDATA
Authors:
  • Peter Richardson
References:
  • Phys.Rev.D 102 (2020) 1, 012009
Beams: e+ e-
Beam energies: (1.9, 1.9); (1.9, 1.9); (2.0, 2.0); (2.0, 2.0); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.1, 2.1); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.2, 2.2); (2.3, 2.3); (2.3, 2.3); (2.3, 2.3) GeV
Run details:
  • e+ e- to hadrons, pi0 set stable

Measurement of the cross section for $e^+e^-\to \pi^0\pi^0 J/\psi$ for $\sqrt{s}=3.808$ to $4.6$ GeV by BESIII. The mass distributions are also measured at three energy points together with the cross section for $Z_c(3900)^0\pi^0$. As there is no PDG code for the $Z_c(3900)^0$ we take it to be the first unused exotic $c\bar{c}$ value 9030443, although this can be changed using the PID option.

Source code: BESIII_2020_I1793431.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/FinalState.hh"
  4
  5namespace Rivet {
  6
  7
  8  /// @brief e+ e- -> pi0 pi0 J/psi
  9  class BESIII_2020_I1793431 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2020_I1793431);
 14
 15
 16    /// @name Analysis methods
 17    /// @{
 18
 19    /// Book histograms and initialise projections before the run
 20    void init() {
 21      // set the PDG code
 22      _pid = getOption<double>("PID", 9030443);
 23      // projections
 24      declare(FinalState(), "FS");
 25      // histos
 26      unsigned int iloc=0;
 27      if      (isCompatibleWithSqrtS(4.226)) iloc=1;
 28      else if (isCompatibleWithSqrtS(4.236)) iloc=2;
 29      else if (isCompatibleWithSqrtS(4.244)) iloc=3;
 30      else if (isCompatibleWithSqrtS(4.258)) iloc=4;
 31      if (iloc>0) {
 32        for (unsigned int iy=0; iy<2; ++iy) {
 33          book(_h[iy], 2, iloc, 1+iy);
 34        }
 35      }
 36      book(_sigma[0], 1, 1, 1);
 37      book(_sigma[1],"TMP/numer", refData<YODA::BinnedEstimate<string>>(1,1,2));
 38      book(_sigma[2],"TMP/denom", refData<YODA::BinnedEstimate<string>>(1,1,2));
 39      book(_sigma[3], 3, 1, 1);
 40
 41      for (unsigned int ix=0; ix<3; ++ix) {
 42        for (const string& en : _sigma[ix].binning().edges<0>()) {
 43          const double end = std::stod(en)*GeV;
 44          if (isCompatibleWithSqrtS(end)) {
 45            _ecms[ix] = en;
 46            break;
 47          }
 48        }
 49      }
 50      if (_ecms[0].empty()) {
 51        MSG_ERROR("Beam energy incompatible with analysis.");
 52      }
 53    }
 54
 55
 56    /// Perform the per-event analysis
 57    void analyze(const Event& event) {
 58      Particles fs = apply<FinalState>(event, "FS").particles();
 59      Particles Jpsi, other;
 60      for (const Particle& p : fs) {
 61        Particle parent=p;
 62        while (!parent.parents().empty()) {
 63          parent=parent.parents()[0];
 64          if (parent.abspid()==PID::JPSI) break;
 65        }
 66        if (parent.abspid()!=PID::JPSI) {
 67          other.push_back(p);
 68          continue;
 69        }
 70        bool found=false;
 71        for (auto & psi : Jpsi) {
 72          // J/psi already in list
 73          if (fuzzyEquals(psi.momentum(),parent.momentum())) {
 74            found=true;
 75            break;
 76          }
 77        }
 78        if (!found) Jpsi.push_back(parent);
 79      }
 80      if (Jpsi.size()!=1 || other.size()!=2) vetoEvent;
 81      if (other[0].pid()==PID::PI0 && other[1].pid()==PID::PI0) {
 82        _sigma[0]->fill(_ecms[0]);
 83        if(!_ecms[1].empty()) _sigma[1]->fill(_ecms[1]);
 84        if (Jpsi[0].parents()[0].pid()==_pid && !_ecms[2].empty()) _sigma[3]->fill(_ecms[2]);
 85        if (_h[0]) {
 86          for (unsigned int iy=0; iy<2; ++iy) {
 87            _h[0]->fill((Jpsi[0].mom()+other[iy].mom()).mass());
 88          }
 89          _h[1]->fill((other[0].mom()+other[1].mom()).mass());
 90        }
 91      }
 92      else if (other[0].pid()==-other[1].pid() &&
 93               other[0].abspid()==PID::PIPLUS &&
 94               !_ecms[1].empty()) {
 95        _sigma[2]->fill(_ecms[1]);
 96      }
 97    }
 98
 99
100    /// Normalise histograms etc., after the run
101    void finalize() {
102      for (unsigned int iy=0; iy<2; ++iy) {
103	      if (_h[iy]) normalize(_h[iy], 1.0, false);
104      }
105      scale(_sigma, crossSection()/ sumOfWeights() /picobarn);
106      BinnedEstimatePtr<string> tmp;
107      book(tmp,1,1,2);
108      divide(_sigma[1], _sigma[2], tmp);
109    }
110
111    /// @}
112
113
114    /// @name Histograms
115    /// @{
116    int _pid;
117    Histo1DPtr _h[2];
118    BinnedHistoPtr<string> _sigma[4];
119    string _ecms[3];
120    /// @}
121
122
123  };
124
125
126  RIVET_DECLARE_PLUGIN(BESIII_2020_I1793431);
127
128}