Rivet analyses

title: BESIII_2019_I1724671

Cross section for $e^+e^-\to\gamma X(3872)$ between 4 and 4.6 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1724671

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 122 (2019) 23, 232002

Beams: e+ e-

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

Run details: - e+ e-

Measurement of the cross section for $e^+e^-\to\gamma X(3872)$ times the branching ratio into the $\omega J\psi$ and $J/\psi\pi^+\pi^-$ modes for $X(3872)$. There is no consensus as to the nature of the $X(3872)$ $c\bar{c}$ state and therefore we taken its PDG code to be 9030443, i.e. the first unused code for an undetermined spin one $c\bar{c}$ state. This can be changed using the PID option if a different code is used by the event generator performing the simulation.

Source code:BESIII_2019_I1724671.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+ e- -> gamma X(3872) class BESIII_2019_I1724671 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1724671);


/// @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");
  // counters
  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);
    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");
  int mode = -1;
  for (const Particle& psi : ufs.particles(Cuts::pid == 443)) {
    if (psi.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(psi, nRes, ncount);
    // check for J/psi pi+pi- mode
    if (ncount == 3) {
      bool matched = true;
      for (const auto& val : nRes) {
        if (abs(val.first) == 211 || val.first == PID::GAMMA) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        mode = 1;
        break;
      }
    }
    bool matched = false;
    for (const Particle& omega : ufs.particles(Cuts::pid == 223)) {
      if (omega.children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(omega, nRes2, ncount2);
      if (ncount2 != 1) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.first == PID::GAMMA) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) break;
    }
    if (matched) {
      mode = 0;
      break;
    }
  }
  if (mode < 0) vetoEvent;
  // find the intermediate
  for (const Particle& XX : ufs.particles(Cuts::pid == _pid)) {
    if (XX.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(XX, nRes, ncount);
    if (ncount != 1) continue;
    bool matched = true;
    for (const auto& val : nRes) {
      if (val.first == PID::GAMMA) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (matched) {
      _sigma[mode]->fill(_sqs);
      break;
    }
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BESIII_2019_I1724671);

} ```