Rivet analyses

title: BESIII_2013_I1225275

$e^+e^-\to\pi^+\pi^-J/\psi$ at $\sqrt{s}=4.26\,$GeV

Experiment: BESIII (BEPC)

Inspire ID: 1225275

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 110 (2013) 252001

Beams: e+ e-

Beam energies: (2.1, 2.1)GeV

Run details: - e+ e- to hadrons

Cross section and measurement of mass distributions for $e^+e^-\to\pi^+\pi^-J/\psi$ at $\sqrt{s}=4.26\,$GeV.

Source code:BESIII_2013_I1225275.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+e- to pi+pi- J/psi class BESIII_2013_I1225275 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2013_I1225275);


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

/// Book histograms and initialise projections before the run
void init() {
  // projections
  declare(FinalState(), "FS");
  // histograms
  book(_h_sigma, 1, 1, 1);
  for (unsigned int ix = 0; ix < 3; ++ix) {
    book(_h_mass[ix], 2, 1, 1 + ix);
  }
  book(_h_mass[3], 3, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles fs = apply<FinalState>(event, "FS").particles();
  Particles PSI, other;
  for (const Particle& p : fs) {
    Particle parent = p;
    while (!parent.parents().empty()) {
      parent = parent.parents()[0];
      if (parent.abspid() == PID::JPSI) break;
    }
    if (parent.abspid() != PID::JPSI) {
      other.push_back(p);
      continue;
    }
    bool found = false;
    for (auto& p : PSI) {
      // psi already in list
      if (fuzzyEquals(p.mom(), parent.mom())) {
        found = true;
        break;
      }
    }
    if (!found) PSI.push_back(parent);
  }
  // J/psi + 2 other particles
  if (PSI.size() != 1 || other.size() != 2) vetoEvent;
  // other particles pi+ pi-
  if (!(other[0].pid() == -other[1].pid() && other[0].abspid() == PID::PIPLUS)) vetoEvent;
  if (other[0].pid() < 0) swap(other[0], other[1]);
  // cross secrtion
  _h_sigma->fill("4.26"s);
  // fill the mass plots
  double mJpsipi[2] = {(other[0].mom() + PSI[0].mom()).mass(), (other[1].mom() + PSI[0].mom()).mass()};
  for (unsigned int ix = 0; ix < 2; ++ix) {
    _h_mass[ix]->fill(mJpsipi[ix]);
  }
  _h_mass[2]->fill((other[0].mom() + other[1].mom()).mass());
  _h_mass[3]->fill(max(mJpsipi[0], mJpsipi[1]));
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_h_sigma, crossSection() / sumOfWeights() / picobarn);
  normalize(_h_mass, 1.0, false);
}

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _h_sigma;
Histo1DPtr _h_mass[4];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2013_I1225275);

} ```