Rivet analyses

title: BESIII_2020_I1800404

Charged particle multiplicity distributions in $J/\psi$ and $\chi_{c0,1,2}$ decays

Experiment: BESIII (BEPC)

Inspire ID: 1800404

Status: VALIDATED

Authors: - Peter Richardson

References: - arXiv: 2006.05588

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/Psi, and chi_{0,1,2}c, originally e+e-

Measurement of the charged particle multiplicity distributions in $J/\psi$ and $\chi_{c0,1,2}$ decays by the BES collaboration.

Source code:BESIII_2020_I1800404.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/Psi chi_c charged particle multiplicities class BESIII_2020_I1800404 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2020_I1800404);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(Cuts::pid == 443 or Cuts::pid == 445 or Cuts::pid == 10441
                            or Cuts::pid == 20443),
          "UFS");
  // histograms
  book(_h_chi0, 1, 1, 1);
  book(_h_chi1, 1, 1, 2);
  book(_h_chi2, 1, 1, 3);
  book(_h_jpsi[0], 2, 1, 1);
  book(_h_jpsi[1], 2, 1, 2);
}

void findChildren(const Particle& p, int& nCharged) {
  for (const Particle& child : p.children()) {
    if (child.children().empty()) {
      if (PID::isCharged(child.pid())) ++nCharged;
    }
    else
      findChildren(child, nCharged);
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // loop over particles
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
    // skip radiative modes
    if (p.children().size() == 2) {
      if (p.children()[0].pid() == PID::GAMMA) {
        if (p.children()[1].pid() == PID::GAMMA || p.children()[1].pid() == PID::JPSI) continue;
      }
      if (p.children()[1].pid() == PID::GAMMA) {
        if (p.children()[0].pid() == PID::GAMMA || p.children()[0].pid() == PID::JPSI) continue;
      }
    }
    // get the charged particle multiplicity
    int nCharged(0);
    findChildren(p, nCharged);
    if (p.pid() == PID::JPSI) {
      _h_jpsi[0]->fill(nCharged);
      _h_jpsi[1]->fill(nCharged);
    }
    else if (p.pid() == 10441)
      _h_chi0->fill(nCharged);
    else if (p.pid() == 20443)
      _h_chi1->fill(nCharged);
    else if (p.pid() == 445)
      _h_chi2->fill(nCharged);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // percentage and bin width
  normalize(_h_jpsi[0], 100.);
  normalize(_h_jpsi[1], 100.);
  normalize(_h_chi0, 100.);
  normalize(_h_chi1, 100.);
  normalize(_h_chi2, 100.);
}

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<int> _h_jpsi[2], _h_chi0, _h_chi1, _h_chi2;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2020_I1800404);

} ```