Rivet analyses

title: CMD2_1999_I483994

Cross section for $e^+e^-\to2\pi^+2\pi^-$ and $\pi^+\pi^-2\pi^0$ below 1.38 GeV

Experiment: CMD2 (VEPP-2M)

Inspire ID: 483994

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B466 (1999) 392-402, 199 9

Beams: e+ e-

Beam energies: ANY

Run details: - e+e- to hadrons

Measurement of the cross section for $e^+e^-\to2\pi^+2\pi^-$ and $\pi^+\pi^-2\pi^0$ below 1.38 GeV

Source code:CMD2_1999_I483994.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- -> 4 pions class CMD2_1999_I483994 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CMD2_1999_I483994);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_ncharged, 1, 1, 1);
  book(_nneutral, 2, 1, 1);
  book(_nomega, 3, 1, 1);
}

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;
  }
  if (ntotal == 4) {
    if (nCount[211] == 2 && nCount[-211] == 2)
      _ncharged->fill(round(sqrtS() / MeV));
    else if (nCount[211] == 1 && nCount[-211] == 1 && nCount[111] == 2)
      _nneutral->fill(round(sqrtS() / MeV));
  }


  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty()) continue;
    // find the omega
    if (p.pid() == 223) {
      map<long, int> nRes = nCount;
      int ncount = ntotal;
      findChildren(p, nRes, ncount);
      // omega pi+pi-
      if (ncount != 1) continue;
      bool matched = true;
      for (const auto& val : nRes) {
        if (abs(val.first) == 111) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) _nomega->fill(round(sqrtS() / MeV));
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  double fact = crossSection() / sumOfWeights() / nanobarn;
  scale(_ncharged, fact);
  scale(_nneutral, fact);
  scale(_nomega, fact);
}

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<int> _ncharged, _nneutral, _nomega;
/// @}

};

RIVET_DECLARE_PLUGIN(CMD2_1999_I483994);

} ```