Rivet analyses

title: BESIII_2018_I1651451

Cross section for $e^+e^- \to \phi\pi^+\pi^-$ and $\phi\pi^0\pi^0$ at 2.125 GeV

Experiment: BESIII (BEPC)

Inspire ID: 1651451

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 99 (2019) 1, 011101

Beams: e+ e-

Beam energies: (1.1, 1.1)GeV

Run details: - e+e- to hadrons

Measurement of the cross section for $e^+e^-\to\phi\pi^+\pi^-$ and $\phi\pi^0\pi^0$ at 2.125 GeV.

Source code:BESIII_2018_I1651451.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- -> phi pi+ pi- and phi pi0 pi0 class BESIII_2018_I1651451 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1651451);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  // histograms
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_h[ix], 1, 1, ix + 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()] -= 1;
      --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");
  for (const Particle& p : ufs.particles(Cuts::pid == PID::PHI)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // phi pi+pi-
    if (ncount == 2) {
      bool matched = true;
      for (const auto& val : nRes) {
        if (abs(val.first) == 211) {
          if (val.second != 1) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        _h[0]->fill("2.125"s);
        break;
      }
      matched = true;
      for (const auto& val : nRes) {
        if (abs(val.first) == 111) {
          if (val.second != 2) {
            matched = false;
            break;
          }
        }
        else if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        _h[1]->fill("2.125"s);
        break;
      }
    }
  }
}


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

/// @}


/// @name Histograms
/// @{
BinnedHistoPtr<string> _h[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1651451);

} ```