Rivet analyses

title: SND_2020_I1800477

Cross section for $e^+e^-\to\eta\pi^0\gamma$ between 1.05 and 2 GeV

Experiment: SND (VEPP-2000)

Inspire ID: 1800477

Status: VALIDATED

Authors: - Peter Richardson

References: - arXiv: 2006.05465

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons

Cross section for $e^+e^-\to\eta\pi^0\gamma$ between 1.05 and 2 GeV by SND. The $\omega\eta$ and radiative subprocesses are also measured.

Source code:SND_2020_I1800477.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+e- > eta pi0 gamma class SND_2020_I1800477 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(SND_2020_I1800477);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_c_omega, "/TMP/omega", refData(1, 1, 1));
  book(_c_total, "/TMP/total", refData(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;
  }
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty()) continue;
    // find the eta
    if (p.pid() != 221) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // check eta pi0 gamma FS
    if (ncount != 2) continue;
    bool matched = true;
    for (const auto& val : nRes) {
      if (abs(val.first) == 22 || val.first == 111) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (!matched) continue;
    _c_total->fill(sqrtS());
    // check if from omega
    for (const Particle& p2 : ufs.particles()) {
      if (p2.pid() != 223) continue;
      map<long, int> nResB = nRes;
      int ncountB = ncount;
      findChildren(p2, nResB, ncountB);
      if (ncountB != 0) continue;
      bool matched2 = true;
      for (const auto& val : nResB) {
        if (val.second != 0) {
          matched2 = false;
          break;
        }
      }
      if (matched2) _c_omega->fill(sqrtS());
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  double fact = crossSection() / picobarn / sumOfWeights();
  scale(_c_total, fact);
  Estimate1DPtr mult;
  book(mult, 1, 1, 1);
  barchart(_c_total, mult);
  scale(_c_omega, fact);
  book(mult, 3, 1, 1);
  barchart(_c_omega, mult);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _c_total, _c_omega;
/// @}

};

RIVET_DECLARE_PLUGIN(SND_2020_I1800477);

} ```