Rivet analyses

title: BABAR_2018_I1647139

Cross section for $e^+e^-\to\eta\pi^+\pi^-$ between 1.2 and 3.5 GeV

Experiment: BABAR (PEP-II)

Inspire ID: 1647139

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev. D97 (2018) 052007

Beams: e+ e-

Beam energies: ANY

Run details: - e+e- to hadrons

Measurement of the cross section for $e^+e^-\to\eta\pi^+\pi^-$ at energies between 1.2 and 3.5 GeV using radiative events.

Source code:BABAR_2018_I1647139.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Add a short analysis description here class BABAR_2018_I1647139 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2018_I1647139);


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

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

  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  book(_numEtaPiPi, "TMP/EtaPiPi");
}


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 omega
    if (p.pid() == 221) {
      map<long, int> nRes = nCount;
      int ncount = ntotal;
      findChildren(p, nRes, ncount);
      // eta pi+pi-
      if (ncount != 2) continue;
      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) _numEtaPiPi->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {

  double sigma = _numEtaPiPi->val();
  double error = _numEtaPiPi->err();
  sigma *= crossSection() / sumOfWeights() / nanobarn;
  error *= crossSection() / sumOfWeights() / nanobarn;
  Estimate1DPtr mult;
  book(mult, 1, 1, 1);
  for (auto& b : mult->bins()) {
    if (inRange(sqrtS() / GeV, b.xMin(), b.xMax())) {
      b.set(sigma, error);
    }
  }
}

/// @}


/// @name Histograms
/// @{
CounterPtr _numEtaPiPi;
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2018_I1647139);

} ```