Rivet analyses

title: BELLE_2018_I1672149

$\gamma\gamma\to\eta^\prime\pi^+\pi^-$ for centre-of-mass energies between 1.4 and 3.8 GeV

Experiment: BELLE (KEKB)

Inspire ID: 1672149

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 7, 072001

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the cross section for $\gamma\gamma\to\eta^\prime\pi^+\pi^-$ for centre-of-mass energies between 1.4 and 3.8 GeV. The resonance subprocess $\gamma\gamma\to f_2(1270)\eta^\prime$ is alos measured, and subtracted from the final result above 2.26 GeV. The contribution from $\eta_c(1)$ is also subtracted between 2.62,3.06 GeV.

Source code:BELLE_2018_I1672149.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> eta' pi+pi- class BELLE_2018_I1672149 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2018_I1672149);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  // counters
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_sigma[ix], "TMP/sigma_" + toString(ix + 1), refData(1 + ix, 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()] -= 1;
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& fs = apply<FinalState>(event, "FS");
  // find the final-state particles
  map<long, int> nCount;
  int ntotal(0);
  for (const Particle& p : fs.particles()) {
    nCount[p.pid()] += 1;
    ++ntotal;
  }
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  // first check for f_2 eta'
  bool foundf2(false);
  Particles f2s = ufs.particles(Cuts::pid == 225);
  Particles etaps = ufs.particles(Cuts::pid == 331);
  for (const Particle& f2 : f2s) {
    bool matched = false;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(f2, nRes, ncount);
    for (const Particle& etap : etaps) {
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(etap, nRes2, ncount2);
      if (ncount2 != 0) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        foundf2 = true;
        break;
      }
    }
    if (foundf2) break;
  }
  // if we have the f_2 eta' state
  if (foundf2) {
    _sigma[0]->fill(sqrtS());
    if (sqrtS() > 2.26 * GeV) vetoEvent;
  }
  // see if we have eta' pi+pi-
  bool foundetap(false);
  for (const Particle& etap : etaps) {
    bool matched = false;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(etap, nRes, ncount);
    if (ncount != 2) continue;
    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) {
      foundetap = true;
      break;
    }
  }
  // check there's no eta_c
  if (foundetap && sqrtS() > 2.62 && sqrtS() < 3.06) {
    for (const Particle& etac : ufs.particles(Cuts::pid == 441)) {
      bool matched = false;
      map<long, int> nRes = nCount;
      int ncount = ntotal;
      findChildren(etac, nRes, ncount);
      for (const auto& val : nRes) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        foundetap = false;
        break;
      }
    }
  }
  if (foundetap) _sigma[1]->fill(sqrtS());
}

/// Normalise histograms etc., after the run
void finalize() {
  const double fact = crossSection() / nanobarn / sumOfWeights();
  // loop over tables in paper
  for (unsigned int ix = 0; ix < 2; ++ix) {
    scale(_sigma[ix], fact);
    Estimate1DPtr tmp;
    book(tmp, ix + 1, 1, 1);
    barchart(_sigma[ix], tmp);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _sigma[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BELLE_2018_I1672149);

} ```