Rivet analyses

title: BELLE_2009_I822474

$\gamma\gamma\to\eta\pi^0$ for centre-of-mass energies between 0.84 and 4.0 GeV

Experiment: BELLE (KEKB)

Inspire ID: 822474

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 80 (2009) 032001, 2009

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, pi0 must be set stable

Measurement of the differential cross section for $\gamma\gamma\to\eta\pi^0$ for $0.84 \text{GeV} < W < 4.0 \text{GeV}$. Both the cross section as a function of the centre-of-mass energy of the photonic collision, and the differential cross section with respect to the pion scattering angle are measured.

Source code:BELLE_2009_I822474.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> eta pi0 class BELLE_2009_I822474 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2009_I822474);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(FinalState(), "FS");
  declare(UnstableParticles(), "UFS");
  // book histos
  if (inRange(sqrtS() / GeV, 0.84, 4.)) {
    book(_sigmaEtaPi, "TMP/nEtaPi", refData(1, 1, 1));
    double sMin = 0.84, step = 0.02;
    unsigned int ihist = 2;
    while (sMin < 4.) {
      if (inRange(sqrtS() / GeV, sMin, sMin + step)) {
        break;
      }
      sMin += step;
      ihist += 1;
      if (fuzzyEquals(1.6, sMin))
        step = 0.04;
      else if (fuzzyEquals(2.4, sMin))
        step = 0.1;
    }
    book(_h_cTheta, ihist, 1, 1);
  }
  else {
    throw Error("Invalid CMS energy for BELLE_2009_I822474");
  }
}

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 FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles(Cuts::pid == PID::ETA)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    if (ncount != 1) continue;
    bool matched = true;
    for (const auto& val : nRes) {
      if (val.first == PID::PI0) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (matched) {
      double cTheta = abs(p.momentum().z() / p.momentum().p3().mod());
      if (cTheta <= 0.8) _sigmaEtaPi->fill(sqrtS());
      if (_h_cTheta) _h_cTheta->fill(cTheta);
      break;
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  const double fact = crossSection() / nanobarn / sumOfWeights();
  if (_h_cTheta) scale(_h_cTheta, fact);
  scale(_sigmaEtaPi, fact);
  Estimate1DPtr tmp;
  book(tmp, 1, 1, 1);
  barchart(_sigmaEtaPi, tmp);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _sigmaEtaPi, _h_cTheta;
/// @}

};

RIVET_DECLARE_PLUGIN(BELLE_2009_I822474);

} ```