Rivet analyses

title: CRYSTAL_BALL_1986_I217547

$\gamma\gamma\to\eta\pi^0$ for centre-of-mass energies between 0.683 and 2.08 GeV

Experiment: CRYSTAL_BALL (PETRA)

Inspire ID: 217547

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 33 (1986) 1847

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 $0.683 \text{GeV} < W < 2.08 \text{GeV}$. 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:CRYSTAL_BALL_1986_I217547.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> etapi0 class CRYSTAL_BALL_1986_I217547 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CRYSTAL_BALL_1986_I217547);


/// @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, 0.683, 2.08)) {
    book(_nEtaPi, "TMP/nEta", refData(1, 1, 1));
    if (inRange(sqrtS() / GeV, 0.9, 1.1)) {
      book(_h_cTheta, 2, 1, 1);
    }
    else if (inRange(sqrtS() / GeV, 1.1, 1.480)) {
      book(_h_cTheta, 2, 1, 2);
    }
  }
  else {
    throw Error("Invalid CMS energy for CRYSTAL_BALL_1986_I217547");
  }
}

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 <= 0.9) _nEtaPi->fill(sqrtS() / MeV);
      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(_nEtaPi, fact);
  Estimate1DPtr tmp;
  book(tmp, 1, 1, 1);
  barchart(_nEtaPi, tmp);
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(CRYSTAL_BALL_1986_I217547);

} ```