Rivet analyses

title: BELLE_2010_I862260

$\gamma\gamma\to\eta\eta$ for centre-of-mass energies between 1.096 and 3.3 GeV

Experiment: BELLE (KEKB)

Inspire ID: 862260

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 82 (2010) 114031

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the differential cross section for $\gamma\gamma\to\eta\eta$ for $1.096 \text{GeV} < W < 3.3 \text{GeV}$. 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 $\eta$ scattering angle are measured.

Source code:BELLE_2010_I862260.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> eta eta class BELLE_2010_I862260 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2010_I862260);


/// @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, 1.096, 4.)) {
    book(_sigmaEtaEta[0], "TMP/nEtaPi_1", refData(1, 1, 1));
    if (sqrtS() <= 2.) book(_sigmaEtaEta[1], "TMP/nEtaPi_2", refData(1, 1, 2));
    double sMin = 1.096, sMax = 1.12, step = 0.04;
    unsigned int ihist = 2;
    while (sMin < 3.3) {
      if (inRange(sqrtS() / GeV, sMin, sMax)) {
        break;
      }
      sMin = sMax;
      sMax += step;
      ihist += 1;
      if (fuzzyEquals(2.4, sMin)) step = 0.1;
    }
    if (ihist <= 43) book(_h_cTheta, ihist, 1, 1);
  }
  else {
    throw Error("Invalid CMS energy for BELLE_2010_I862260");
  }
}

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 UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  Particles etas = ufs.particles(Cuts::pid == PID::ETA);
  if (etas.size() < 2) vetoEvent;
  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;
  }
  for (unsigned int ix = 0; ix < etas.size(); ++ix) {
    if (etas[ix].children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(etas[ix], nRes, ncount);
    for (unsigned int iy = ix + 1; iy < etas.size(); ++iy) {
      if (etas[iy].children().empty()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(etas[iy], nRes2, ncount2);
      if (ncount2 != 0) continue;
      bool matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) {
        double cTheta = abs(etas[iy].momentum().z() / etas[iy].momentum().p3().mod());
        if (cTheta <= 0.9) _sigmaEtaEta[0]->fill(sqrtS());
        if (_sigmaEtaEta[1]) _sigmaEtaEta[1]->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);
  for (unsigned int ix = 0; ix < 2; ++ix) {
    if (!_sigmaEtaEta[ix]) continue;
    scale(_sigmaEtaEta[ix], fact);
    Estimate1DPtr tmp;
    book(tmp, 1, 1, 1 + ix);
    barchart(_sigmaEtaEta[ix], tmp);
  }
}
/// @}


/// @name Histograms
/// @{
Histo1DPtr _sigmaEtaEta[2], _h_cTheta;
/// @}

};

RIVET_DECLARE_PLUGIN(BELLE_2010_I862260);

} ```