Rivet analyses

title: CELLO_1989_I267081

$\gamma\gamma\to \rho^+\rho^-$ between 1.2 and 3.0 GeV

Experiment: CELLO (PETRA)

Inspire ID: 267081

Status: UNVALIDATED

Authors: - Peter Richardson

References: - Phys.Lett.B 218 (1989) 493-498, 1989.

Beams: 22 22

Beam energies: (0.7, 0.7); (0.8, 0.8); (1.0, 1.0); (1.1, 1.1); (1.3, 1.3); (1.4, 1.4)GeV

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

Measurement of the differential cross section for $\gamma\gamma\to \rho^+\rho^-$ for $1.2 \text{GeV} < W < 3.0 \text{GeV}$. The cross section is measured as a function of the centre-of-mass energy of the photonic collision using the $\pi^+\pi^-\pi^0\pi^0$ final state.

Source code:CELLO_1989_I267081.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> rho+ rho- class CELLO_1989_I267081 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CELLO_1989_I267081);


/// @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
  book(_mult[0], 1, 1, 1);
  book(_mult[1], 2, 1, 1);
  book(_nRho, "TMP/nRho");
  for (const string& en : _mult[0].binning().edges<0>()) {
    const double eval = stod(en) * GeV;
    if (isCompatibleWithSqrtS(eval)) {
      _sqs = en;
      break;
    }
  }
  raiseBeamErrorIf(_sqs.empty());
}

void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) const {
  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;
  }
  // find any rho mesons
  Particles rho = apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == 213);
  for (size_t ix = 0; ix < rho.size(); ++ix) {
    if (rho[ix].children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(rho[ix], nRes, ncount);
    bool matched = false;
    for (size_t iy = ix + 1; iy < rho.size(); ++iy) {
      if (rho[iy].children().empty()) continue;
      if (rho[ix].pid() != -rho[iy].pid()) continue;
      map<long, int> nRes2 = nRes;
      int ncount2 = ncount;
      findChildren(rho[iy], nRes2, ncount2);
      if (ncount2 != 0) continue;
      matched = true;
      for (const auto& val : nRes2) {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
      if (matched) break;
    }
    if (matched) {
      _nRho->fill();
      break;
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_nRho, crossSection() / nanobarn / sumOfWeights());
  _mult[0]->binAt(_sqs).set(_nRho->val(), _nRho->err(), "stats");
  _mult[1]->binAt(_sqs).set(_nRho->val(), _nRho->err(), "stats");
}

/// @}


/// @name Histograms
/// @{
string _sqs = "";
CounterPtr _nRho;
BinnedEstimatePtr<string> _mult[2];
/// @}

};

RIVET_DECLARE_PLUGIN(CELLO_1989_I267081); } ```