Rivet analyses

title: ARGUS_1997_I420421

$\gamma\gamma\to\pi^+\pi^-\pi^0$ between 0.8 and 2.1 GeV

Experiment: ARGUS (DORIS)

Inspire ID: 420421

Status: VALIDATED

Authors: - Peter Richardson

References: - Z.Phys.C 74 (1997) 469-477, 1997

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, K0S and pi0 mesons must be set stable

Measurement of the differential cross section for $\gamma\gamma\to\pi^+\pi^-\pi^0$for $0.8 \text{GeV} < W < 2.1 \text{GeV}$. The cross section is measured as a function of the centre-of-mass energy of the photonic collision for the final, and a range of intermediate states.

Source code:ARGUS_1997_I420421.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief gamma gamma -> pi+pi-pi0 class ARGUS_1997_I420421 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ARGUS_1997_I420421);


/// @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, 0.8, 2.1)) {
    for (unsigned int ix = 0; ix < 5; ++ix) {
      book(_nMeson[ix], "TMP/nMeson_" + toString(ix + 1));
    }
  }
  else {
    throw Error("Invalid CMS energy for ARGUS_1997_I420421");
  }
}

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;
  }
  // check the 3 meson final state
  if (ntotal != 3) vetoEvent;
  if (nCount[PID::PI0] == 1 && nCount[PID::PIPLUS] == 1 && nCount[PID::PIMINUS] == 1) {
    _nMeson[0]->fill();
  }
  else
    vetoEvent;
  // now the intermediate states
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  // f0 and f2 mesons and rho
  bool nonRes = true;
  for (const Particle& p :
       ufs.particles(Cuts::pid == 225 || Cuts::pid == 9010221 || Cuts::abspid == 213)) {
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    int sign = p.pid() / p.abspid();
    // id of the pion not from the resonance decay
    int idother = p.abspid() == 213 ? -sign * 211 : 111;
    bool matched = true;
    for (const auto& val : nRes) {
      if (val.first == idother) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else {
        if (val.second != 0) {
          matched = false;
          break;
        }
      }
    }
    if (matched) {
      nonRes = false;
      if (p.abspid() == 213) {
        _nMeson[2]->fill();
      }
      else if (p.pid() == 225) {
        _nMeson[3]->fill();
      }
      else {
        _nMeson[4]->fill();
      }
      break;
    }
  }
  if (nonRes) _nMeson[1]->fill();
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_nMeson, crossSection() / nanobarn / sumOfWeights());
  // loop over tables in paper
  for (unsigned int ix = 0; ix < 5; ++ix) {
    unsigned int iy = 1;
    if (ix == 2)
      iy = 5;
    else if (ix == 3)
      iy = 3;
    Estimate1DPtr mult;
    book(mult, ix + 1, 1, iy);
    for (auto& b : mult->bins()) {
      if (inRange(sqrtS() / MeV, b.xMin(), b.xMax())) {
        b.setVal(_nMeson[ix]->val());
        b.setErr(_nMeson[ix]->err());
      }
    }
  }
}

/// @}


/// @name Histograms
/// @{
CounterPtr _nMeson[5];
/// @}

};

RIVET_DECLARE_PLUGIN(ARGUS_1997_I420421);

} ```