Rivet analyses

title: KLOE_2009_I818106

Mass spectrum of $\eta\pi$ in $\phi\to\eta\pi^0\gamma$ decays

Experiment: KLOE (DAPHNE)

Inspire ID: 818106

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B681 (2009) 5-13

Beams: * *

Beam energies: ANY

Run details: - Any process producing phi mesons

Mass spectra for $\eta\pi$ in $\phi$ decays to $\eta\pi^0\gamma$ measured by KLOE. Useful for teting the treatment of the a_0(980) meson.

Source code:KLOE_2009_I818106.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief phi -> eta pi0 gamma class KLOE_2009_I818106 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(KLOE_2009_I818106);


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

/// Book histograms and initialise projections before the run
void init() {
  declare(UnstableParticles(), "UFS");
  book(_h_etapi, 1, 1, 1);
  book(_nPhi, "TMP/PhiCounter");
}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       unsigned int& neta,
                       unsigned int& npi,
                       unsigned int& ngamma,
                       FourMomentum& ptot) {
  for (const Particle& p : mother.children()) {
    int id = p.pid();
    if (id == PID::ETA) {
      ++neta;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::PI0) {
      ++npi;
      ++nstable;
      ptot += p.momentum();
    }
    else if (id == PID::GAMMA) {
      ++ngamma;
      ++nstable;
    }
    else if (id == PID::PIPLUS || id == PID::PIMINUS) {
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, neta, npi, ngamma, ptot);
    }
    else
      ++nstable;
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {

  // Loop over phis
  for (const Particle& phi : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == PID::PHI)) {
    _nPhi->fill();
    unsigned int nstable(0), neta(0), npi(0), ngamma(0);
    FourMomentum p_tot(0, 0, 0, 0);
    findDecayProducts(phi, nstable, neta, npi, ngamma, p_tot);
    if (nstable != 3) continue;
    if (neta == 1 && npi == 1 && ngamma == 1) {
      _h_etapi->fill(p_tot.mass() / MeV);
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  // normalise to total no of phi mesons
  // and mult by 10^7 due normalisation in paper
  scale(_h_etapi, 1. / _nPhi->sumW() * 1e7);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_etapi;
CounterPtr _nPhi;
/// @}

};

RIVET_DECLARE_PLUGIN(KLOE_2009_I818106);

} ```