Rivet analyses

title: KLOE2_2014_I1317236

Form factor for the decay $\phi\to\eta e^+e^-$

Experiment: KLOE2 (DAPHNE)

Inspire ID: 1317236

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B742 (2015) 1-6

Beams: * *

Beam energies: ANY

Run details: - phi production with both the dalitz decay and radiative decay for normalization

Measurement of the form factor $\left|F_{\phi\eta }(q^2)\right|$ for the decay $\phi\to\eta e^+e^-$ by the KLOE experiment at DAPHNE

Source code:KLOE2_2014_I1317236.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief form factor for phi-> eta class KLOE2_2014_I1317236 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(KLOE2_2014_I1317236);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  // Book histograms
  book(_h_m, 1, 1, 1);
  book(_weight, "TMP/weight");
}

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


/// Perform the per-event analysis
void analyze(const Event& event) {
  static double me = 0.5109989461 * MeV;
  static double mphi = 1019.461 * MeV;
  static double meta = 547.862 * MeV;

  // Loop over phi mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 333)) {
    unsigned nstable(0), neta(0), nep(0), nem(0), ngamma(0);
    FourMomentum ptot;
    findDecayProducts(p, nstable, neta, nep, nem, ngamma, ptot);
    if (nstable == 3 && nem == 1 && nem == 1 && neta == 1) {
      double q = ptot.mass();
      double beta = sqrt(1. - 4 * sqr(me / q));
      double p = sqrt(sqr(1. + sqr(q) / (sqr(mphi) - sqr(meta)))
                      - 4. * sqr(mphi * q / (sqr(mphi) - sqr(meta))));
      double fact = beta * MeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
      _h_m->fill(q / MeV, 1. / fact);
    }
    else if (nstable == 2 && ngamma == 1 && neta == 1) {
      _weight->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  static double alpha = 7.2973525664e-3;
  scale(_h_m, 1.5 * M_PI / alpha / *_weight);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_m;
CounterPtr _weight;
/// @}

};

RIVET_DECLARE_PLUGIN(KLOE2_2014_I1317236);

} ```