Rivet analyses

title: A2_2017_I1498079

Form factor for the decay $\pi^0\to\gamma e^+e^-$

Experiment: A2 (MAMI)

Inspire ID: 1498079

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.C 95 (2017) 2, 025202

Beams: * *

Beam energies: ANY

Run details: - pi0 production with both the dalitz decay and radiative decay for normalization

Measurement of the form factor $\left|F_{\pi^0}(q^2)\right|$ for the decay $\pi^0\to\gamma e^+e^-$ by the A2 experiment at MAMI.

Source code:A2_2017_I1498079.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief form factors in pi0->gamma gamma(e+e-) class A2_2017_I1498079 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(A2_2017_I1498079);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  book(_h_pi0, 1, 1, 1);
  book(_weight_pi0, "TMP/weight_pi0");
}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       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::GAMMA && p.children().empty()) {
      ++ngamma;
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, nep, nem, ngamma, ptot);
    }
    else
      ++nstable;
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static double me = 0.5109989461 * MeV;
  static double mpi = 134.9770 * MeV;

  // Loop over pi0 and omega mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 111)) {
    unsigned nstable(0), nep(0), nem(0), ngamma(0);
    FourMomentum ptot;
    findDecayProducts(p, nstable, nep, nem, ngamma, ptot);
    if (nstable == 3 && nem == 1 && nem == 1 && ngamma == 1) {
      double q = ptot.mass();
      double beta = sqrt(1. - 4 * sqr(me / q));
      double p = 1. - sqr(q / mpi);
      double fact = beta * MeV / q * (1. + 2. * sqr(me / q)) * pow(p, 3);
      _h_pi0->fill(q / MeV, 1. / fact);
    }
    else if (nstable == 2 && ngamma == 2) {
      _weight_pi0->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  static double alpha = 7.2973525664e-3;
  scale(_h_pi0, 0.75 * M_PI / alpha / *_weight_pi0);
}

///@}


/// @name Histograms
///@{
Histo1DPtr _h_pi0;
CounterPtr _weight_pi0;

///@}

};

RIVET_DECLARE_PLUGIN(A2_2017_I1498079);

} ```