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| // -*- 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);
}
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