1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/UnstableParticles.hh"
 4#include "Rivet/Projections/DecayedParticles.hh"
 5
 6namespace Rivet {
 7
 8
 9  /// @brief J/psi -> eta' e+e-
10  class BESIII_2019_I1692688 : public Analysis {
11  public:
12
13    /// Constructor
14    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1692688);
15
16
17    /// @name Analysis methods
18    /// @{
19
20    /// Book histograms and initialise projections before the run
21    void init() {
22      UnstableParticles ufs = UnstableParticles(Cuts::abspid==PID::JPSI);
23      declare(ufs, "UFS");
24      DecayedParticles psi(ufs);
25      psi.addStable(PID::PI0);
26      psi.addStable(PID::ETAPRIME);
27      declare(psi, "PSI");
28      for(unsigned int ix=0;ix<2;++ix)
29	book(_h[ix], 1, 1, ix+1);
30    }
31
32
33    /// Perform the per-event analysis
34    void analyze(const Event& event) {
35      // define the decay mode
36      static const map<PdgId,unsigned int> & mode   = { {PID::ETAPRIME,1},{ 11,1}, { -11,1}};
37      DecayedParticles psi = apply<DecayedParticles>(event, "PSI");
38      // loop over particles
39      for(unsigned int ix=0;ix<psi.decaying().size();++ix) {
40	if(!psi.modeMatches(ix,3,mode)) continue;
41	const Particle & em = psi.decayProducts()[ix].at( 11)[0];
42	const Particle & ep = psi.decayProducts()[ix].at(-11)[0];
43	double mee = (ep.momentum()+em.momentum()).mass();
44	for(unsigned int ix=0;ix<2;++ix) _h[ix]->fill(mee);
45      }
46    }
47
48
49    /// Normalise histograms etc., after the run
50    void finalize() {
51      for(unsigned int ix=0;ix<2;++ix)
52	normalize(_h[ix],1.,false);
53    }
54
55    /// @}
56
57
58    /// @name Histograms
59    /// @{
60    Histo1DPtr _h[2];
61    /// @}
62
63
64  };
65
66
67  RIVET_DECLARE_PLUGIN(BESIII_2019_I1692688);
68
69}