1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/UnstableParticles.hh"
 4
 5namespace Rivet {
 6
 7
 8  /// @brief $J/\psi\to e^+e^-\eta$ decays
 9  class BESIII_2018_I1697377 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1697377);
14
15
16    /// @name Analysis methods
17    /// @{
18
19    /// Book histograms and initialise projections before the run
20    void init() {
21      // Initialise and register projections
22      declare(UnstableParticles(), "UFS");
23      // Book histograms
24      book(_h_m, 1, 1, 1);
25    }
26
27
28    void findDecayProducts(const Particle & mother, unsigned int & nstable, unsigned int & neta,
29                           unsigned int & nep, unsigned int & nem, FourMomentum & ptot) {
30      for(const Particle & p : mother.children()) {
31        int id = p.pid();
32        if (id == PID::EMINUS ) {
33          ++nem;
34          ++nstable;
35          ptot += p.momentum();
36        }
37        else if (id == PID::EPLUS) {
38          ++nep;
39          ++nstable;
40          ptot += p.momentum();
41        }
42        else if (id == PID::ETA) {
43          ++neta;
44          ++nstable;
45        }
46        else if ( !p.children().empty() ) {
47          findDecayProducts(p, nstable, neta,nep,nem,ptot);
48        }
49        else
50          ++nstable;
51      }
52    }
53
54
55    /// Perform the per-event analysis
56    void analyze(const Event& event) {
57
58      // Loop over J/psi mesons
59      for (const Particle& p :  apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==443)) {
60        unsigned nstable(0),neta(0),nep(0),nem(0);
61        FourMomentum ptot;
62        findDecayProducts(p,nstable,neta,nep,nem,ptot);
63        if(nstable==3 && nem==1 && nem==1 && neta==1) {
64          _h_m->fill(ptot.mass());
65        }
66      }
67
68    }
69
70
71    /// Normalise histograms etc., after the run
72    void finalize() {
73      normalize(_h_m); // normalize to unity
74    }
75
76    /// @}
77
78
79    /// @name Histograms
80    /// @{
81    Histo1DPtr _h_m;
82    /// @}
83
84  };
85
86
87  RIVET_DECLARE_PLUGIN(BESIII_2018_I1697377);
88
89}