1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/UnstableParticles.hh"
 4
 5namespace Rivet {
 6
 7
 8  /// @brief Add a short analysis description here
 9  class BESIII_2018_I1641075 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1641075);
14
15
16    /// @name Analysis methods
17    //@{
18
19    /// Book histograms and initialise projections before the run
20    void init() {
21
22      // Initialise and register projections
23      declare(UnstableParticles(), "UFS");
24
25      // Book histograms
26      book(_h_m, 1, 1, 5);
27
28    }
29    
30    void findDecayProducts(const Particle & mother, unsigned int & nstable, unsigned int & ngamma, 
31                           unsigned int & npip, unsigned int & npim, FourMomentum & ptot) {
32      for(const Particle & p : mother.children()) {
33        int id = p.pid();
34        if (id == PID::PIMINUS ) {
35	  ++npim;
36          ++nstable;
37	  ptot += p.momentum();
38	}
39        else if (id == PID::PIPLUS) {
40          ++npip;
41          ++nstable;
42	  ptot += p.momentum();
43        }
44        else if ( !p.children().empty() ) {
45          findDecayProducts(p, nstable, ngamma,npip,npim,ptot);
46        }
47        else if (id == PID::GAMMA) {
48	  ++ngamma;
49          ++nstable;
50        }
51        else
52          ++nstable;
53      }
54    }
55
56
57    /// Perform the per-event analysis
58    void analyze(const Event& event) {
59
60      // Loop over eta' mesons
61      for (const Particle& p :  apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==331)) {
62	unsigned nstable(0),ngamma(0),npip(0),npim(0);
63	FourMomentum ptot;
64	findDecayProducts(p,nstable,ngamma,npip,npim,ptot);
65	if(nstable==3 && npim==1 && npip==1 && ngamma==1)
66	  _h_m->fill(ptot.mass());
67      }
68    }
69
70
71    /// Normalise histograms etc., after the run
72    void finalize() {
73
74      normalize(_h_m);
75
76    }
77
78    //@}
79
80
81    /// @name Histograms
82    //@{
83    Histo1DPtr _h_m;
84    //@}
85
86
87  };
88
89
90  // The hook for the plugin system
91  RIVET_DECLARE_PLUGIN(BESIII_2018_I1641075);
92
93
94}