1// -*- C++ -*-
 2#include "Rivet/Analysis.hh"
 3#include "Rivet/Projections/FinalState.hh"
 4
 5namespace Rivet {
 6
 7
 8  /// @brief e+ e- > n nbar
 9  class BESIII_2021_I1966612 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1966612);
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(FinalState(), "FS");
24
25      // Book histograms
26      book(_nneutron, "TMP/neutron");
27    }
28
29
30    /// Perform the per-event analysis
31    void analyze(const Event& event) {
32      const FinalState& fs = apply<FinalState>(event, "FS");
33      if(fs.particles().size()!=2) vetoEvent;
34      for (const Particle& p :  fs.particles()) {
35	if(abs(p.pid())!=PID::NEUTRON) vetoEvent;
36      }
37      _nneutron->fill();
38    }
39
40
41    /// Normalise histograms etc., after the run
42    void finalize() {
43      double sigma = _nneutron->val();
44      double error = _nneutron->err();
45      sigma *= crossSection()/ sumOfWeights() /picobarn;
46      error *= crossSection()/ sumOfWeights() /picobarn; 
47      Scatter2D temphisto(refData(1, 1, 1));
48      Scatter2DPtr  mult;
49      book(mult, 1, 1, 1);
50      for (size_t b = 0; b < temphisto.numPoints(); b++) {
51	const double x  = temphisto.point(b).x();
52	pair<double,double> ex = temphisto.point(b).xErrs();
53	pair<double,double> ex2 = ex;
54	if(ex2.first ==0.) ex2. first=0.0001;
55	if(ex2.second==0.) ex2.second=0.0001;
56	if (inRange(sqrtS()/GeV, x-ex2.first, x+ex2.second)) {
57	  mult->addPoint(x, sigma, ex, make_pair(error,error));
58	}
59	else {
60	  mult->addPoint(x, 0., ex, make_pair(0.,.0));
61	}
62      }
63    }
64
65    ///@}
66
67
68    /// @name Histograms
69    ///@{
70    CounterPtr _nneutron;
71    ///@}
72
73
74  };
75
76
77  RIVET_DECLARE_PLUGIN(BESIII_2021_I1966612);
78
79}