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_I1853007 : public Analysis {
10  public:
11
12    /// Constructor
13    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2021_I1853007);
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
31    /// Perform the per-event analysis
32    void analyze(const Event& event) {
33      const FinalState& fs = apply<FinalState>(event, "FS");
34      if(fs.particles().size()!=2) vetoEvent;
35      for (const Particle& p : fs.particles()) {
36	if(abs(p.pid())!=PID::NEUTRON) vetoEvent;
37      }
38      _nneutron->fill();
39    }
40
41
42    /// Normalise histograms etc., after the run
43    void finalize() {
44      double sigma = _nneutron->val();
45      double error = _nneutron->err();
46      sigma *= crossSection()/ sumOfWeights() /picobarn;
47      error *= crossSection()/ sumOfWeights() /picobarn; 
48      Scatter2D temphisto(refData(1, 1, 1));
49      Scatter2DPtr  mult;
50      book(mult, 1, 1, 1);
51      for (size_t b = 0; b < temphisto.numPoints(); b++) {
52	const double x  = temphisto.point(b).x();
53	pair<double,double> ex = temphisto.point(b).xErrs();
54	pair<double,double> ex2 = ex;
55	if(ex2.first ==0.) ex2. first=0.0001;
56	if(ex2.second==0.) ex2.second=0.0001;
57	if (inRange(sqrtS()/GeV, x-ex2.first, x+ex2.second)) {
58	  mult->addPoint(x, sigma, ex, make_pair(error,error));
59	}
60	else {
61	  mult->addPoint(x, 0., ex, make_pair(0.,.0));
62	}
63      }
64    }
65
66    ///@}
67
68
69    /// @name Histograms
70    ///@{
71    CounterPtr _nneutron;
72    ///@}
73
74
75  };
76
77
78  RIVET_DECLARE_PLUGIN(BESIII_2021_I1853007);
79
80}