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 chi_c -> J/psi mu+mu-
10  class BESIII_2019_I1716256 : public Analysis {
11  public:
12
13    /// Constructor
14    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1716256);
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::pid==20443||Cuts::pid==445);
23      declare(ufs, "UFS");
24      DecayedParticles chi(ufs);
25      chi.addStable(PID::JPSI);
26      declare(chi, "chi");
27      // Initialise and register projections
28      for(unsigned int ix=0;ix<2;++ix) {
29      	for(unsigned int iy=0;iy<2;++iy) {
30      	  book(_h[ix][iy],1+ix,1,1+iy);
31      	  book(_n[ix][iy],"TMP/n_"+toString(ix+1)+"_"+toString(iy+1));
32      	}
33      }
34    }
35
36
37    /// Perform the per-event analysis
38    void analyze(const Event& event) {
39      static double alpha= 7.2973525664e-3;
40      static const map<PdgId,unsigned int> & mode1 = { { 443,1}, {22,1} };
41      static const map<PdgId,unsigned int> & mode2 = { { 443,1}, {13,1}, {-13,1} };
42      DecayedParticles chi = apply<DecayedParticles>(event, "chi");
43      // loop over particles
44      for(unsigned int ix=0;ix<chi.decaying().size();++ix) {
45	unsigned int iloc=chi.decaying()[ix].pid()==20443 ? 0 : 1;
46	_n[iloc][0]->fill();
47	if(chi.modeMatches(ix,2,mode1)) {
48	  _n[iloc][1]->fill();
49	}
50	else if(chi.modeMatches(ix,3,mode2)) {
51	  const Particle & mum  = chi.decayProducts()[ix].at( 13)[0];
52	  const Particle & mup  = chi.decayProducts()[ix].at(-13)[0];
53	  const Particle & Jpsi = chi.decayProducts()[ix].at(443)[0];
54	  double q = (mum.momentum()+mup.momentum()).mass();
55	  // br
56	  _h[iloc][0]->fill(q);
57	  double M2 = chi.decaying()[ix].mass2();
58	  double m  = Jpsi.mass();
59	  // factor for form factor (eqn 2 arXiv:1709.02444)
60	  double fact = alpha/3./M_PI/sqr(q)*
61	    sqrt((1.-sqr(m+q)/M2)*(1.-sqr(m-q)/M2))/(1.-sqr(m)/M2)*
62	    (1.+2.*sqr(mum.mass()/q))*sqrt(1.-4.*sqr(mum.mass()/q));
63	  // additional factor of 2q from dq^2 = 2q dq
64	  _h[iloc][1]->fill(q,1./fact/2/q);
65	}
66      }
67    }
68
69
70    /// Normalise histograms etc., after the run
71    void finalize() {
72      for(unsigned int ix=0;ix<2;++ix) {
73	for(unsigned int iy=0;iy<2;++iy) {
74	  scale(_h[ix][iy], 1./ *_n[ix][iy]);
75	}
76	scale(_h[ix][0], 1e5);
77      }
78    }
79    /// @}
80
81    /// @name Histograms
82    ///@{
83    Histo1DPtr _h[2][2];
84    CounterPtr _n[2][2];
85
86    ///@}
87
88  };
89
90
91  RIVET_DECLARE_PLUGIN(BESIII_2019_I1716256);
92
93}