Rivet analyses referenceBESIII_2013_I1209121Mass and anglar distributuions in $J/\psi\to\gamma\eta\eta$Experiment: BESIII (BEPC) Inspire ID: 1209121 Status: VALIDATED NOHEPDATA Authors:
Beam energies: (1.6, 1.6) GeV Run details:
Measurement of mass and anglar distributuions in $J/\psi\to\gamma\eta\eta$ by BESIII Source code: BESIII_2013_I1209121.cc 1// -*- C++ -*-
2#include "Rivet/Analysis.hh"
3#include "Rivet/Projections/Beam.hh"
4#include "Rivet/Projections/UnstableParticles.hh"
5#include "Rivet/Projections/DecayedParticles.hh"
6
7namespace Rivet {
8
9
10 /// @brief J/psi -> gamma eta eta
11 class BESIII_2013_I1209121 : public Analysis {
12 public:
13
14 /// Constructor
15 RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2013_I1209121);
16
17
18 /// @name Analysis methods
19 /// @{
20
21 /// Book histograms and initialise projections before the run
22 void init() {
23 // Initialise and register projections
24 UnstableParticles ufs = UnstableParticles(Cuts::abspid==443);
25 declare(ufs, "UFS");
26 DecayedParticles PSI(ufs);
27 PSI.addStable(PID::ETA);
28 declare(PSI, "PSI");
29 declare(Beam(), "Beams");
30 // book histograms
31 for(unsigned int ix=0;ix<4;++ix)
32 book(_h[ix],1,1,1+ix);
33 }
34
35 // angle cuts due regions of BES calorimeter
36 bool vetoPhoton(const double & cTheta) {
37 return cTheta>0.92 || (cTheta>0.8 && cTheta<0.86);
38 }
39
40 /// Perform the per-event analysis
41 void analyze(const Event& event) {
42 // get the axis, direction of incoming electron
43 const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
44 Vector3 axis;
45 if(beams.first.pid()>0)
46 axis = beams.first .momentum().p3().unit();
47 else
48 axis = beams.second.momentum().p3().unit();
49 // find the J/psi decays
50 static const map<PdgId,unsigned int> & mode = { { 221,2},{ 22,1}};
51 DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
52 if( PSI.decaying().size()!=1) vetoEvent;
53 if(!PSI.modeMatches(0,3,mode)) vetoEvent;
54 // particles
55 const Particles & eta = PSI.decayProducts()[0].at(221);
56 const Particle & gam = PSI.decayProducts()[0].at( 22)[0];
57 _h[0]->fill((eta[0].momentum()+eta[1].momentum()).mass());
58 double cTheta = axis.dot(gam.p3().unit());
59 if(vetoPhoton(abs(cTheta))) vetoEvent;
60 _h[1]->fill(cTheta);
61 // remaining angles
62 LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(PSI.decaying()[0].momentum().betaVec());
63 FourMomentum pGamma = boost1.transform(gam.momentum());
64 FourMomentum pEtaEta= boost1.transform(eta[0].momentum()+eta[1].momentum());
65 Vector3 e1z = pGamma.p3().unit();
66 Vector3 e1y = e1z.cross(axis).unit();
67 Vector3 e1x = e1y.cross(e1z).unit();
68 LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pEtaEta.betaVec());
69 Vector3 axis2 = boost2.transform(boost1.transform(eta[0].momentum())).p3().unit();
70 _h[2]->fill(e1z.dot(axis2));
71 _h[3]->fill(atan2(e1y.dot(axis2),e1x.dot(axis2)));
72 }
73
74
75 /// Normalise histograms etc., after the run
76 void finalize() {
77 for(unsigned int ix=0;ix<4;++ix)
78 normalize(_h[ix],1.,false);
79 }
80
81 /// @}
82
83
84 /// @name Histograms
85 /// @{
86 Histo1DPtr _h[4];
87 /// @}
88
89
90 };
91
92
93 RIVET_DECLARE_PLUGIN(BESIII_2013_I1209121);
94
95}
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