Rivet Analyses Reference

BESIII_2012_I1121378

Analysis of $J/psi\to \Lambda^0\bar\Sigma^0+\text{c.c.}$
Experiment: BESIII (BEPC)
Inspire ID: 1121378
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Rev. D86 (2012) 032008
Beams: e- e+
Beam energies: (1.6, 1.6) GeV
Run details:
  • e+e- > J/psi

Analysis of the angular distribution of the baryons produced in $e^+e^-\to J/\psi\to\Lambda^0\bar\Sigma^0+\text{c.c.}$. Gives information about the decay and is useful for testing correlations in hadron decays.

Source code: BESIII_2012_I1121378.cc
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief JPsi -> sigma0 lambda0 analysis
  class BESIII_2012_I1121378 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1121378);


    /// @name Analysis methods
    //@{

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(), "UFS");
      declare(FinalState(), "FS");
      
      // Book histograms
      book(_h_lam, 1, 1, 2);
      book(_h_bar, 1, 1, 1);
      book(_h_all, "/TMP/h_all",20,-1.,1.);

    }

    void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
      for(const Particle &child : p.children()) {
	if(child.children().empty()) {
	  nRes[child.pid()]-=1;
	  --ncount;
	}
	else
	  findChildren(child,nRes,ncount);
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // get the axis, direction of incoming electron
      const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
      Vector3 axis;
      if(beams.first.pid()>0)
	axis = beams.first .momentum().p3().unit();
      else
	axis = beams.second.momentum().p3().unit();
      // types of final state particles
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long,int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
	nCount[p.pid()] += 1;
	++ntotal;
      }
      // loop over lambda0 and sigma0 baryons
      const UnstableParticles & ufs = apply<UnstableParticles>(event, "UFS");
      for (const Particle& p : ufs.particles(Cuts::abspid==3122)) {
	int sign = p.pid()/3122;
       	if(p.children().empty()) continue;
       	map<long,int> nRes=nCount;
       	int ncount = ntotal;
       	findChildren(p,nRes,ncount);
	bool matched=false;
	// check for Sigma0 or Sigma0bar
	for (const Particle& p2 : ufs.particles(Cuts::pid==sign*3212)) {
	  if(p2.children().empty()) continue;
	  map<long,int> nRes2=nRes;
	  int ncount2 = ncount;
	  findChildren(p2,nRes2,ncount2);
	  if(ncount2==0) {
	    matched = true;
	    for(auto const & val : nRes2) {
	      if(val.second!=0) {
		matched = false;
		break;
	      }
	    }
	    // fond baryon and antibaryon
	    if(matched) {
	      // calc cosine
	      double ctheta = p .momentum().p3().unit().dot(axis);
	      if(p.pid()==3122)
		_h_lam->fill(ctheta);
	      else
		_h_bar->fill(ctheta);
	      _h_all->fill(ctheta);
	      break;
	    }
	  }
	}
	if(matched) break;
      }
    }

    pair<double,pair<double,double> > calcAlpha(Histo1DPtr hist) {
      if(hist->numEntries()==0.) return make_pair(0.,make_pair(0.,0.));
      double sum1(0.),sum2(0.),sum3(0.),sum4(0.),sum5(0.);
      for (auto bin : hist->bins() ) {
       	double Oi = bin.area();
	if(Oi==0.) continue;
	double a =  1.5*(bin.xMax() - bin.xMin());
	double b = 0.5*(pow(bin.xMax(),3) - pow(bin.xMin(),3));
       	double Ei = bin.areaErr();
	sum1 +=   a*Oi/sqr(Ei);
	sum2 +=   b*Oi/sqr(Ei);
	sum3 += sqr(a)/sqr(Ei);
	sum4 += sqr(b)/sqr(Ei);
	sum5 +=    a*b/sqr(Ei);
      }
      // calculate alpha
      double alpha = (-3*sum1 + 9*sum2 + sum3 - 3*sum5)/(sum1 - 3*sum2 + 3*sum4 - sum5);
      // and error
      double cc = -pow((sum3 + 9*sum4 - 6*sum5),3);
      double bb = -2*sqr(sum3 + 9*sum4 - 6*sum5)*(sum1 - 3*sum2 + 3*sum4 - sum5);
      double aa =  sqr(sum1 - 3*sum2 + 3*sum4 - sum5)*(-sum3 - 9*sum4 + sqr(sum1 - 3*sum2 + 3*sum4 - sum5) + 6*sum5);      
      double dis = sqr(bb)-4.*aa*cc;
      if(dis>0.) {
	dis = sqrt(dis);
	return make_pair(alpha,make_pair(0.5*(-bb+dis)/aa,-0.5*(-bb-dis)/aa));
      }
      else {
	return make_pair(alpha,make_pair(0.,0.));
      }
    }

    /// Normalise histograms etc., after the run
    void finalize() {

      normalize(_h_lam);
      normalize(_h_bar);
      normalize(_h_all);
      pair<double,pair<double,double> > alpha = calcAlpha(_h_all);
      Scatter2DPtr h_alpha_lam;
      book(h_alpha_lam, 5,1,1);
      h_alpha_lam->addPoint(0.5, alpha.first, make_pair(0.5,0.5), make_pair(alpha.second.first,alpha.second.second) );

    }

    //@}


    /// @name Histograms
    //@{
    Histo1DPtr _h_lam,_h_bar,_h_all;
    //@}


  };


  // The hook for the plugin system
  RIVET_DECLARE_PLUGIN(BESIII_2012_I1121378);


}