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BESIII_2016_I1495838

Cross section for $e^+e^-\to J/\psi\pi^+\pi^-$ at energies between 3.77 and 4.6 GeV
Experiment: BESIII (BEPC)
Inspire ID: 1495838
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Rev.Lett. 118 (2017) no.9, 092001
Beams: e+ e-
Beam energies: ANY
Run details:
  • e+e- to hadrons

Measurement of the cross section for $e^+e^-\to J/\psi\pi^+\pi^-$ at energies between 3.77 and 4.6 GeV.

Source code: BESIII_2016_I1495838.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/FinalState.hh"
  4#include "Rivet/Projections/UnstableParticles.hh"
  5
  6namespace Rivet {
  7
  8
  9  /// @brief Add a short analysis description here
 10  class BESIII_2016_I1495838 : public Analysis {
 11  public:
 12
 13    /// Constructor
 14    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1495838);
 15
 16
 17    /// @name Analysis methods
 18    //@{
 19
 20    /// Book histograms and initialise projections before the run
 21    void init() {
 22      declare(FinalState(), "FS");
 23      declare(UnstableParticles(), "UFS");
 24      book(_nPsi, "TMP/psi");
 25    }
 26
 27    void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
 28      for (const Particle &child : p.children()) {
 29	if(child.children().empty()) {
 30	  --nRes[child.pid()];
 31	  --ncount;
 32	}
 33	else
 34	  findChildren(child,nRes,ncount);
 35      }
 36    }
 37
 38    /// Perform the per-event analysis
 39    void analyze(const Event& event) {
 40      const FinalState& fs = apply<FinalState>(event, "FS");
 41      map<long,int> nCount;
 42      int ntotal(0);
 43      for (const Particle& p: fs.particles()) {
 44	nCount[p.pid()] += 1;
 45	++ntotal;
 46      }
 47      const FinalState& ufs = apply<FinalState>(event, "UFS");
 48      // find the psis
 49      for (const Particle& p :  ufs.particles(Cuts::pid==443)) {
 50	if(p.children().empty()) continue;
 51	map<long,int> nRes = nCount;
 52	int ncount = ntotal;
 53	findChildren(p,nRes,ncount);
 54	// psi pi+pi-
 55	if(ncount!=2) continue;
 56	bool matched = true;
 57	for(auto const & val : nRes) {
 58	  if(abs(val.first)==211) {
 59	    if(val.second !=1) {
 60	      matched = false;
 61	      break;
 62	    }
 63	  }
 64	  else if(val.second!=0) {
 65	    matched = false;
 66	    break;
 67	  }
 68	}
 69	if(matched) {
 70	  _nPsi->fill();
 71	  break;
 72	}
 73      }
 74    }
 75
 76    /// Normalise histograms etc., after the run
 77    void finalize() {
 78      double fact =  crossSection()/ sumOfWeights() /picobarn;
 79      double sigma = _nPsi->val()*fact;
 80      double error = _nPsi->err()*fact;
 81      for(unsigned int ix=1;ix<3;++ix) {
 82	Scatter2D temphisto(refData(ix, 1, 1));
 83	Scatter2DPtr  mult;
 84        book(mult, ix, 1, 1);
 85	for (size_t b = 0; b < temphisto.numPoints(); b++) {
 86	  const double x  = temphisto.point(b).x();
 87	  pair<double,double> ex = temphisto.point(b).xErrs();
 88	  pair<double,double> ex2 = ex;
 89	  if(ex2.first ==0.) ex2. first=0.0001;
 90	  if(ex2.second==0.) ex2.second=0.0001;
 91	  if (inRange(sqrtS()/GeV, x-ex2.first, x+ex2.second)) {
 92	    mult->addPoint(x, sigma, ex, make_pair(error,error));
 93	  }
 94	  else {
 95	    mult->addPoint(x, 0., ex, make_pair(0.,.0));
 96	  }
 97	}
 98      }
 99    }
100
101    //@}
102
103
104    /// @name Histograms
105    //@{
106    CounterPtr _nPsi;
107    //@}
108
109
110  };
111
112
113  // The hook for the plugin system
114  RIVET_DECLARE_PLUGIN(BESIII_2016_I1495838);
115
116
117}