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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
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Add a short analysis description here
  class BESIII_2016_I1495838 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(BESIII_2016_I1495838);


    /// @name Analysis methods
    //@{

    /// Book histograms and initialise projections before the run
    void init() {
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_nPsi, "TMP/psi");
    }

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

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      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;
      }
      const FinalState& ufs = apply<FinalState>(event, "UFS");
      // find the psis
      for (const Particle& p :  ufs.particles(Cuts::pid==443)) {
	if(p.children().empty()) continue;
	map<long,int> nRes = nCount;
	int ncount = ntotal;
	findChildren(p,nRes,ncount);
	// psi pi+pi-
	if(ncount!=2) continue;
	bool matched = true;
	for(auto const & val : nRes) {
	  if(abs(val.first)==211) {
	    if(val.second !=1) {
	      matched = false;
	      break;
	    }
	  }
	  else if(val.second!=0) {
	    matched = false;
	    break;
	  }
	}
	if(matched) {
	  _nPsi->fill();
	  break;
	}
      }
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      double fact =  crossSection()/ sumOfWeights() /picobarn;
      double sigma = _nPsi->val()*fact;
      double error = _nPsi->err()*fact;
      for(unsigned int ix=1;ix<3;++ix) {
	Scatter2D temphisto(refData(ix, 1, 1));
	Scatter2DPtr  mult;
        book(mult, ix, 1, 1);
	for (size_t b = 0; b < temphisto.numPoints(); b++) {
	  const double x  = temphisto.point(b).x();
	  pair<double,double> ex = temphisto.point(b).xErrs();
	  pair<double,double> ex2 = ex;
	  if(ex2.first ==0.) ex2. first=0.0001;
	  if(ex2.second==0.) ex2.second=0.0001;
	  if (inRange(sqrtS()/GeV, x-ex2.first, x+ex2.second)) {
	    mult->addPoint(x, sigma, ex, make_pair(error,error));
	  }
	  else {
	    mult->addPoint(x, 0., ex, make_pair(0.,.0));
	  }
	}
      }
    }

    //@}


    /// @name Histograms
    //@{
    CounterPtr _nPsi;
    //@}


  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(BESIII_2016_I1495838);


}