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BESIII_2016_I1384778

Azimuthal asymmetries in inclusive charged pion-pair production at $\sqrt{s}=3.65$ GeV
Experiment: BESIII (BEPC)
Inspire ID: 1384778
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Rev.Lett. 116 (2016) no.4, 042001
Beams: e+ e-
Beam energies: (1.8, 1.8) GeV
Run details:
  • e+e- to hadrons

Measurement of azimuthal asymmetries in inclusive charged pion-pair production at $\sqrt{s}=3.65$ GeV by the BESII experiment

Source code: BESIII_2016_I1384778.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/Beam.hh"

namespace Rivet {


  /// @brief Collins assymmetry
  class BESIII_2016_I1384778 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(BESIII_2016_I1384778);


    /// @name Analysis methods
    //@{

    /// Book histograms and initialise projections before the run
    void init() {
      declare(Beam(), "Beams");
      declare(FinalState(Cuts::abspid==PID::PIPLUS), "FS");
      // book the histograms
      _h_L = vector<Histo1DPtr>(6,Histo1DPtr());
      _h_U = vector<Histo1DPtr>(6,Histo1DPtr());
      _h_C = vector<Histo1DPtr>(6,Histo1DPtr());
      for(unsigned int ix=0;ix<6;++ix) {
	std::ostringstream title;
	title << "/TMP/h_z1z2_" << ix+1;
	book(_h_L[ix],title.str()+"_L",20,0.,M_PI);
	book(_h_U[ix],title.str()+"_U",20,0.,M_PI);
	book(_h_C[ix],title.str()+"_C",20,0.,M_PI);
      }
      double xbin[6]={0.,.2,.3,.45,.8,1.4};
      for(unsigned int ix=0;ix<5;++ix) {
	std::ostringstream title;
	title << "/TMP/h_pT_" << ix+1;
	Histo1DPtr hL,hU,hC;
	book(hL,title.str()+"_L",20,0.,M_PI);
	_h_pT_L.add(xbin[ix],xbin[ix+1], hL);
	book(hU,title.str()+"_U",20,0.,M_PI);
	_h_pT_U.add(xbin[ix],xbin[ix+1], hU);
	book(hC,title.str()+"_C",20,0.,M_PI);
	_h_pT_C.add(xbin[ix],xbin[ix+1], hC);
      }
    }


    /// 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();
      // loop over pions pair, using index to avoid double counting
      Particles pions = apply<FinalState>(event, "FS").particles();
      for(unsigned int i1=0;i1<pions.size();++i1) {
	const double x1=2.*pions[i1].momentum().t()/sqrtS();
	// cut on z1
	if(x1<0.2||x1>0.9) continue;
	// cos theta cut
	if(abs(cos(pions[i1].momentum().p3().polarAngle()))>0.93) continue;
	for(unsigned int i2=i1+1;i2<pions.size();++i2) {
	  // cut on z2
	  const double x2=2.*pions[i2].momentum().t()/sqrtS();
	  if(x2<0.2||x2>0.9) continue;
	  // cos theta cut
	  if(abs(cos(pions[i2].momentum().p3().polarAngle()))>0.93) continue;
	  // cut on opening angle (>120 degrees)
	  if(pions[i1].momentum().p3().angle(pions[i2].momentum().p3())>2.*M_PI/3.)
	    continue;
	  Particle p1=pions[i1], p2=pions[i2];
	  double z1(x1),z2(x2);
	  // randomly order the particles
	  if(rand()/static_cast<double>(RAND_MAX) < 0.5 ) {
	    swap(p1,p2);
	    swap(z1,z2);
	  }
	  // particle 2 defines the z axis
	  Vector3 ez = p2.momentum().p3().unit();
          // beam and 2 define the plane (y is normal to plane) 
          Vector3 ey = ez.cross(axis).unit();
          // x by cross product 
          Vector3 ex = ey.cross(ez).unit();
          // phi
          double phi = ex.angle(p1.momentum().p3());
	  // hists vs z1,z2
	  unsigned int ibin=0;
	  if(z1<=.3&&z2<=.3) {
	    ibin=0;
	  }
	  else if(z1>0.5&&z2>0.5) {
	    ibin=5;
	  }
	  else if(min(z1,z2)<=0.3) {
	    if(max(z1,z2)>0.5)
	      ibin=2;
	    else
	      ibin=1;
	  }
	  else {
	    if(max(z1,z2)>0.5)
	      ibin=4;
	    else
	      ibin=3;
	  }
	  _h_C[ibin]->fill(phi);
	  if(p1.pid()==p2.pid())
	    _h_L[ibin]->fill(phi);
	  else
	    _h_U[ibin]->fill(phi);
	  // hists vs pT
	  double pPar2 = sqr(ez.dot(p1.momentum().p3()));
	  double pPerp = sqrt(p1.momentum().p3().mod2()-pPar2);
	  _h_pT_C.fill(pPerp,phi);
	  if(p1.pid()==p2.pid()) 
	    _h_pT_L.fill(pPerp,phi);
	  else 
	    _h_pT_U.fill(pPerp,phi);
	}
      }
    }
    
    pair<double,double> calcAsymmetry(Scatter2DPtr hist) {
      double sum1(0.),sum2(0.);
      for (auto point : hist->points() ) {
	double Oi = point.y();
	if(Oi==0. || std::isnan(Oi) ) continue;
	double ai = 1.;
	double bi = 0.5*(sin(2.*point.xMax())-sin(2.*point.xMin()))/(point.xMax()-point.xMin());
	double Ei = point.yErrAvg();
	sum1 += sqr(bi/Ei);
	sum2 += bi/sqr(Ei)*(Oi-ai);
      }
      return make_pair(sum2/sum1,sqrt(1./sum1));
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      // ratios
      Scatter2DPtr _h_z_UL,_h_z_UC;
      book(_h_z_UL,1,1,5);
      book(_h_z_UC,1,1,6);
      for(unsigned int ix=0;ix<6;++ix) {
	normalize(_h_L[ix]);
	normalize(_h_U[ix]);
	normalize(_h_C[ix]);
	std::ostringstream title;
	title << "/TMP/R_z1z2_" << ix+1;
	Scatter2DPtr R1;
	book(R1,title.str()+"_UL");
	divide(_h_U[ix],_h_L[ix],R1);
	Scatter2DPtr R2;
	book(R2,title.str()+"_UC");
	divide(_h_U[ix],_h_C[ix],R2);
	pair<double,double> asym1 = calcAsymmetry(R1);
	_h_z_UL->addPoint(double(ix)+1., asym1.first, make_pair(0.5,0.5),
			  make_pair(asym1.second,asym1.second) );
	pair<double,double> asym2 = calcAsymmetry(R2);
	_h_z_UC->addPoint(double(ix)+1., asym2.first, make_pair(0.5,0.5),
			  make_pair(asym2.second,asym2.second) );
      }
      Scatter2DPtr _h_pT_UL,_h_pT_UC;
      book(_h_pT_UL,2,1,4);
      book(_h_pT_UC,2,1,5);
      Scatter2D temphisto(refData(2, 1, 4));
      for(unsigned int ix=0;ix<5;++ix) {
	normalize(_h_pT_L.histos()[ix]);
	normalize(_h_pT_U.histos()[ix]);
	normalize(_h_pT_C.histos()[ix]);
	std::ostringstream title;
	title << "/TMP/R_pT_" << ix+1;
	Scatter2DPtr R1;
	book(R1,title.str()+"_UL");
	divide(_h_U[ix],_h_L[ix],R1);
	Scatter2DPtr R2;
	book(R2,title.str()+"_UC");
	divide(_h_U[ix],_h_C[ix],R2);
	const double x  = temphisto.point(ix).x();
	pair<double,double> ex = temphisto.point(ix).xErrs();
	pair<double,double> asym1 = calcAsymmetry(R1);
	_h_pT_UL->addPoint(x, asym1.first, ex,
			  make_pair(asym1.second,asym1.second) );
	pair<double,double> asym2 = calcAsymmetry(R2);
	_h_pT_UC->addPoint(x, asym2.first, ex,
			   make_pair(asym2.second,asym2.second) );
      }
    }
    //@}


    /// @name Histograms
    //@{
    vector<Histo1DPtr> _h_L,_h_U,_h_C;
    BinnedHistogram _h_pT_L,_h_pT_U,_h_pT_C;
    //@}


  };


  DECLARE_RIVET_PLUGIN(BESIII_2016_I1384778);

}