Rivet analyses

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

namespace Rivet {


  /// @brief psi2S baryon decay analysis
  class BESIII_2018_I1658762 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1658762);


    /// @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_proton ,"ctheta_p",20,-1.,1.);
      book(_h_neutron,"ctheta_n",20,-1.,1.);

    }


    /// 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);
      Particle outgoing;
      for (const Particle& p :  fs.particles()) {
        nCount[p.pid()] += 1;
        if(p.pid()==2212 || p.pid()==2112)
          outgoing = p;
        ++ntotal;
      }
      if(ntotal==2) {
        if(nCount[2212]==1 && nCount[-2212]==1) {
          _h_proton->fill(outgoing.momentum().p3().unit().dot(axis));
        }
        else if(nCount[2112]==1 && nCount[-2112]==1) {
          _h_neutron->fill(outgoing.momentum().p3().unit().dot(axis));
        }
      }
    }

    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 (const auto& bin : hist->bins() ) {
        double Oi = bin.sumW();
        if(Oi==0.) continue;
        double a =  1.5*bin.xWidth();
        double b = 0.5*(pow(bin.xMax(),3) - pow(bin.xMin(),3));
        double Ei = bin.errW();
        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() {
      // proton
      normalize(_h_proton );
      pair<double,pair<double,double> > alpha = calcAlpha(_h_proton);
      Estimate0DPtr _h_alpha_proton;
      book(_h_alpha_proton,1,1,1);
      _h_alpha_proton->set(alpha.first, alpha.second);
      // neutron
      normalize(_h_neutron);
      alpha = calcAlpha(_h_neutron);
      Estimate0DPtr _h_alpha_neutron;
      book(_h_alpha_neutron,1,1,2);
      _h_alpha_neutron->set(alpha.first, alpha.second);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_proton,_h_neutron;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2018_I1658762);


}