Rivet analyses

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

namespace Rivet {


  /// @brief Xi_c0 -> Xi-pi+ asymmetry
  class BELLE_2021_I1851126 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2021_I1851126);


    /// @name Analysis methods
    ///@{

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::abspid==4132), "UFS" );
      // Book histograms
      book(_h_c_P,1,1,1);
      book(_h_c_M,1,1,2);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Xi_c0 baryons
      for( const Particle& Xic : apply<UnstableParticles>(event, "UFS").particles()) {
    int sign = Xic.pid()/4132;
    if(Xic.children().size()!=2) continue;
    Particle baryon1,meson1;
    if(Xic.children()[0].pid()==sign*3312 && 
       Xic.children()[1].pid()==sign*211) {
      baryon1 = Xic.children()[0];
      meson1  = Xic.children()[1];
    }
    else if(Xic.children()[1].pid()==sign*3312 && 
        Xic.children()[0].pid()==sign*211) {
      baryon1 = Xic.children()[1];
      meson1  = Xic.children()[0];
    }
    else
      continue;
    Particle baryon2,meson2;
    if(baryon1.children()[0].pid()== sign*3122 && 
       baryon1.children()[1].pid()==-sign*211) {
      baryon2 = baryon1.children()[0];
      meson2  = baryon1.children()[1];
    }
    else if(baryon1.children()[1].pid()== sign*3122 && 
        baryon1.children()[0].pid()==-sign*211) {
      baryon2 = baryon1.children()[1];
      meson2  = baryon1.children()[0];
    }
    else
      continue;
    // first boost to the Xic rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xic.momentum().betaVec());
    FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
    FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
    // to lambda rest frame
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
    Vector3 axis = pbaryon1.p3().unit();
    FourMomentum pp = boost2.transform(pbaryon2);
    // calculate angle
    double cTheta = pp.p3().unit().dot(axis);
    if(sign>0)
      _h_c_P->fill(cTheta,1.);
    else
      _h_c_M->fill(cTheta,1.);
      }
    }

    pair<double,double> calcAlpha(Histo1DPtr hist) {
      if(hist->numEntries()==0.) return make_pair(0.,0.);
      double sum1(0.),sum2(0.);
      for (const auto& bin : hist->bins() ) {
        double Oi = bin.sumW();
        if(Oi==0.) continue;
        double ai = 0.5*(bin.xMax()-bin.xMin());
        double bi = 0.5*ai*(bin.xMax()+bin.xMin());
        double Ei = bin.errW();
        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() {
      // first mode
      normalize(_h_c_P);
      Estimate0DPtr _h_alpha_P;
      book(_h_alpha_P,2,1,1);
      pair<double,double> alphaP = calcAlpha(_h_c_P);
      alphaP.first /= -0.401;
      alphaP.second/= -0.401;
      _h_alpha_P->set(alphaP.first, alphaP.second);
      // second mode
      normalize(_h_c_M);
      Estimate0DPtr _h_alpha_M;
      book(_h_alpha_M,2,1,2);
      pair<double,double> alphaM = calcAlpha(_h_c_M);
      alphaM.first /= 0.389;
      alphaM.second/= 0.389;
      _h_alpha_M->set(alphaM.first, alphaM.second);
      // average
      double aver = 0.5*(-alphaP.first+alphaM.first);
      double err  = 0.5*sqrt(sqr(alphaP.second)+sqr(alphaM.second));
      Estimate0DPtr _h_alpha_aver;
      book(_h_alpha_aver,2,1,3);
      _h_alpha_aver->set(aver, err);
      // asymetry
      double asym = (alphaP.first+alphaM.first)/(alphaP.first-alphaM.first);
      err         = 2./sqr(alphaP.first-alphaM.first)*sqrt(sqr(alphaM.first *alphaP.second)+
                               sqr(alphaM.second*alphaP.first ));
      Estimate0DPtr _h_alpha_asym;
      book(_h_alpha_asym,2,1,4);
      _h_alpha_asym->set(asym, err);
    }

    ///@}


    /// @name Histograms
    ///@{
    Histo1DPtr _h_c_M,_h_c_P;
    ///@}


  };


  RIVET_DECLARE_PLUGIN(BELLE_2021_I1851126);

}