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 e+ e- > Lambda_c+ Lambda_c-
  class BESIII_2023_I2677290 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2677290);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(Cuts::abspid==4122), "UFS");
      declare(FinalState(), "FS");
      // histograms
      for (size_t ih=0; ih<5; ++ih) {
        book(_est[ih], 1, 1, 1+ih);
      }
      for (const string& en : _est[0].binning().edges<0>()) {

        double eval = stod(en);
        if (isCompatibleWithSqrtS(eval))  _sqs = en;

        book(_h[en+"cThetaL"], "TMP/cThetaL_"+en, 20,-1.,1.);

      }
      raiseBeamErrorIf(_sqs.empty());
    }

    void findChildren(const Particle& p, map<long,int>& nRes, int& ncount) const {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          nRes[child.pid()]-=1;
          --ncount;
        }
        else {
          findChildren(child,nRes,ncount);
        }
      }
    }

    /// 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 .mom().p3().unit();
      else                      axis = beams.second.mom().p3().unit();
      // types of final state particles
      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 UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
      Particle Lambda, LamBar;
      bool matched(false);
      for (const Particle& p :  ufs.particles()) {
        if (p.children().empty()) continue;
        map<long,int> nRes=nCount;
        int ncount = ntotal;
        findChildren(p,nRes,ncount);
        matched=false;
        // check for antiparticle
        for (const Particle& p2 :  ufs.particles(Cuts::pid==-p.pid())) {
          if (p2.children().empty()) continue;
          map<long,int> nRes2=nRes;
          int ncount2 = ncount;
          findChildren(p2,nRes2,ncount2);
          if (ncount2==0) {
            matched = true;
            for (const auto& val : nRes2) {
              if (val.second!=0) {
                matched = false;
                break;
              }
            }
            // found baryon and antibaryon
            if (matched) {
              if (p.pid()>0) {
                Lambda = p;
                LamBar = p2;
              }
              else {
                Lambda = p2;
                LamBar = p;
              }
              break;
            }
          }
        }
        if (matched) break;
      }
      if (!matched) vetoEvent;
      const double cosL = axis.dot(Lambda.mom().p3().unit());
      _h[_sqs+"cThetaL"]->fill(cosL);
    }

    pair<double,pair<double,double> > calcAlpha0(const Histo1DPtr& hist) const {
      if (hist->numEntries()==0.)  return make_pair(0.,make_pair(0.,0.));
      const double d = 3./(pow(hist->xMax(),3)-pow(hist->xMin(),3));
      const double c = 3.*(hist->xMax()-hist->xMin())/(pow(hist->xMax(),3)-pow(hist->xMin(),3));
      double sum1(0.),sum2(0.),sum3(0.),sum4(0.),sum5(0.);
      for (const auto& bin : hist->bins()) {
        const double Oi = bin.sumW();
        if (Oi==0.) continue;
        const double a =  d*(bin.xMax() - bin.xMin());
        const double b = d/3.*(pow(bin.xMax(),3) - pow(bin.xMin(),3));
        const 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
      const double alpha = (-c*sum1 + sqr(c)*sum2 + sum3 - c*sum5)/(sum1 - c*sum2 + c*sum4 - sum5);
      // and error
      const double cc = -pow((sum3 + sqr(c)*sum4 - 2*c*sum5),3);
      const double bb = -2*sqr(sum3 + sqr(c)*sum4 - 2*c*sum5)*(sum1 - c*sum2 + c*sum4 - sum5);
      const double aa =  sqr(sum1 - c*sum2 + c*sum4 - sum5)*(-sum3 - sqr(c)*sum4 + sqr(sum1 - c*sum2 + c*sum4 - sum5) + 2*c*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() {
      // storage of the values to fill histos
      const double mn    = 2.28646;
      const double alpha = 7.2973525693e-3;
      const double GeV2pb = 0.3893793721e9;

      for (const string& en : _est[0].binning().edges<0>()) {

        if (isZero(_h[en+"cThetaL"]->effNumEntries()))  continue;
        double eval = stod(en);

        const double tau  = 4.*sqr(mn/eval);
        const double beta = sqrt(1.-tau);
        // prefactor and sigma
        double sigma0 = 4.*M_PI*sqr(alpha/eval)*beta*GeV2pb;
        // calculate alpha0
        pair<double,pair<double,double> > alpha0 = calcAlpha0(_h[en+"cThetaL"]);
        scale(_h[en+"cThetaL"], crossSection()/ sumOfWeights()/picobarn);
        const double wsum = _h[en+"cThetaL"]->integral();
        const double werr = _h[en+"cThetaL"]->integralError();
        // Geff
        pair<double,double> Geff = make_pair(1e2*sqrt(3.*wsum/(sigma0*(1 + 0.5*tau))),
                                             1e2*1.5*wsum/(sigma0*(1 + 0.5*tau)));
        // GM
        double GMv = 1e2*sqrt(6.*((1+alpha0.first)*wsum)/((3+alpha0.first)*sigma0));
        double GMe1 = 1e2*sqrt((3*(sqr(werr)*sqr(1 + alpha0.first)*sqr(3 + alpha0.first) + 4*sqr(alpha0.second.first)*sqr(wsum)))/
                               (2.*(1 + alpha0.first)*pow(3 + alpha0.first,3)*wsum*sigma0));
        double GMe2 = 1e2*sqrt((3*(sqr(werr)*sqr(1 + alpha0.first)*sqr(3 + alpha0.first) + 4*sqr(alpha0.second.second)*sqr(wsum)))/
                               (2.*(1 + alpha0.first)*pow(3 + alpha0.first,3)*wsum*sigma0));
        pair<double,pair<double,double> > GM = make_pair(std::move(GMv), make_pair(std::move(GMe1), std::move(GMe2)));
        // ratio
        pair<double,pair<double,double>> R;
        R.first = sqrt((1 - alpha0.first)/(tau + alpha0.first*tau));
        R.second.first  = R.first*alpha0.second.first /(1.-sqr(alpha0.first));
        R.second.second = R.first*alpha0.second.second/(1.-sqr(alpha0.first));
        for (size_t ix=1; ix<6; ++ix) {
          double val;
          pair<double,double> err;
          if (ix==1) {
            val = wsum;
            err = make_pair(werr, werr);
          }
          else if (ix==2) {
            val = Geff.first;
            err = make_pair(Geff.second,Geff.second);
          }
          else if (ix==3) {
            val = alpha0.first;
            err = alpha0.second;
          }
          else if (ix==4) {
            val = R.first;
            err = R.second;
          }
          else {
            val = GM.first;
            err = GM.second;
          }
          _est[ix-1]->binAt(en).set(val,err);
        }
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    map<string,Histo1DPtr> _h;
    BinnedEstimatePtr<string> _est[5];
    string _sqs = "";
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2023_I2677290);

}