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 Lambda Lambdabar cross section
  class BESIII_2019_I1726357 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2019_I1726357);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // histograms
      book(_h_sigma, 1, 1, 1);
      book(_h_cTheta, 2, 1, 1);
      book(_h_pol, {-1, -0.8, -0.6, -0.4, -0.2, 0., 0.2, 0.4, 0.6, 0.8, 1.0});
      for (auto& b : _h_pol->bins()) {
        book(b, "/TMP/h_pol_" + to_string(b.index() + 1), 20, -1.0, 1.0);
      }
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
      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.momentum().p3().unit();
      else
        axis = beams.second.momentum().p3().unit();
      const FinalState& fs = apply<FinalState>(event, "FS");
      // total hadronic and muonic cross sections
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      // find the Lambdas
      bool matched = false;
      const FinalState& ufs = apply<UnstableParticles>(event, "UFS");
      Particle Lambda;
      for (unsigned int ix = 0; ix < ufs.particles().size(); ++ix) {
        const Particle& p1 = ufs.particles()[ix];
        if (abs(p1.pid()) != 3122) continue;
        // check fs
        bool fs = true;
        for (const Particle& child : p1.children()) {
          if (child.pid() == p1.pid()) {
            fs = false;
            break;
          }
        }
        if (!fs) continue;
        // find the children
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p1, nRes, ncount);
        for (unsigned int iy = ix + 1; iy < ufs.particles().size(); ++iy) {
          matched = false;
          const Particle& p2 = ufs.particles()[iy];
          if (abs(p2.pid()) != 3122) continue;
          // check fs
          bool fs = true;
          for (const Particle& child : p2.children()) {
            if (child.pid() == p2.pid()) {
              fs = false;
              break;
            }
          }
          if (!fs) continue;
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(p2, nRes2, ncount2);
          if (ncount2 != 0) continue;
          matched = true;
          for (const auto& val : nRes2) {
            if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            _h_sigma->fill("2.396"s);
            if (p1.pid() == PID::LAMBDA) {
              Lambda = p1;
            }
            else {
              Lambda = p2;
            }
            break;
          }
        }
        if (matched) break;
      }
      // now for the polarization measurements
      if (matched) {
        double cTheta = Lambda.momentum().p3().unit().dot(axis);
        _h_cTheta->fill(cTheta);
        Particle proton;
        if (Lambda.children().size() != 2) return;
        if (Lambda.children()[0].pid() == PID::PROTON && Lambda.children()[1].pid() == PID::PIMINUS)
          proton = Lambda.children()[0];
        else if (Lambda.children()[1].pid() == PID::PROTON && Lambda.children()[0].pid() == PID::PIMINUS)
          proton = Lambda.children()[1];
        else
          return;
        LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambda.momentum().betaVec());
        Vector3 axis1 = boost1.transform(proton.momentum()).p3().unit();
        double cPhi = axis1.dot(Lambda.momentum().p3().unit());
        _h_pol->fill(cTheta, cPhi);
      }
    }

    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.xWidth();
        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() {
      scale(_h_sigma, crossSection() / sumOfWeights() / picobarn);
      normalize(_h_cTheta);
      Estimate1DPtr _h_alpha;
      book(_h_alpha, 3, 1, 1);
      for (auto& b : _h_pol->bins()) {
        normalize(b);
        pair<double, double> alpha = calcAlpha(b);
        _h_alpha->bin(b.index()).set(alpha.first, alpha.second);
      }
    }
    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _h_sigma;
    Histo1DPtr _h_cTheta;
    Histo1DGroupPtr _h_pol;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2019_I1726357);

}