Rivet analyses

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

namespace Rivet {


  /// @brief Lambda Lambdabar cross section
  class BESIII_2023_I2642062 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2642062);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      book(_sigmaLambda, "TMP/sigma", refData(1, 1, 1));
    }

    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) {
      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
      const FinalState& ufs = apply<UnstableParticles>(event, "UFS");
      for (unsigned int ix = 0; ix < ufs.particles().size(); ++ix) {
        const Particle& p1 = ufs.particles()[ix];
        if (abs(p1.pid()) != 3122) continue;
        bool matched = false;
        // 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) {
          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) {
            _sigmaLambda->fill(sqrtS());
            break;
          }
        }
        if (matched) break;
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_sigmaLambda, crossSection() / sumOfWeights() / picobarn);
      Estimate1DPtr tmp;
      book(tmp, 1, 1, 1);
      barchart(_sigmaLambda, tmp);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _sigmaLambda;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2023_I2642062);

}