Rivet analyses

γγ → ρ0ρ0 between 1.2 and 2.0 GeV

Experiment: TASSO (PETRA)

Inspire ID: 180755

Status: VALIDATED

Authors: - Peter Richardson

References: - Z.Phys.C 16 (1982) 13, 1982

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons, pi0 mesons must be set stable

Measurement of the differential cross section for γγ → ρ0ρ0 for 1.2GeV < W < 2.0GeV. The cross section is measured as a function of the centre-of-mass energy of the photonic collision using the 2π+2π− final state.

Source code:TASSO_1982_I180755.cc

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

namespace Rivet {


  /// @brief gamma gamma -> rho0 rho0
  class TASSO_1982_I180755 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(TASSO_1982_I180755);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // book histos
      if (inRange(sqrtS() / GeV, 1.2, 2.0)) {
        book(_nRho, "TMP/nrho", refData(1, 1, 1));
      }
      else {
        throw Error("Invalid CMS energy for TASSO_1982_I180755");
      }
    }

    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");
      // find the final-state particles
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      // find any rho mesons
      Particles rho = apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 113);
      for (unsigned int ix = 0; ix < rho.size(); ++ix) {
        if (rho[ix].children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(rho[ix], nRes, ncount);
        bool matched = false;
        for (unsigned int iy = ix + 1; iy < rho.size(); ++iy) {
          if (rho[iy].children().empty()) continue;
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(rho[iy], nRes2, ncount2);
          if (ncount2 != 0) continue;
          matched = true;
          for (const auto& val : nRes2) {
            if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            break;
          }
        }
        if (matched) {
          _nRho->fill(sqrtS() / GeV);
          break;
        }
      }
    }


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

    /// @}


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


  RIVET_DECLARE_PLUGIN(TASSO_1982_I180755);

}