Rivet analyses

γγ → η′π+π− for centre-of-mass energies between 1.4 and 3.8 GeV

Experiment: BELLE (KEKB)

Inspire ID: 1672149

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 7, 072001

Beams: 22 22

Beam energies: ANY

Run details: - gamma gamma to hadrons

Measurement of the cross section for γγ → η′π+π− for centre-of-mass energies between 1.4 and 3.8 GeV. The resonance subprocess γγ → f2(1270)η′ is alos measured, and subtracted from the final result above 2.26 GeV. The contribution from ηc(1) is also subtracted between 2.62,3.06 GeV.

Source code:BELLE_2018_I1672149.cc

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

namespace Rivet {


  /// @brief gamma gamma -> eta' pi+pi-
  class BELLE_2018_I1672149 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2018_I1672149);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(FinalState(), "FS");
      declare(UnstableParticles(), "UFS");
      // counters
      for (unsigned int ix = 0; ix < 2; ++ix) {
        book(_sigma[ix], "TMP/sigma_" + toString(ix + 1), refData(1 + ix, 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");
      // find the final-state particles
      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");
      // first check for f_2 eta'
      bool foundf2(false);
      Particles f2s = ufs.particles(Cuts::pid == 225);
      Particles etaps = ufs.particles(Cuts::pid == 331);
      for (const Particle& f2 : f2s) {
        bool matched = false;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(f2, nRes, ncount);
        for (const Particle& etap : etaps) {
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(etap, nRes2, ncount2);
          if (ncount2 != 0) continue;
          matched = true;
          for (const auto& val : nRes2) {
            if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            foundf2 = true;
            break;
          }
        }
        if (foundf2) break;
      }
      // if we have the f_2 eta' state
      if (foundf2) {
        _sigma[0]->fill(sqrtS());
        if (sqrtS() > 2.26 * GeV) vetoEvent;
      }
      // see if we have eta' pi+pi-
      bool foundetap(false);
      for (const Particle& etap : etaps) {
        bool matched = false;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(etap, nRes, ncount);
        if (ncount != 2) continue;
        matched = true;
        for (const auto& val : nRes) {
          if (abs(val.first) == 211) {
            if (val.second != 1) {
              matched = false;
              break;
            }
          }
          else if (val.second != 0) {
            matched = false;
            break;
          }
        }
        if (matched) {
          foundetap = true;
          break;
        }
      }
      // check there's no eta_c
      if (foundetap && sqrtS() > 2.62 && sqrtS() < 3.06) {
        for (const Particle& etac : ufs.particles(Cuts::pid == 441)) {
          bool matched = false;
          map<long, int> nRes = nCount;
          int ncount = ntotal;
          findChildren(etac, nRes, ncount);
          for (const auto& val : nRes) {
            if (val.second != 0) {
              matched = false;
              break;
            }
          }
          if (matched) {
            foundetap = false;
            break;
          }
        }
      }
      if (foundetap) _sigma[1]->fill(sqrtS());
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      const double fact = crossSection() / nanobarn / sumOfWeights();
      // loop over tables in paper
      for (unsigned int ix = 0; ix < 2; ++ix) {
        scale(_sigma[ix], fact);
        Estimate1DPtr tmp;
        book(tmp, ix + 1, 1, 1);
        barchart(_sigma[ix], tmp);
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _sigma[2];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2018_I1672149);

}