Rivet analyses

title: BABAR_2018_I1691222

$e^+e^-\to e^+e^-\eta^\prime$ via intermediate photons at 10.58 GeV

Experiment: BABAR (PEP-II)

Inspire ID: 1691222

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 11, 112002

Beams: e+ e-

Beam energies: (5.3, 5.3)GeV

Run details: - e+ e- > e+e- meson via photon photon -> meson

Measurement of the cross section for the production of $\eta^\prime$ in photon-photon collisions, i.e. $e^+e^-\to \gamma\gamma e^+e^-$ followed by $\gamma\gamma\to\eta^\prime$, by the Babar experiment at 10.58 GeV. This measurement is doubly differential in the virtuality of the two photons.

Source code:BABAR_2018_I1691222.cc

```c++ // -- 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- > e+e- eta' class BABAR_2018_I1691222 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2018_I1691222);


/// @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");
  // book the histograms
  book(_h_etap, 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()];
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

bool findScattered(Particle beam, double& q2) {
  bool found = false;
  Particle scat = beam;
  while (!scat.children().empty()) {
    found = false;
    for (const Particle& p : scat.children()) {
      if (p.pid() == scat.pid()) {
        scat = p;
        found = true;
        break;
      }
    }
    if (!found) break;
  }
  if (!found) return false;
  q2 = -(beam.momentum() - scat.momentum()).mass2();
  return true;
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  if (_edges.empty()) _edges = _h_etap->xEdges();
  // find scattered leptons and calc Q2
  const Beam& beams = apply<Beam>(event, "Beams");
  double q12 = -1, q22 = -1;
  if (!findScattered(beams.beams().first, q12)) vetoEvent;
  if (!findScattered(beams.beams().second, q22)) vetoEvent;
  if (q22 > q12) swap(q12, q22);
  // check the final state
  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;
  }
  // find the meson
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles(Cuts::pid == 331)) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    bool matched = true;
    for (const auto& val : nRes) {
      if (abs(val.first) == 11) {
        if (val.second != 1) {
          matched = false;
          break;
        }
      }
      else if (val.second != 0) {
        matched = false;
        break;
      }
    }
    if (matched) {
      // 2<Q2<10 for both photons bin
      if (q12 > 2. && q12 < 10. && q22 > 2. && q22 < 10.) {
        _h_etap->fill(_edges[0], 1. / sqr(8.));
      }
      // 10<Q2<30 for both photons bin
      else if (q12 > 10 && q12 < 30. && q22 > 10. && q22 < 30.)
        _h_etap->fill(_edges[1], 1. / sqr(20.));
      // 10<Q12<30 2<Q22<10
      else if (q22 > 2. && q22 < 10. && q12 > 10. && q12 < 30.)
        _h_etap->fill(_edges[2], 1. / 8. / 20. / 2.);
      // 2<Q22<30 30<Q12<60
      else if (q22 > 2. && q22 < 30. && q12 > 30. && q12 < 60.)
        _h_etap->fill(_edges[3], 1. / 28. / 30. / 2.);
      // 30<Q2<60 for both photons
      else if (q22 > 30. && q22 < 60. && q12 > 30. && q12 < 60.)
        _h_etap->fill(_edges[4], 1. / sqr(30.));
    }
  }
}

/// Normalise histograms etc., after the run
void finalize() {
  scale(_h_etap, 1e4 * crossSection() / femtobarn / sumW());
}

///@}


/// @name Histograms
///@{
BinnedHistoPtr<string> _h_etap;
vector<string> _edges;
///@}

};

RIVET_DECLARE_PLUGIN(BABAR_2018_I1691222);

} ```