Rivet analyses

title: BABAR_2012_I1107905

Cross section for $e^+e^-\to J/\psi\pi^+\pi^-$ at energies between 3.75 and 5.5 GeV

Experiment: BABAR (PEP-II)

Inspire ID: 1107905

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 86 (2012) 051102

Beams: e+ e-

Beam energies: ANY

Run details: - e+ e- to hadrons

Measurement of the cross section for $e^+e^-\to J/\psi\pi^+\pi^-$ at energies between 3.75 and 5.5 GeV using radiative return by BABAR.

Source code:BABAR_2012_I1107905.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief e+ e- > pi+ pi- J/psi class BABAR_2012_I1107905 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2012_I1107905);


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

/// Book histograms and initialise projections before the run
void init() {
  declare(FinalState(), "FS");
  declare(UnstableParticles(Cuts::pid == 443), "UFS");
  book(_sigmaJPsi, "TMP/jpsi", 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()];
      --ncount;
    }
    else {
      findChildren(child, nRes, ncount);
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  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;
  }
  const FinalState& ufs = apply<FinalState>(event, "UFS");
  for (const Particle& p : ufs.particles()) {
    if (p.children().empty()) continue;
    map<long, int> nRes = nCount;
    int ncount = ntotal;
    findChildren(p, nRes, ncount);
    // J/psi pi+pi-
    if (ncount != 2) continue;
    bool 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) {
      _sigmaJPsi->fill(sqrtS());
      break;
    }
  }
}


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

/// @}


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

};

RIVET_DECLARE_PLUGIN(BABAR_2012_I1107905);

} ```