Rivet analyses

title: BESIII_2017_I1604892

$e^+e^-\to \pi^+\pi^- J/\psi$ for $\sqrt{s}=4.23$ and $4.26$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 1604892

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 119 (2017) 7, 072001

Beams: e+ e-

Beam energies: (2.1, 2.1); (2.1, 2.1)GeV

Run details: none listed

Measurement of mass distributions in $e^+e^-\to \pi^+\pi^- J/\psi$ for $\sqrt{s}=4.23$ and $4.26$ GeV.

Source code:BESIII_2017_I1604892.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FinalState.hh"

namespace Rivet {

/// @brief e+ e- -> pi+ pi- J/psi class BESIII_2017_I1604892 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1604892);


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

/// Book histograms and initialise projections before the run
void init() {
  // projections
  declare(FinalState(), "FS");
  size_t ih = 0;
  for (double eVal : allowedEnergies()) {

    const string en = toString(round(eVal / MeV));
    if (isCompatibleWithSqrtS(eVal)) _sqs = en;

    book(_h[en + "mass1"], 1, 1 + ih, 1);
    book(_h[en + "mass2"], 1, 1 + ih, 2);
    ++ih;
  }
  raiseBeamErrorIf(_sqs.empty());
  book(_h["cTheta"], 2, 1, 1);
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles fs = apply<FinalState>(event, "FS").particles();
  Particles Jpsi, other;
  for (const Particle& p : fs) {
    Particle parent = p;
    while (!parent.parents().empty()) {
      parent = parent.parents()[0];
      if (parent.abspid() == PID::JPSI) break;
    }
    if (parent.abspid() != PID::JPSI) {
      other.push_back(p);
      continue;
    }
    bool found = false;
    for (const auto& psi : Jpsi) {
      // J/psi already in list
      if (fuzzyEquals(psi.mom(), parent.mom())) {
        found = true;
        break;
      }
    }
    if (!found) Jpsi.push_back(parent);
  }
  if (Jpsi.size() != 1 || other.size() != 2) vetoEvent;
  if (other[0].pid() != -other[1].pid() || other[0].abspid() != PID::PIPLUS) vetoEvent;
  _h[_sqs + "mass1"]->fill((other[0].mom() + other[1].mom()).mass());
  for (size_t ix = 0; ix < 2; ++ix) {
    FourMomentum pZ = Jpsi[0].mom() + other[ix].mom();
    double mZ = pZ.mass() / GeV;
    _h[_sqs + "mass2"]->fill(mZ);
    if (mZ > 3.86 && mZ < 3.92) {
      LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(pZ.betaVec());
      Vector3 axis1 = pZ.p3().unit();
      Vector3 axis2 = boost.transform(other[ix].mom()).p3().unit();
      _h["cTheta"]->fill(abs(axis1.dot(axis2)));
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h, 1.0, false);
}

/// @}


/// @name Histograms
/// @{
map<string, Histo1DPtr> _h;
string _sqs = "";
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2017_I1604892);

} ```