Rivet analyses

title: BESII_2004_I652399

Mass and angular distributions in $J/\psi\to\omega \pi^+\pi^-$

Experiment: BESII (BEPC)

Inspire ID: 652399

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Lett.B 598 (2004) 149-158

Beams: e- e+

Beam energies: (1.6, 1.6)GeV

Run details: - e+e- > J/psi

Mass and angular distributions in $J/\psi\to\omega \pi^+\pi^-$. The data were read from the figures in the paper and are not corrected for acceptance.

Source code:BESII_2004_I652399.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief J/psi -> omega pi+ pi- class BESII_2004_I652399 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESII_2004_I652399);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 443);
  declare(ufs, "UFS");
  DecayedParticles PSI(ufs);
  PSI.addStable(223);
  declare(PSI, "PSI");
  declare(Beam(), "Beams");
  // histogram
  for (unsigned int ix = 0; ix < 2; ++ix) {
    book(_h_mass[ix], 1, 1, 1 + ix);
    for (unsigned int iy = 0; iy < 4; ++iy) {
      book(_h_angle[iy][ix], 2, 1 + iy, 1 + ix);
    }
  }
}

void findChildren(const Particle& p,
                  Particles& pim,
                  Particles& pip,
                  Particles& pi0,
                  unsigned int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.pid() == PID::PIPLUS) {
      pip.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PIMINUS) {
      pim.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PI0) {
      pi0.push_back(child);
      ncount += 1;
    }
    else if (child.children().empty()) {
      ncount += 1;
    }
    else {
      findChildren(child, pim, pip, pi0, ncount);
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // get the axis, direction of incoming electron
  const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
  Vector3 axis;
  if (beams.first.pid() > 0) {
    axis = beams.first.mom().p3().unit();
  }
  else {
    axis = beams.second.mom().p3().unit();
  }
  // J/psi decay products
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  if (PSI.decaying().size() != 1) vetoEvent;
  if (!PSI.modeMatches(0, 3, mode)) vetoEvent;
  const Particle& Pip = PSI.decayProducts()[0].at(211)[0];
  const Particle& Pim = PSI.decayProducts()[0].at(-211)[0];
  const Particle& omega = PSI.decayProducts()[0].at(223)[0];
  // mass histograms
  FourMomentum pPiPi = Pip.mom() + Pim.mom();
  const double mPiPi = pPiPi.mass();
  _h_mass[0]->fill(mPiPi / GeV);
  _h_mass[1]->fill((Pip.mom() + omega.mom()).mass() / GeV);
  _h_mass[1]->fill((Pim.mom() + omega.mom()).mass() / GeV);
  unsigned int iplot = 0;
  if (mPiPi > .3 && mPiPi < .5)
    iplot = 0;
  else if (mPiPi > .5 && mPiPi < .7)
    iplot = 1;
  else
    return;
  if (abs(Pip.p3().unit().dot(axis)) > 0.84) return;
  if (abs(Pim.p3().unit().dot(axis)) > 0.84) return;
  LorentzTransform boost;
  if (PSI.decaying()[0].mom().p3().mod() > 1 * MeV) {
    boost = LorentzTransform::mkFrameTransformFromBeta(PSI.decaying()[0].mom().betaVec());
  }
  pPiPi = boost.transform(pPiPi);
  // omega decay
  unsigned int ncount = 0;
  Particles pip, pim, pi0;
  findChildren(omega, pim, pip, pi0, ncount);
  if (ncount != 3 || !(pim.size() == 1 && pip.size() == 1 && pi0.size() == 1)) return;
  if (abs(pip[0].p3().unit().dot(axis)) > 0.84) return;
  if (abs(pim[0].p3().unit().dot(axis)) > 0.84) return;
  FourMomentum pOmega = boost.transform(omega.mom());
  Vector3 e1Z = pOmega.p3().unit();
  Vector3 e1Y = e1Z.cross(axis).unit();
  Vector3 e1X = e1Y.cross(e1Z).unit();
  const double cOmega = e1Z.dot(axis);
  _h_angle[1][iplot]->fill(cOmega);
  // pion angles
  LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pPiPi.betaVec());
  FourMomentum pPi = boost2.transform(boost.transform(Pip.mom()));
  const double cPi = pPi.p3().unit().dot(e1Z);
  _h_angle[2][iplot]->fill(cPi);
  Vector3 trans1 = pPi.p3().unit() - cPi * e1Z;
  // omega angles
  LorentzTransform boost3 = LorentzTransform::mkFrameTransformFromBeta(pOmega.betaVec());
  FourMomentum ppip = boost3.transform(boost.transform(pip[0].mom()));
  FourMomentum ppim = boost3.transform(boost.transform(pim[0].mom()));
  Vector3 nW = ppip.p3().cross(ppim.p3()).unit();
  const double bW = nW.dot(e1X);
  _h_angle[3][iplot]->fill(bW);
  Vector3 trans2 = ppip.p3() - ppip.p3().dot(e1Z) * e1Z;
  const double chi = abs(atan2(trans1.cross(trans2).dot(e1Z), trans1.dot(trans2)));
  _h_angle[0][iplot]->fill(chi / M_PI * 180.);
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_mass, 1.0, false);
  for (unsigned int iy = 0; iy < 4; ++iy) {
    normalize(_h_angle[iy], 1.0, false);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_mass[2], _h_angle[4][2];
const map<PdgId, unsigned int> mode = {{223, 1}, {211, 1}, {-211, 1}};
/// @}

};

RIVET_DECLARE_PLUGIN(BESII_2004_I652399);

} ```