Rivet analyses

title: BES_1999_I507637

$\psi(2S)\to J/\psi\pi^+\pi^-$

Experiment: BES (BEPC)

Inspire ID: 507637

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D62 (2000) 032002

Beams: * *

Beam energies: ANY

Run details: - In principle any process producing psi(2S), but e+e- -> psi(2S) needs for some dists

Measurement of the $\pi^+\pi^-$ mass Spectrum and other distributions in $\psi(2S)\to J/\psi\pi^+\pi^-$ decays.

Source code:BES_1999_I507637.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief psi(2S) -> J/Psi pi+pi- class BES_1999_I507637 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BES_1999_I507637);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  book(_h_mpipi, 1, 1, 1);
  book(_h_cosl, 1, 1, 2);
  book(_h_cosX, 1, 1, 3);
  book(_h_cospi, 1, 1, 4);
}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       Particles& pip,
                       Particles& pim,
                       Particles& onium) {
  for (const Particle& p : mother.children()) {
    int id = p.pid();
    if (id == PID::PIMINUS) {
      pim.push_back(p);
      ++nstable;
    }
    else if (id == PID::PIPLUS) {
      pip.push_back(p);
      ++nstable;
    }
    else if (id == 443) {
      onium.push_back(p);
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, pip, pim, onium);
    }
    else
      ++nstable;
  }
}

void findLeptons(const Particle& mother, unsigned int& nstable, Particles& lp, Particles& lm) {
  for (const Particle& p : mother.children()) {
    int id = p.pid();
    if (id == 11 || id == 13) {
      lm.push_back(p);
      ++nstable;
    }
    else if (id == -11 || id == -13) {
      lp.push_back(p);
      ++nstable;
    }
    else if (!p.children().empty()) {
      findLeptons(p, nstable, lp, lm);
    }
    else
      ++nstable;
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // loop over unstable particles
  for (const Particle& psi : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == 100443)) {
    unsigned int nstable(0);
    Particles pip, pim, onium;
    findDecayProducts(psi, nstable, pip, pim, onium);
    // check for onium
    if (onium.size() != 1 || nstable != 3 || pip.size() != 1 || pim.size() != 1) continue;
    FourMomentum q = pip[0].momentum() + pim[0].momentum();
    _h_mpipi->fill(q.mass());
    _h_cosX->fill(cos(q.polarAngle()));
    // leptons from J/psi decay
    nstable = 0;
    Particles lp, lm;
    findLeptons(onium[0], nstable, lp, lm);
    if (nstable == 2 && lp.size() == 1 && lm.size() == 1) {
      LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(onium[0].momentum().betaVec());
      FourMomentum pl = boost.transform(lp[0].momentum());
      _h_cosl->fill(cos(pl.polarAngle()));
    }
    // pions in rest frame
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(q.betaVec());
    FourMomentum ppi = boost.transform(pip[0].momentum());
    Vector3 axis1 = q.p3().unit();
    _h_cospi->fill(axis1.dot(ppi.p3().unit()));
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_mpipi);
  normalize(_h_cosl, 1., false);
  normalize(_h_cosX);
  normalize(_h_cospi);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_mpipi, _h_cosl, _h_cosX, _h_cospi;
/// @}

};

RIVET_DECLARE_PLUGIN(BES_1999_I507637);

} ```