Rivet analyses

title: MARKII_1979_I144382

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

Experiment: MARKII (SPEAR)

Inspire ID: 144382

Status: VALIDATED

Authors: - Peter Richardson

References: none listed

Beams: * *

Beam energies: ANY

Run details: none listed

Measurement of the mass spectrum for $\pi^+\pi^-$ in $\psi(2S)\to\pi^+\pi^-J/\psi$ decays by the MARK-II experiment.

Source code:MARKII_1979_I144382.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief psi(2S) -> J/psi pi+pi- class MARKII_1979_I144382 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(MARKII_1979_I144382);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  declare(UnstableParticles(Cuts::pid == 100443), "UFS");
  for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 1 + ix, 1, 1);
}


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;
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // loop over unstable particles
  for (const Particle& ups : apply<UnstableParticles>(event, "UFS").particles()) {
    unsigned int nstable(0);
    Particles pip, pim, onium;
    findDecayProducts(ups, nstable, pip, pim, onium);
    // check for onium
    if (onium.size() != 1 || nstable != 3 || pip.size() != 1 || pim.size() != 1) continue;
    double mpipi = (pip[0].momentum() + pim[0].momentum()).mass();
    _h[0]->fill(sqr(mpipi));
    double mpi = 2. * pip[0].mass();
    double x = (mpipi - mpi) / (ups.mass() - onium[0].mass() - mpi);
    _h[1]->fill(x);
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  for (unsigned int ix = 0; ix < 2; ++ix) normalize(_h[ix], 1., false);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[2];
/// @}

};

RIVET_DECLARE_PLUGIN(MARKII_1979_I144382);

} ```