Rivet analyses

title: BESII_2005_I678943

$\pi^+\pi^-$ mass distribution in $J/\psi\to 2\pi^+2\pi^-$

Experiment: BESII (BEPC)

Inspire ID: 678943

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 72 (2005) 012002

Beams: * *

Beam energies: ANY

Run details: - Any process producing J/psi, originally e+e-

$\pi^+\pi^-$ mass distribution in $J/\psi\to 2\pi^+2\pi^-$. The uncorrected data was read from the figures in the paper, given the lack of corrections and background subtraction only the low background $J/\psi\to 2\pi^+2\pi^-$ mode is implemented and not the $\psi(2S)\to3\pi^+\pi^-$ distributions which have a large background.

Source code:BESII_2005_I678943.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief $\pi^+\pi^-$ mass distribution in $J/\psi\to 2\pi^+2\pi^-$ class BESII_2005_I678943 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESII_2005_I678943);


/// @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);
  declare(PSI, "PSI");
  // histos
  book(_h, 1, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // find the J/psi decays
  DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
  for (unsigned int ix = 0; ix < PSI.decaying().size(); ++ix) {
    if (!PSI.modeMatches(ix, 4, mode)) continue;
    const Particles& pip = PSI.decayProducts()[0].at(211);
    const Particles& pim = PSI.decayProducts()[0].at(-211);
    bool kVeto = false;
    vector<double> mpipi;
    mpipi.reserve(4);
    for (unsigned int iy = 0; iy < 2; ++iy) {
      for (unsigned int iy = 0; iy < 2; ++iy) {
        double mpp = (pip[ix].mom() + pim[0].mom()).mass();
        kVeto |= (mpp > 0.47 && mpp < 0.53);
        mpipi.push_back(mpp);
      }
    }
    if (kVeto) continue;
    for (const double& m : mpipi) _h->fill(m);
  }
}


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

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h;
const map<PdgId, unsigned int> mode = {{211, 2}, {-211, 2}};
/// @}

};

RIVET_DECLARE_PLUGIN(BESII_2005_I678943);

} ```