Rivet analyses

title: BESIII_2018_I1703033

Dalitz plot analysis of $\omega\to\pi^+\pi^-\pi^0$

Experiment: BESIII (BEPC)

Inspire ID: 1703033

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 98 (2018) 11, 112007

Beams: * *

Beam energies: ANY

Run details: - Any process producing omega -> pi+pi-pi0

Measurement of kinematic distributions in the decay $\omega\to\pi^+\pi^-\pi^0$ by BESIII. The data were read from the plots in the paper and therefore for most points the error bars are the size of the point. It is also not clear that any resolution/acceptance effects have been unfolded.

Source code:BESIII_2018_I1703033.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief omega -> pi+pi-pi0 class BESIII_2018_I1703033 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1703033);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::pid == 223);
  declare(ufs, "UFS");
  DecayedParticles OMEGA(ufs);
  OMEGA.addStable(PID::PI0);
  declare(OMEGA, "OMEGA");
  // histograms
  book(_h_z, 1, 1, 1);
  book(_h_phi, 1, 1, 2);
  book(_h_cos, 1, 1, 3);
  book(_h_ppi, 1, 1, 4);
  book(_dalitz, "dalitz", 50, -1, 1, 50, -1.1, 0.9);
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode = {{211, 1}, {-211, 1}, {111, 1}};
  static const std::complex<double> ii(0., 1.);
  DecayedParticles OMEGA = apply<DecayedParticles>(event, "OMEGA");
  // loop over particles
  for (unsigned int ix = 0; ix < OMEGA.decaying().size(); ++ix) {
    // pi+ pi- pi0
    if (!OMEGA.modeMatches(ix, 3, mode)) continue;
    // apply omega mass cut
    if (abs(OMEGA.decaying()[ix].mass() - 0.78265) > 0.04) continue;
    // decay products
    const Particle& pip = OMEGA.decayProducts()[ix].at(211)[0];
    const Particle& pim = OMEGA.decayProducts()[ix].at(-211)[0];
    const Particle& pi0 = OMEGA.decayProducts()[ix].at(111)[0];
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(
        OMEGA.decaying()[ix].momentum().betaVec());
    FourMomentum pp = boost.transform(pip.momentum());
    FourMomentum pm = boost.transform(pim.momentum());
    FourMomentum p0 = boost.transform(pi0.momentum());
    // variables from eqn 1 in paper (use version from Eqn 2.1 of 1010.3946)
    double Qc = OMEGA.decaying()[ix].mass() - pi0.mass() - pim.mass() - pip.mass();
    double x = sqrt(3.) * (pm.E() - pp.E()) / Qc;
    double y = 3. / Qc * (p0.E() - pi0.mass()) - 1.;
    _dalitz->fill(x, y);
    // plot arg and phase of variable
    std::complex<double> zz = x + ii * y;
    _h_z->fill(norm(zz));
    double phi = arg(zz);
    if (phi < 0.) phi += 2. * M_PI;
    _h_phi->fill(phi);
    double ctheta = -pp.p3().unit().dot(p0.p3().unit());
    _h_cos->fill(ctheta);
    _h_ppi->fill(p0.p3().mod());
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_z);
  normalize(_h_phi);
  normalize(_h_cos);
  normalize(_h_ppi);
  normalize(_dalitz);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_z, _h_phi, _h_cos, _h_ppi;
Histo2DPtr _dalitz;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2018_I1703033);

} ```