Rivet analyses

title: KLOE2_2016_I1416990

Measurement of the Dalitz plot for $\eta\to\pi^+\pi^-\pi^0$

Experiment: KLOE2 (DAPHNE)

Inspire ID: 1416990

Status: VALIDATED

Authors: - Peter Richardson

References: - JHEP 1605 (2016) 019

Beams: * *

Beam energies: ANY

Run details: - Any process producing eta mesons

Measurement of the dalitz plot for the decay $\eta\to\pi^+\pi^-\pi^0$ by the KLOE2 collaboration. The distributions are normalised such that the bin at $X=0$, $Y=0.05$ is $1$.

Source code:KLOE2_2016_I1416990.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief eta -> 3 pi analysis class KLOE2_2016_I1416990 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(KLOE2_2016_I1416990);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");

  // Book histograms
  book(_dalitz, 1, 1, 1);
  book(_h_dalitz,
       {-0.9, -0.8, -0.7, -0.6, -0.5, -0.4, -0.3, -0.2, -0.1, 0., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7,
        0.8});
  for (unsigned int i = 1; i <= _h_dalitz->numBins(); ++i) {
    book(_h_dalitz->bins()[i], 2, 1, i);
  }
  book(_norm, "TMP/norm");
}

void findDecayProducts(const Particle& mother,
                       unsigned int& nstable,
                       Particles& pi0,
                       Particles& pip,
                       Particles& pim) {
  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 == PID::PI0) {
      pi0.push_back(p);
      ++nstable;
    }
    else if (!p.children().empty()) {
      findDecayProducts(p, nstable, pi0, pip, pim);
    }
    else {
      ++nstable;
    }
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  // Loop over eta mesons
  for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid == PID::ETA)) {
    Particles pi0, pip, pim;
    unsigned nstable(0);
    findDecayProducts(p, nstable, pi0, pip, pim);
    if (nstable == 3 && pi0.size() == 1 && pip.size() == 1 && pim.size() == 1) {
      // massesx
      double meta = p.mass();
      double mpip = pip[0].mass();
      double mpim = pim[0].mass();
      double mpi0 = pi0[0].mass();
      // kinetic energies
      double Q = meta - mpip - mpim - mpi0;
      double Ep = 0.5 / meta * (sqr(meta) + sqr(mpip) - (p.momentum() - pip[0].momentum()).mass2())
          - mpip;
      double Em = 0.5 / meta * (sqr(meta) + sqr(mpim) - (p.momentum() - pim[0].momentum()).mass2())
          - mpim;
      double E0 = 0.5 / meta * (sqr(meta) + sqr(mpi0) - (p.momentum() - pi0[0].momentum()).mass2())
          - mpi0;
      // X and Y variables
      double X = sqrt(3.) / Q * (Ep - Em);
      double Y = 3. * E0 / Q - 1.;
      _dalitz->fill(X, Y);
      _h_dalitz->fill(Y, X);
      if (fabs(X) < 0.03225806451612903 && Y > 0. && Y < 0.1) _norm->fill();
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_dalitz, 0.06451612903225806 * 0.1 / *_norm);
  scale(_h_dalitz, 0.06451612903225806 / *_norm);
}

/// @}


/// @name Histograms
/// @{
Histo2DPtr _dalitz;
Histo1DGroupPtr _h_dalitz;
CounterPtr _norm;
/// @}

};

RIVET_DECLARE_PLUGIN(KLOE2_2016_I1416990);

} ```