Rivet analyses

Measurement of the Dalitz plot for η → π+π−π0

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 η → π+π−π0 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++ -*-
#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);


}