Rivet analyses

Mass distributions in D0 → KS0π0π0 decays

Experiment: BESIII (BEPC)

Inspire ID: 3074754

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - 2510.25111 [hep-ex]

Beams: * *

Beam energies: ANY

Run details: - Any process producing D0 mesons, original e+e-

Measurement of the mass distributions in the decay D0 → KS0π0π0 by BES. The data were read from the plots in the paper and backgrounds given subtracted therefore for some points the error bars are the size of the point. It is also not clear that any resolution effects have been unfolded.

Source code:BESIII_2025_I3074754.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#include "Rivet/Projections/DecayedParticles.hh"

namespace Rivet {


  /// @brief D0 -> KS0 pi+pi-
  class BESIII_2025_I3074754 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2025_I3074754);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      UnstableParticles ufs = UnstableParticles(Cuts::abspid==421);
      declare(ufs, "UFS");
      DecayedParticles D0(ufs);
      D0.addStable(PID::PI0);
      D0.addStable(PID::K0S);
      D0.addStable(PID::ETA);
      D0.addStable(PID::ETAPRIME);
      declare(D0, "D0");
      // histos
      for (size_t ix=0; ix<3; ++ix) {
        book(_h[ix],1+ix,1,1);
      }
    }

    void findDecayProducts(const Particle & mother, unsigned int & nstable,
               Particles& pip , Particles& pim , Particles& K0) const {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::PIPLUS) {
          pip.push_back(p);
          ++nstable;
        }
        else if (id == PID::PIMINUS) {
          pim.push_back(p);
          ++nstable;
        }
        else if (id == PID::K0S) {
          K0.push_back(p);
          ++nstable;
        }
        else if (id == PID::KPLUS || id == PID::KMINUS || id == PID::PI0||id == PID::K0L) {
          ++nstable;
        }
        else if ( !p.children().empty() ) {
          findDecayProducts(p, nstable, pip, pim, K0);
        }
        else {
          ++nstable;
        }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      static const map<PdgId,unsigned int> & mode = { { 111,2}, {PID::K0S,1} };
      DecayedParticles D0 = apply<DecayedParticles>(event, "D0");
      for (size_t ix=0; ix<D0.decaying().size(); ++ix) {
        if (! D0.modeMatches(ix,3,mode)) continue;
        const Particle& KS0 = D0.decayProducts()[ix].at(PID::K0S)[0];
        const Particles& pi0 = D0.decayProducts()[ix].at(PID::PI0);
        double mkpi2[2] = {(KS0.momentum()+pi0[0].momentum()).mass2(),
                           (KS0.momentum()+pi0[1].momentum()).mass2()};
        double mpipi2 = (pi0[0].momentum()+pi0[1].momentum()).mass2();
        _h[2]->fill(mpipi2);

        LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(D0.decaying()[ix].momentum().betaVec());
        double ppi[2] = {boost.transform(pi0[0].momentum()).p3().mod(),
                         boost.transform(pi0[1].momentum()).p3().mod()};
        if (ppi[1]>ppi[0]) swap(mkpi2[0],mkpi2[1]);
        for (size_t ix=0; ix<2; ++ix) _h[ix]->fill(mkpi2[ix]);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h,1.,false);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[3];
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BESIII_2025_I3074754);

}