Rivet analyses

title: BESIII_2025_I3074754

Mass distributions in $D^0\to K^0_S\pi^0\pi^0$ 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 $D^0\to K^0_S\pi^0\pi^0$ 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++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.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);

} ```