Rivet analyses

title: BELLE_2024_I2796026

Decay asymmetries in $\Xi^0_c\to\Xi^0\pi^0$

Experiment: BELLE (KEKB)

Inspire ID: 2796026

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - JHEP 10 (2024) 045

Beams: * *

Beam energies: ANY

Run details: - Any process producing Xi_c0

Measurement of the decay asymmetries in $\Xi^0_c\to\Xi^0\pi^0$ by the BELLE experiment. The asymmetry parameter is extracted by fitting to normalised angular distribution. This analysis is useful for testing spin correlations in hadron decays.

Source code:BELLE_2024_I2796026.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @briefXi_c0 -> Xi0pi0 asymmetry class BELLE_2024_I2796026 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2024_I2796026);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(Cuts::abspid == 4132), "UFS");
  // Book histograms
  for (unsigned int ix = 0; ix < 2; ++ix) book(_h[ix], 2, 1, 1 + ix);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // loop over Xi_c0 baryons
  for (const Particle& Xic : apply<UnstableParticles>(event, "UFS").particles()) {
    int sign = Xic.pid() / 4132;
    if (Xic.children().size() != 2) continue;
    Particle baryon1, meson1;
    if (Xic.children()[0].pid() == sign * 3322 && Xic.children()[1].pid() == 111) {
      baryon1 = Xic.children()[0];
      meson1 = Xic.children()[1];
    }
    else if (Xic.children()[1].pid() == sign * 3322 && Xic.children()[0].pid() == 111) {
      baryon1 = Xic.children()[1];
      meson1 = Xic.children()[0];
    }
    else
      continue;
    Particle baryon2, meson2;
    if (baryon1.children()[0].pid() == sign * 3122 && baryon1.children()[1].pid() == 111) {
      baryon2 = baryon1.children()[0];
      meson2 = baryon1.children()[1];
    }
    else if (baryon1.children()[1].pid() == sign * 3122 && baryon1.children()[0].pid() == 111) {
      baryon2 = baryon1.children()[1];
      meson2 = baryon1.children()[0];
    }
    else
      continue;
    // first boost to the Xic rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xic.momentum().betaVec());
    FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
    FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
    // to lambda rest frame
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
    Vector3 axis = pbaryon1.p3().unit();
    FourMomentum pp = boost2.transform(pbaryon2);
    // calculate angle
    double cTheta = pp.p3().unit().dot(axis);
    for (unsigned int ix = 0; ix < 2; ++ix) _h[ix]->fill(cTheta);
  }
}

pair<double, double> calcAlpha(Histo1DPtr hist) {
  if (hist->numEntries() == 0.) return make_pair(0., 0.);
  double sum1(0.), sum2(0.);
  for (const auto& bin : hist->bins()) {
    double Oi = bin.sumW();
    if (Oi == 0.) continue;
    double ai = 0.5 * (bin.xMax() - bin.xMin());
    double bi = 0.5 * ai * (bin.xMax() + bin.xMin());
    double Ei = bin.errW();
    sum1 += sqr(bi / Ei);
    sum2 += bi / sqr(Ei) * (Oi - ai);
  }
  return make_pair(sum2 / sum1, sqrt(1. / sum1));
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h, 1.);
  Estimate0DPtr _h_alpha;
  book(_h_alpha, 1, 1, 1);
  pair<double, double> alphaP = calcAlpha(_h[0]);
  alphaP.first /= -0.349;
  alphaP.second /= -0.349;
  _h_alpha->set(alphaP.first, alphaP.second);
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BELLE_2024_I2796026);

} ```