Rivet analyses

Decay asymmetries in Ξc0 → Ξ0π0

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 Ξc0 → Ξ0π0 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++ -*-
#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);

}