Rivet analyses

Decay asymmetries in Ξc0 → Ξ−π+

Experiment: CLEO (CESR)

Inspire ID: 537236

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B634 (2006) 165-172

Beams: * *

Beam energies: ANY

Run details: - Any process producing Lambda_c baryons

Measurement of the decay asymmetries in Ξc0 → Ξ−π+ by the CLEO 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:CLEO_2000_I537236.cc

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

namespace Rivet {


  /// @brief Xi_c0 -> Xi-pi+ asymmetry
  class CLEO_2000_I537236 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CLEO_2000_I537236);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      // Book histograms
      book(_h_ctheta, "ctheta", 20, -1, 1);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Xi_c0 baryons
      for (const Particle& Xic : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == 4132)) {
        int sign = Xic.pid() / 4132;
        if (Xic.children().size() != 2) continue;
        Particle baryon1, meson1;
        if (Xic.children()[0].pid() == sign * 3312 && Xic.children()[1].pid() == sign * 211) {
          baryon1 = Xic.children()[0];
          meson1 = Xic.children()[1];
        }
        else if (Xic.children()[1].pid() == sign * 3312 && Xic.children()[0].pid() == sign * 211) {
          baryon1 = Xic.children()[1];
          meson1 = Xic.children()[0];
        }
        else
          continue;
        Particle baryon2, meson2;
        if (baryon1.children()[0].pid() == sign * 3122 && baryon1.children()[1].pid() == -sign * 211) {
          baryon2 = baryon1.children()[0];
          meson2 = baryon1.children()[1];
        }
        else if (baryon1.children()[1].pid() == sign * 3122 && baryon1.children()[0].pid() == -sign * 211) {
          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);
        _h_ctheta->fill(cTheta, 1.);
      }
    }

    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_ctheta);
      Estimate0DPtr _h_alpha;
      book(_h_alpha, 1, 1, 1);
      pair<double, double> alpha = calcAlpha(_h_ctheta);
      _h_alpha->set(alpha.first, alpha.second);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_ctheta;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(CLEO_2000_I537236);


}