Rivet analyses

Mass distributions and Λ̄0 polarization in B0 → Λ̄0pπ−

Experiment: BABAR (PEP-II)

Inspire ID: 819092

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 79 (2009) 112009

Beams: * *

Beam energies: ANY

Run details: - Any process producing B0 mesons, originally Upsilon(4S) decays

Measurement of the Λ̄0p mass spectrum and Λ̄0 energy in the decay B0 → Λ̄0pπ−. The polarization of the Λ̄0 is also measured. The data were read from the plots/tables in the paper but are efficiency corrected and background subtracted. In additon the values of the polarization were adjusted to use the PDG 2022 value of αΛ as there has been a significant change due to due measuremnts.

Source code:BABAR_2009_I819092.cc

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

namespace Rivet {


  /// @brief B0 -> lambdabar p pi-
  class BABAR_2009_I819092 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2009_I819092);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      UnstableParticles ufs = UnstableParticles(Cuts::pid==511);
      declare(ufs, "UFS");
      DecayedParticles B0(ufs);
      B0.addStable( 3122);
      B0.addStable(-3122);
      declare(B0, "B0");
      book(_h_pol1,2,1,1);
      for(unsigned int ix=0;ix<3;++ix) {
    if(ix<2) book(_h_mass[ix],1,1,1+ix);
    book(_h_pol2[ix],3,1,1+ix);
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      double alpha = -0.732;
      static const map<PdgId,unsigned int> & mode   = { { 2212,1},{-3122,1}, {-211,1}};
      DecayedParticles B0 = apply<DecayedParticles>(event, "B0");
      // loop over particles
      for(unsigned int ix=0;ix<B0.decaying().size();++ix) {
    if (!B0.modeMatches(ix,3,mode)) continue;
        const Particle & pp     = B0.decayProducts()[ix].at( 2212)[0];
        const Particle & LamBar = B0.decayProducts()[ix].at(-3122)[0];
    _h_mass[0]->fill( (pp.momentum()+LamBar.momentum()).mass());
    // boost to B rest frame
    LorentzTransform boost =
      LorentzTransform::mkFrameTransformFromBeta(B0.decaying()[ix]. momentum().betaVec());
    FourMomentum pLam    = boost.transform(LamBar.momentum());
    FourMomentum pProton = boost.transform(pp    .momentum());
    _h_mass[1]->fill(pLam.E());
    // Lambda decay products
    if(LamBar.children().size()!=2) continue;
    Particle pbar;
    if(LamBar.children()[0].pid()==-2212 &&
       LamBar.children()[1].pid()== 211) {
      pbar = LamBar.children()[0];
    }
    else if(LamBar.children()[1].pid()==-2212 &&
        LamBar.children()[0].pid()== 211) {
      pbar = LamBar.children()[1];
    }
    else
      continue;
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pLam.betaVec());
    Vector3 axisP = boost2.transform(boost.transform(pbar.momentum())).p3().unit();
    Vector3 axis1 = pLam.p3().unit();
    double cTheta = axisP.dot(axis1);
    _h_pol1   ->fill(pLam.E(),3.*cTheta);
    _h_pol2[0]->fill(pLam.E(),3.*cTheta/alpha);
    Vector3 axis2 = pLam.p3().cross(pProton.p3()).unit();
    cTheta = axisP.dot(axis2);
    _h_pol2[1]->fill(pLam.E(),3.*cTheta/alpha);
    Vector3 axis3 = axis1.cross(axis2);
    cTheta = axisP.dot(axis3);
    _h_pol2[2]->fill(pLam.E(),3.*cTheta/alpha);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for(unsigned int ix=0;ix<2;++ix)
    normalize(_h_mass[ix],1.,false);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_mass[2];
    Profile1DPtr _h_pol1,_h_pol2[3];
    /// @}


  };


  RIVET_DECLARE_PLUGIN(BABAR_2009_I819092);

}