Rivet analyses

Mass distributions and $\bar{\Lambda}^0$ polarization in $B^0\to \bar{\Lambda}^0 p\pi^-$

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 $\bar\Lambda^0 p$ mass spectrum and $\bar\Lambda^0$ energy in the decay $B^0\to\bar{\Lambda}^0 p \pi^-$. The polarization of the $\bar\Lambda^0$ 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 $\alpha_\Lambda$ 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/DecayedParticles.hh"
#include "Rivet/Projections/UnstableParticles.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);

}