Rivet analyses

title: BABAR_2009_I819092

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++ // -- 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);

} ```