Rivet analyses

title: BABAR_2008_I758472

Longitudinal Polarization in $B^0\to K^{0}\bar{K}^{0}$

Experiment: BABAR (PEP-II)

Inspire ID: 758472

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 100 (2008) 081801

Beams: * *

Beam energies: ANY

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

Measurement of the longitudinal polarization of the $K^{0}$ mesons in the decay $B^0\to K^{0}\bar{K}^{*0}$ by the BaBar collaboration. Only the longitudinal polarization is implemented due to the large backgrounds in the kinematic distributions in the paper.

Source code:BABAR_2008_I758472.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Longitudinal Polarization in B -> K0 K0 class BABAR_2008_I758472 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2008_I758472);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 511);
  declare(ufs, "UFS");
  DecayedParticles BB(ufs);
  BB.addStable(313);
  BB.addStable(-313);
  declare(BB, "BB");
  // histos
  book(_p[0], 1, 1, 1);
  book(_p[1], "TMP/wgt");
}

bool isK(int id) {
  return id == 321 || id == 311 || id == 310 || id == 130;
}

bool isPi(int id) {
  return id == 211 || id == 111;
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode1 = {{-313, 1}, {313, 1}};

  DecayedParticles BB = apply<DecayedParticles>(event, "BB");
  // loop over particles
  for (unsigned int ix = 0; ix < BB.decaying().size(); ++ix) {
    if (!BB.modeMatches(ix, 2, mode1)) continue;
    int sign = BB.decaying()[ix].pid() / BB.decaying()[ix].abspid();
    const Particle& Kstar = BB.decayProducts()[ix].at(sign * 313)[0];
    Particle KK;
    if (isK(Kstar.children()[0].abspid()) && isPi(Kstar.children()[1].abspid()))
      KK = Kstar.children()[0];
    else if (isK(Kstar.children()[1].abspid()) && isPi(Kstar.children()[0].abspid()))
      KK = Kstar.children()[1];
    else
      continue;
    // boost to B rest frame
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(
        BB.decaying()[ix].momentum().betaVec());
    FourMomentum pKstar = boost.transform(Kstar.momentum());
    FourMomentum pK = boost.transform(KK.momentum());
    const LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pKstar.betaVec());
    pK = boost2.transform(pK);
    FourMomentum pB = boost2.transform(boost.transform(BB.decaying()[ix].momentum()));
    double cosK = -pB.p3().unit().dot(pK.p3().unit());
    _p[0]->fill(0.5 * (5. * sqr(cosK) - 1));
    _p[1]->fill();
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_p[0], 1. / *_p[1]);
}

/// @}


/// @name Histograms
/// @{
CounterPtr _p[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2008_I758472);

} ```