Rivet analyses

Longitudinal Polarization in B0 → K*0K̄*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 B0 → K*0K̄*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++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/DecayedParticles.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Longitudinal Polarization in B -> K*0 K*0
  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);

}