Rivet analyses

title: BABAR_2005_I686355

Helicity angle and polarization in $B^0\to D^{+}D^{-}$

Experiment: BABAR (PEP-II)

Inspire ID: 686355

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 95 (2005) 151804

Beams: * *

Beam energies: ANY

Run details: - Any process producing B0, original Upsilon(4S) decay

Helicity angle distributions and polarization in $B^0\to D^{+}D^{-}$ decays

Source code:BABAR_2005_I686355.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief B0 -> D+ D- class BABAR_2005_I686355 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2005_I686355);


/// @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");
  // histograms
  book(_p[0], 1, 1, 1);
  book(_p[1], "TMP/wgt");
  book(_h, 2, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles B0 = apply<UnstableParticles>(event, "UFS").particles();
  for (const Particle& p : B0) {
    if (p.children().size() != 2) continue;
    if (p.children()[0].pid() != -p.children()[1].pid()) continue;
    if (p.children()[0].abspid() != 413) continue;
    Particle Dp = p.children()[0];
    Particle Dm = p.children()[1];
    if (p.pid() > 0 && Dp.pid() < 0)
      swap(Dp, Dm);
    else if (p.pid() < 0 && Dp.pid() > 0)
      swap(Dp, Dm);
    // find children of the D mesons
    Particle pip, pim;
    if (Dp.children().size() != 2) continue;
    if (Dp.children()[0].abspid() == PID::PIPLUS && Dp.children()[1].abspid() == PID::D0)
      pip = Dp.children()[0];
    else if (Dp.children()[1].abspid() == PID::PIPLUS && Dp.children()[0].abspid() == PID::D0)
      pip = Dp.children()[1];
    else
      continue;
    if (Dm.children().size() != 2) continue;
    if (Dm.children()[0].abspid() == PID::PIPLUS && Dm.children()[1].abspid() == PID::D0) {
      pim = Dm.children()[0];
    }
    else if (Dm.children()[1].abspid() == PID::PIPLUS && Dm.children()[0].abspid() == PID::D0) {
      pim = Dm.children()[1];
    }
    else
      continue;
    // boost to rest frame
    LorentzTransform boostB = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
    FourMomentum pDstarPlus = boostB.transform(Dp.momentum());
    FourMomentum pDstarMinus = boostB.transform(Dm.momentum());
    LorentzTransform boostDp = LorentzTransform::mkFrameTransformFromBeta(pDstarPlus.betaVec());
    FourMomentum ppip = boostDp.transform(boostB.transform(pip.momentum()));
    LorentzTransform boostDm = LorentzTransform::mkFrameTransformFromBeta(pDstarMinus.betaVec());
    FourMomentum ppim = boostDm.transform(boostB.transform(pim.momentum()));
    Vector3 axisX = pDstarPlus.p3().unit();
    // y and z axis
    ppim = boostDp.transform(boostB.transform(pim.momentum()));
    Vector3 axisZ = axisX.cross(ppim.p3()).unit();
    // cThetaTr
    double cThetaTr = axisZ.dot(ppip.p3().unit());
    _h->fill(cThetaTr);
    _p[0]->fill(0.5 * (5. * sqr(cThetaTr) - 1));
    _p[1]->fill();
  }
}


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

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h;
CounterPtr _p[2];
/// @}

};

RIVET_DECLARE_PLUGIN(BABAR_2005_I686355);

} ```