Rivet analyses

title: BABAR_2009_I825406

Polarization in $B^0\to a_1^+a_1^-$

Experiment: BABAR (PEP-II)

Inspire ID: 825406

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 80 (2009) 092007

Beams: * *

Beam energies: ANY

Run details: - Any process producing B0, originally Upsilon94S) decays

Measurement of the polarization in n $B^0\to a_1^+a_1^-$ decays

Source code:BABAR_2009_I825406.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief B0 -> a1+ a1- class BABAR_2009_I825406 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2009_I825406);


/// @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, "B0");
  // histograms
  book(_p[0], 1, 1, 1);
  book(_p[1], "TMP/wgt");
}

void findChildren(const Particle& p,
                  Particles& pim,
                  Particles& pip,
                  Particles& pi0,
                  unsigned int& ncount) {
  for (const Particle& child : p.children()) {
    if (child.pid() == PID::PIPLUS) {
      pip.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PIMINUS) {
      pim.push_back(child);
      ncount += 1;
    }
    else if (child.pid() == PID::PI0) {
      pi0.push_back(child);
      ncount += 1;
    }
    else if (child.children().empty()) {
      ncount += 1;
    }
    else
      findChildren(child, pim, pip, pi0, ncount);
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  Particles B0 = apply<UnstableParticles>(event, "B0").particles();
  for (const Particle& p : B0) {
    // skip cases with mixing
    if (p.children().size() == 1 && p.children()[0].abspid() == p.abspid()) continue;
    // particle antiparticle pair of a_1
    if (p.children().size() != 2 || p.children()[0].pid() != -p.children()[1].pid()
        || p.children()[0].abspid() != 20213)
      continue;
    Particle a1p = p.children()[0], a1m = p.children()[1];
    if ((p.pid() > 0 && a1p.pid() < 0) || (p.pid() < 0 && a1p.pid() > 0)) swap(a1p, a1m);
    Particles pip, pim, pi0;
    unsigned int ncount = 0;
    findChildren(a1p, pim, pip, pi0, ncount);
    if (ncount != 3 || pip.size() != 2 || pim.size() != 1) continue;
    LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
    FourMomentum pa1p = boost.transform(a1p.momentum());
    FourMomentum pa1m = boost.transform(a1m.momentum());
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pa1p.betaVec());
    FourMomentum ppip = boost2.transform(boost.transform(pip[0].momentum()));
    FourMomentum ppim = boost2.transform(boost.transform(pim[0].momentum()));
    Vector3 n = ppip.p3().cross(ppim.p3()).unit();
    double cTheta = n.dot(pa1m.p3().unit());
    _p[0]->fill((2. - 5. * sqr(cTheta)));
    _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_2009_I825406);

} ```