Rivet analyses

title: BABAR_2004_I632736

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

Experiment: BABAR (PEP-II)

Inspire ID: 632736

Status: VALIDATED NOHEPDATA

Authors: - Peter Richardson

References: - Phys.Rev.D 69 (2004) 031102

Beams: * *

Beam energies: ANY

Run details: none listed

Measurement of the polarization in $B^0\to\rho^+\rho^-$.

Source code:BABAR_2004_I632736.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/DecayedParticles.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief B0 -> rho+ rho- class BABAR_2004_I632736 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2004_I632736);


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

/// Book histograms and initialise projections before the run
void init() {
  // projections
  UnstableParticles ufs = UnstableParticles(Cuts::abspid == 511);
  declare(ufs, "UFS");
  DecayedParticles B0(ufs);
  B0.addStable(213);
  B0.addStable(-213);
  declare(B0, "B0");
  // histos
  book(_p[0], 1, 1, 1);
  book(_p[1], "TMP/wgt");
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  static const map<PdgId, unsigned int>& mode = {{213, 1}, {-213, 1}};
  DecayedParticles B0 = apply<DecayedParticles>(event, "B0");
  for (unsigned int ix = 0; ix < B0.decaying().size(); ++ix) {
    int sign = B0.decaying()[ix].pid() / B0.decaying()[ix].abspid();
    if (!B0.modeMatches(ix, 2, mode)) continue;
    const Particle& rhop = B0.decayProducts()[ix].at(sign * 213)[0];
    //const Particle & rhom = B0.decayProducts()[ix].at(-sign*213)[0];
    // compute the helicity angle
    if (rhop.children().size() != 2) continue;
    Particle pip;
    if (rhop.children()[0].pid() == sign * 211 && rhop.children()[1].pid() == 111)
      pip = rhop.children()[0];
    else if (rhop.children()[1].pid() == sign * 211 && rhop.children()[0].pid() == 111)
      pip = rhop.children()[1];
    else
      continue;
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(
        B0.decaying()[ix].momentum().betaVec());
    FourMomentum prho = boost1.transform(rhop.momentum());
    FourMomentum ppi = boost1.transform(pip);
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(prho.betaVec());
    ppi = boost2.transform(ppi);
    double cTheta = ppi.p3().unit().dot(prho.p3().unit());
    _p[0]->fill(-0.5 * (1. - 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_2004_I632736);

} ```