Rivet analyses

// -*- 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);

}