Rivet analyses

// -*- 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);

}