Rivet analyses

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Exclusive $D^0\to K^-,\pi^-e^+\nu_e$ decays
  class BESIII_2015_I1391138 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2015_I1391138);


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

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(UnstableParticles(), "UFS");

      // Book histograms
      book(_h_q2_K,  1, 1, 3);
      book(_h_q2_pi, 2, 1, 3);
      book(nD0, "TMP/DCounter");
    }


    // Calculate the Q2 using mother and daugher meson
    size_t q2(const Particle& B, int mesonID, size_t species) const {
      FourMomentum q = B.mom() - select(B.children(), Cuts::pid==mesonID)[0];
      return axes[species].index(q*q) - 1;
    }


    // Check for explicit decay into pdgids
    bool isSemileptonicDecay(const Particle& mother, vector<int> ids) const {
      // Trivial check to ignore any other decays but the one in question modulo photons
      const Particles children = mother.children(Cuts::pid!=PID::PHOTON);
      if (children.size()!=ids.size()) return false;
      // Check for the explicit decay
      return all(ids, [&](int i){return count(children, hasPID(i))==1;});
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {

      if (edges_K.empty())   edges_K = _h_q2_K->xEdges();
      if (edges_pi.empty())  edges_pi = _h_q2_pi->xEdges();

      // Loop over D0 mesons
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==PID::D0)) {
        nD0->fill();
        if (isSemileptonicDecay(p, {PID::PIMINUS, PID::POSITRON, PID::NU_E})) {
          _h_q2_pi->fill( edges_pi[q2(p, PID::PIMINUS, 1)] );
        }
        else if (isSemileptonicDecay(p, {PID::KMINUS, PID::POSITRON, PID::NU_E})) {
          _h_q2_K ->fill( edges_K[q2(p, PID::KMINUS, 0)] );
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      // scale by D0 lifetime = 410.1e-6 ps (from PDG 2014 used in paper)
      scale(_h_q2_K , 1./dbl(*nD0)/410.1e-6);
      scale(_h_q2_pi, 1./dbl(*nD0)/410.1e-6);
    }

    /// @}


    /// @name Histograms
    /// @{
    BinnedHistoPtr<string> _h_q2_K, _h_q2_pi;
    CounterPtr nD0;
    vector<string> edges_K, edges_pi;
    YODA::Axis<double> axes[2] = { YODA::Axis<double>(17, 0.0, 1.7),
                                   YODA::Axis<double>(13, 0.0, 2.6) };
    /// @}

  };


  RIVET_DECLARE_PLUGIN(BESIII_2015_I1391138);

}