Rivet analyses

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

namespace Rivet {


  /// @brief B -> D semileptonic
  class BELLE_2022_I2169621 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2022_I2169621);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // projection
      declare(UnstableParticles(Cuts::pid == 511 or Cuts::pid == 521), "UFS");
      // histograms
      for (unsigned int ix = 0; ix < 4; ++ix) book(_h[ix], 1, 1, 1 + ix);
      for (unsigned int ix = 0; ix < 2; ++ix) book(_nB[ix], "TMP/nB_" + toString(ix));
    }

    // Check for explicit decay into pdgids
    bool isSemileptonicDecay(const Particle& mother, vector<int> ids) {
      // 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; });
    }

    // Calculate the recoil w using mother and daugher meson
    double recoilW(const Particle& B, int mesonID) {
      // TODO why does that not work with const?
      Particle D = select(B.children(), Cuts::pid == mesonID)[0];
      FourMomentum q = B.mom() - D.mom();
      return (B.mom() * B.mom() + D.mom() * D.mom() - q * q)
          / (2. * sqrt(B.mom() * B.mom()) * sqrt(D.mom() * D.mom()));
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
        if (p.children().size() <= 1) continue;
        if (p.pid() == PID::BPLUS) {
          _nB[0]->fill();
          if (isSemileptonicDecay(p, {PID::D0BAR, PID::POSITRON, PID::NU_E}))
            _h[0]->fill(recoilW(p, PID::D0BAR));
          if (isSemileptonicDecay(p, {PID::D0BAR, PID::ANTIMUON, PID::NU_MU}))
            _h[1]->fill(recoilW(p, PID::D0BAR));
        }
        else if (p.pid() == PID::B0) {
          _nB[1]->fill();
          if (isSemileptonicDecay(p, {PID::DMINUS, PID::POSITRON, PID::NU_E}))
            _h[2]->fill(recoilW(p, PID::DMINUS));
          if (isSemileptonicDecay(p, {PID::DMINUS, PID::ANTIMUON, PID::NU_MU}))
            _h[3]->fill(recoilW(p, PID::DMINUS));
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      double tau[2] = {1638e-15, 1519e-15};
      double hbar = 6.582119569e-10;
      for (unsigned int ix = 0; ix < 4; ++ix) {
        double fact = ix < 2 ? hbar / tau[0] : hbar / tau[1];
        CounterPtr c = ix < 2 ? _nB[0] : _nB[1];
        scale(_h[ix], fact / *c);
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[4];
    CounterPtr _nB[2];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2022_I2169621);

}