Rivet analyses

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

namespace Rivet {


  /// @brief B -> D* semileptonic
  class BELLE_2023_I2624324 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2023_I2624324);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::abspid == 511 || Cuts::abspid == 521), "UFS");
      // histograms
      for (unsigned int ix = 0; ix < 4; ++ix) {
        book(_h_aver[ix], 2, 1 + ix, 1);
        for (unsigned int iy = 0; iy < 4; ++iy) {
          book(_h[ix][iy], 1, 1 + ix, 1 + iy);
        }
      }
    }

    /// Perform the per-event analysis
    bool analyzeDecay(const Particle& mother, const vector<int>& ids) {
      // There is no point in looking for decays with less particles than to be analysed
      if (mother.children().size() == ids.size()) {
        bool decayfound = true;
        for (int id : ids) {
          if (!contains(mother, id)) decayfound = false;
        }
        return decayfound;
      }
      return false;
    }

    bool contains(const Particle& mother, int id) {
      return any(mother.children(), HasPID(id));
    }

    double recoilW(const Particle& mother) {
      FourMomentum lepton, neutrino, meson, q;
      for (const Particle& c : mother.children()) {
        if (c.isNeutrino()) neutrino = c.mom();
        if (c.isLepton() && !c.isNeutrino()) lepton = c.mom();
        if (c.isHadron()) meson = c.mom();
      }
      q = lepton + neutrino; //no hadron before
      double mb2 = mother.mom() * mother.mom();
      double mD2 = meson * meson;
      return (mb2 + mD2 - q * q) / (2. * sqrt(mb2) * sqrt(mD2));
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      FourMomentum pl, pnu, pB, pD, pDs, ppi;
      // Iterate of B mesons
      for (const Particle& p : apply<UnstableParticles>(event, "UFS").particles()) {
        pB = p.mom();
        // Find semileptonic decays
        int sign = p.pid() / p.abspid();
        int iDStar = sign * (p.abspid() == 511 ? -413 : -423);
        int iloc = -1;
        if (analyzeDecay(p, {iDStar, 12 * sign, -11 * sign}))
          iloc = 0;
        else if (analyzeDecay(p, {iDStar, 14 * sign, -13 * sign}))
          iloc = 1;
        else
          continue;
        if (p.abspid() == 521) iloc += 2;
        double w = recoilW(p);
        _h[0][iloc]->fill(w);
        _h_aver[0]->fill(w);
        // Get the necessary momenta for the angles
        bool foundDdecay = false;
        for (const Particle& c : p.children()) {
          if (c.abspid() == 413 || c.abspid() == 423) {
            if ((c.pid() == -413
                 && (analyzeDecay(c, {PID::PIMINUS, PID::D0BAR}) || analyzeDecay(c, {PID::PI0, PID::DMINUS})))
                || (c.pid() == 413
                    && (analyzeDecay(c, {PID::PIPLUS, PID::D0}) || analyzeDecay(c, {PID::PI0, PID::DPLUS})))
                || (c.pid() == -423 && analyzeDecay(c, {PID::PI0, PID::D0BAR}))
                || (c.pid() == 423 && analyzeDecay(c, {PID::PI0, PID::D0}))) {
              foundDdecay = true;
              pDs = c.mom();
              for (const Particle& dc : c.children()) {
                if (dc.hasCharm())
                  pD = dc.mom();
                else
                  ppi = dc.mom();
              }
            }
          }
          else if (c.abspid() == 11 || c.abspid() == 13)
            pl = c.mom();
          else if (c.abspid() == 12 || c.abspid() == 14)
            pnu = c.mom();
        }
        // This is the angle analysis
        if (!foundDdecay) continue;
        // First boost all relevant momenta into the B-rest frame
        const LorentzTransform B_boost = LorentzTransform::mkFrameTransformFromBeta(pB.betaVec());
        // Momenta in B rest frame:
        FourMomentum lv_brest_Dstar = B_boost.transform(pDs);
        FourMomentum lv_brest_w = B_boost.transform(pB - pDs);
        FourMomentum lv_brest_D = B_boost.transform(pD);
        FourMomentum lv_brest_lep = B_boost.transform(pl);

        const LorentzTransform Ds_boost = LorentzTransform::mkFrameTransformFromBeta(
            lv_brest_Dstar.betaVec());
        FourMomentum lv_Dstarrest_D = Ds_boost.transform(lv_brest_D);
        const LorentzTransform W_boost = LorentzTransform::mkFrameTransformFromBeta(lv_brest_w.betaVec());
        FourMomentum lv_wrest_lep = W_boost.transform(lv_brest_lep);

        double cos_thetaV = cos(lv_brest_Dstar.p3().angle(lv_Dstarrest_D.p3()));
        _h[2][iloc]->fill(cos_thetaV);
        _h_aver[2]->fill(cos_thetaV);

        double cos_thetaL = cos(lv_brest_w.p3().angle(lv_wrest_lep.p3()));
        _h[1][iloc]->fill(cos_thetaL);
        _h_aver[1]->fill(cos_thetaL);

        Vector3 LTrans = lv_wrest_lep.p3() - cos_thetaL * lv_wrest_lep.p3().perp() * lv_brest_w.p3().unit();
        Vector3 VTrans = lv_Dstarrest_D.p3()
            - cos_thetaV * lv_Dstarrest_D.p3().perp() * lv_brest_Dstar.p3().unit();
        float chi = atan2(LTrans.cross(VTrans).dot(lv_brest_w.p3().unit()), LTrans.dot(VTrans));
        if (chi < 0.) chi += 2. * M_PI;
        _h[3][iloc]->fill(chi);
        _h_aver[3]->fill(chi);
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h, 1.0);
      normalize(_h_aver, 1.0);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[4][4];
    Histo1DPtr _h_aver[4];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2023_I2624324);

}