Rivet analyses

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

namespace Rivet {


  /// @brief Xi_c0 -> Xi-pi+ asymmetry
  class BELLE_2021_I1851126 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2021_I1851126);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(Cuts::abspid == 4132), "UFS");
      // Book histograms
      book(_h_c_P, 1, 1, 1);
      book(_h_c_M, 1, 1, 2);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Xi_c0 baryons
      for (const Particle& Xic : apply<UnstableParticles>(event, "UFS").particles()) {
        int sign = Xic.pid() / 4132;
        if (Xic.children().size() != 2) continue;
        Particle baryon1, meson1;
        if (Xic.children()[0].pid() == sign * 3312 && Xic.children()[1].pid() == sign * 211) {
          baryon1 = Xic.children()[0];
          meson1 = Xic.children()[1];
        }
        else if (Xic.children()[1].pid() == sign * 3312 && Xic.children()[0].pid() == sign * 211) {
          baryon1 = Xic.children()[1];
          meson1 = Xic.children()[0];
        }
        else
          continue;
        Particle baryon2, meson2;
        if (baryon1.children()[0].pid() == sign * 3122 && baryon1.children()[1].pid() == -sign * 211) {
          baryon2 = baryon1.children()[0];
          meson2 = baryon1.children()[1];
        }
        else if (baryon1.children()[1].pid() == sign * 3122 && baryon1.children()[0].pid() == -sign * 211) {
          baryon2 = baryon1.children()[1];
          meson2 = baryon1.children()[0];
        }
        else
          continue;
        // first boost to the Xic rest frame
        LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xic.momentum().betaVec());
        FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
        FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
        // to lambda rest frame
        LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
        Vector3 axis = pbaryon1.p3().unit();
        FourMomentum pp = boost2.transform(pbaryon2);
        // calculate angle
        double cTheta = pp.p3().unit().dot(axis);
        if (sign > 0)
          _h_c_P->fill(cTheta, 1.);
        else
          _h_c_M->fill(cTheta, 1.);
      }
    }

    pair<double, double> calcAlpha(Histo1DPtr hist) {
      if (hist->numEntries() == 0.) return make_pair(0., 0.);
      double sum1(0.), sum2(0.);
      for (const auto& bin : hist->bins()) {
        double Oi = bin.sumW();
        if (Oi == 0.) continue;
        double ai = 0.5 * (bin.xMax() - bin.xMin());
        double bi = 0.5 * ai * (bin.xMax() + bin.xMin());
        double Ei = bin.errW();
        sum1 += sqr(bi / Ei);
        sum2 += bi / sqr(Ei) * (Oi - ai);
      }
      return make_pair(sum2 / sum1, sqrt(1. / sum1));
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      // first mode
      normalize(_h_c_P);
      Estimate0DPtr _h_alpha_P;
      book(_h_alpha_P, 2, 1, 1);
      pair<double, double> alphaP = calcAlpha(_h_c_P);
      alphaP.first /= -0.401;
      alphaP.second /= -0.401;
      _h_alpha_P->set(alphaP.first, alphaP.second);
      // second mode
      normalize(_h_c_M);
      Estimate0DPtr _h_alpha_M;
      book(_h_alpha_M, 2, 1, 2);
      pair<double, double> alphaM = calcAlpha(_h_c_M);
      alphaM.first /= 0.389;
      alphaM.second /= 0.389;
      _h_alpha_M->set(alphaM.first, alphaM.second);
      // average
      double aver = 0.5 * (-alphaP.first + alphaM.first);
      double err = 0.5 * sqrt(sqr(alphaP.second) + sqr(alphaM.second));
      Estimate0DPtr _h_alpha_aver;
      book(_h_alpha_aver, 2, 1, 3);
      _h_alpha_aver->set(aver, err);
      // asymetry
      double asym = (alphaP.first + alphaM.first) / (alphaP.first - alphaM.first);
      err = 2. / sqr(alphaP.first - alphaM.first)
          * sqrt(sqr(alphaM.first * alphaP.second) + sqr(alphaM.second * alphaP.first));
      Estimate0DPtr _h_alpha_asym;
      book(_h_alpha_asym, 2, 1, 4);
      _h_alpha_asym->set(asym, err);
    }

    ///@}


    /// @name Histograms
    ///@{
    Histo1DPtr _h_c_M, _h_c_P;
    ///@}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2021_I1851126);

}