Rivet analyses

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

namespace Rivet {


  /// @brief Lambda_c -> Sigma+ pi0,eta,eta' decay asymmetries
  class BELLE_2022_I2140379 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2022_I2140379);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");
      // histograms
      for (unsigned int ix = 0; ix < 3; ++ix) book(_h[ix], 2, 1, 1 + ix);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Lambda_c baryons
      for (const Particle& Lambdac : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == 4122)) {
        int sign = Lambdac.pid() / 4122;
        if (Lambdac.children().size() != 2) continue;
        Particle baryon1;
        int imeson = -1;
        if (Lambdac.children()[0].pid() == sign * 3222 && Lambdac.children()[1].pid() == 111) {
          baryon1 = Lambdac.children()[0];
          imeson = 0;
        }
        else if (Lambdac.children()[1].pid() == sign * 3222 && Lambdac.children()[0].pid() == 111) {
          baryon1 = Lambdac.children()[1];
          imeson = 0;
        }
        else if (Lambdac.children()[0].pid() == sign * 3222 && Lambdac.children()[1].pid() == 221) {
          baryon1 = Lambdac.children()[0];
          imeson = 1;
        }
        else if (Lambdac.children()[1].pid() == sign * 3222 && Lambdac.children()[0].pid() == 221) {
          baryon1 = Lambdac.children()[1];
          imeson = 1;
        }
        else if (Lambdac.children()[0].pid() == sign * 3222 && Lambdac.children()[1].pid() == 331) {
          baryon1 = Lambdac.children()[0];
          imeson = 2;
        }
        else if (Lambdac.children()[1].pid() == sign * 3222 && Lambdac.children()[0].pid() == 331) {
          baryon1 = Lambdac.children()[1];
          imeson = 2;
        }
        else
          continue;
        Particle baryon2;
        if (baryon1.children()[0].pid() == sign * 2212 && baryon1.children()[1].pid() == 111) {
          baryon2 = baryon1.children()[0];
        }
        else if (baryon1.children()[1].pid() == sign * 2212 && baryon1.children()[0].pid() == 111) {
          baryon2 = baryon1.children()[1];
        }
        else
          continue;
        // first boost to the Lambdac rest frame
        LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
        FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
        FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
        // to sigma+ 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);
        _h[imeson]->fill(cTheta);
      }
    }

    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() {
      pair<double, double> aSigma(-.983, 0.013);
      for (unsigned int ix = 0; ix < 3; ++ix) {
        normalize(_h[ix]);
        Estimate0DPtr _h_alpha1;
        book(_h_alpha1, 1, 1 + ix, 1);
        pair<double, double> alpha = calcAlpha(_h[ix]);
        _h_alpha1->set(alpha.first, alpha.second);
        // divide out alpha Sigma
        alpha.second = alpha.first / aSigma.first
            * sqrt(sqr(alpha.second / alpha.first) + sqr(aSigma.second / aSigma.first));
        alpha.first /= aSigma.first;
        Estimate0DPtr _h_alpha2;
        book(_h_alpha2, 1, 1 + ix, 2);
        _h_alpha2->set(alpha.first, alpha.second);
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h[3];
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BELLE_2022_I2140379);

}