Rivet analyses

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

namespace Rivet {


  /// @brief psi(2s) -> Sigma- sigmabar+
  class BESIII_2022_I2158325 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2158325);


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

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

      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(Cuts::abspid == 3112), "UFS");
      declare(FinalState(), "FS");
      // histograms
      book(_h, 1, 1, 1);
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) {
      for (const Particle& child : p.children()) {
        if (child.children().empty()) {
          nRes[child.pid()] -= 1;
          --ncount;
        }
        else
          findChildren(child, nRes, ncount);
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // get the axis, direction of incoming electron
      const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
      Vector3 axis;
      if (beams.first.pid() > 0)
        axis = beams.first.momentum().p3().unit();
      else
        axis = beams.second.momentum().p3().unit();
      // types of final state particles
      const FinalState& fs = apply<FinalState>(event, "FS");
      map<long, int> nCount;
      int ntotal(0);
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        ++ntotal;
      }
      // loop over Sigma+ baryons
      const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
      Particle Sigma, SigBar;
      bool matched(false);
      for (const Particle& p : ufs.particles()) {
        if (p.children().empty()) continue;
        map<long, int> nRes = nCount;
        int ncount = ntotal;
        findChildren(p, nRes, ncount);
        matched = false;
        // check for antiparticle
        for (const Particle& p2 : ufs.particles(Cuts::pid == -p.pid())) {
          if (p2.children().empty()) continue;
          map<long, int> nRes2 = nRes;
          int ncount2 = ncount;
          findChildren(p2, nRes2, ncount2);
          if (ncount2 == 0) {
            matched = true;
            for (const auto& val : nRes2) {
              if (val.second != 0) {
                matched = false;
                break;
              }
            }
            // fond baryon and antibaryon
            if (matched) {
              if (p.pid() > 0) {
                Sigma = p;
                SigBar = p2;
              }
              else {
                Sigma = p2;
                SigBar = p;
              }
              break;
            }
          }
        }
        if (matched) break;
      }
      if (!matched) vetoEvent;
      _h->fill(axis.dot(Sigma.momentum().p3().unit()));
    }

    pair<double, pair<double, double>> calcAlpha0(Histo1DPtr hist) {
      if (hist->numEntries() == 0.) return make_pair(0., make_pair(0., 0.));
      double d = 3. / (pow(hist->xMax(), 3) - pow(hist->xMin(), 3));
      double c = 3. * (hist->xMax() - hist->xMin()) / (pow(hist->xMax(), 3) - pow(hist->xMin(), 3));
      double sum1(0.), sum2(0.), sum3(0.), sum4(0.), sum5(0.);
      for (const auto& bin : hist->bins()) {
        double Oi = bin.sumW();
        if (Oi == 0.) continue;
        double a = d * (bin.xMax() - bin.xMin());
        double b = d / 3. * (pow(bin.xMax(), 3) - pow(bin.xMin(), 3));
        double Ei = bin.errW();
        sum1 += a * Oi / sqr(Ei);
        sum2 += b * Oi / sqr(Ei);
        sum3 += sqr(a) / sqr(Ei);
        sum4 += sqr(b) / sqr(Ei);
        sum5 += a * b / sqr(Ei);
      }
      // calculate alpha
      double alpha = (-c * sum1 + sqr(c) * sum2 + sum3 - c * sum5) / (sum1 - c * sum2 + c * sum4 - sum5);
      // and error
      double cc = -pow((sum3 + sqr(c) * sum4 - 2 * c * sum5), 3);
      double bb = -2 * sqr(sum3 + sqr(c) * sum4 - 2 * c * sum5) * (sum1 - c * sum2 + c * sum4 - sum5);
      double aa = sqr(sum1 - c * sum2 + c * sum4 - sum5)
          * (-sum3 - sqr(c) * sum4 + sqr(sum1 - c * sum2 + c * sum4 - sum5) + 2 * c * sum5);
      double dis = sqr(bb) - 4. * aa * cc;
      if (dis > 0.) {
        dis = sqrt(dis);
        return make_pair(alpha, make_pair(0.5 * (-bb + dis) / aa, -0.5 * (-bb - dis) / aa));
      }
      else {
        return make_pair(alpha, make_pair(0., 0.));
      }
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h);
      // calculate alpha0
      pair<double, pair<double, double>> alpha0 = calcAlpha0(_h);
      Estimate0DPtr _h_alpha0;
      book(_h_alpha0, 2, 1, 1);
      _h_alpha0->set(alpha0.first, make_pair(alpha0.second.first, alpha0.second.second));
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2022_I2158325);

}