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 e+ e- > Lambda_c+ Lambda_c-
  class BESIII_2023_I2677290 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2023_I2677290);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(Cuts::abspid == 4122), "UFS");
      declare(FinalState(), "FS");
      // histograms
      for (size_t ih = 0; ih < 5; ++ih) {
        book(_est[ih], 1, 1, 1 + ih);
      }
      for (const string& en : _est[0].binning().edges<0>()) {

        double eval = stod(en);
        if (isCompatibleWithSqrtS(eval)) _sqs = en;

        book(_h[en + "cThetaL"], "TMP/cThetaL_" + en, 20, -1., 1.);
      }
      raiseBeamErrorIf(_sqs.empty());
    }

    void findChildren(const Particle& p, map<long, int>& nRes, int& ncount) const {
      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.mom().p3().unit();
      else
        axis = beams.second.mom().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;
      }
      const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
      Particle Lambda, LamBar;
      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;
              }
            }
            // found baryon and antibaryon
            if (matched) {
              if (p.pid() > 0) {
                Lambda = p;
                LamBar = p2;
              }
              else {
                Lambda = p2;
                LamBar = p;
              }
              break;
            }
          }
        }
        if (matched) break;
      }
      if (!matched) vetoEvent;
      const double cosL = axis.dot(Lambda.mom().p3().unit());
      _h[_sqs + "cThetaL"]->fill(cosL);
    }

    pair<double, pair<double, double>> calcAlpha0(const Histo1DPtr& hist) const {
      if (hist->numEntries() == 0.) return make_pair(0., make_pair(0., 0.));
      const double d = 3. / (pow(hist->xMax(), 3) - pow(hist->xMin(), 3));
      const 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()) {
        const double Oi = bin.sumW();
        if (Oi == 0.) continue;
        const double a = d * (bin.xMax() - bin.xMin());
        const double b = d / 3. * (pow(bin.xMax(), 3) - pow(bin.xMin(), 3));
        const 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
      const double alpha = (-c * sum1 + sqr(c) * sum2 + sum3 - c * sum5)
          / (sum1 - c * sum2 + c * sum4 - sum5);
      // and error
      const double cc = -pow((sum3 + sqr(c) * sum4 - 2 * c * sum5), 3);
      const double bb = -2 * sqr(sum3 + sqr(c) * sum4 - 2 * c * sum5) * (sum1 - c * sum2 + c * sum4 - sum5);
      const 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() {
      // storage of the values to fill histos
      const double mn = 2.28646;
      const double alpha = 7.2973525693e-3;
      const double GeV2pb = 0.3893793721e9;

      for (const string& en : _est[0].binning().edges<0>()) {

        if (isZero(_h[en + "cThetaL"]->effNumEntries())) continue;
        double eval = stod(en);

        const double tau = 4. * sqr(mn / eval);
        const double beta = sqrt(1. - tau);
        // prefactor and sigma
        double sigma0 = 4. * M_PI * sqr(alpha / eval) * beta * GeV2pb;
        // calculate alpha0
        pair<double, pair<double, double>> alpha0 = calcAlpha0(_h[en + "cThetaL"]);
        scale(_h[en + "cThetaL"], crossSection() / sumOfWeights() / picobarn);
        const double wsum = _h[en + "cThetaL"]->integral();
        const double werr = _h[en + "cThetaL"]->integralError();
        // Geff
        pair<double, double> Geff = make_pair(1e2 * sqrt(3. * wsum / (sigma0 * (1 + 0.5 * tau))),
                                              1e2 * 1.5 * wsum / (sigma0 * (1 + 0.5 * tau)));
        // GM
        double GMv = 1e2 * sqrt(6. * ((1 + alpha0.first) * wsum) / ((3 + alpha0.first) * sigma0));
        double GMe1 = 1e2
            * sqrt((3
                    * (sqr(werr) * sqr(1 + alpha0.first) * sqr(3 + alpha0.first)
                       + 4 * sqr(alpha0.second.first) * sqr(wsum)))
                   / (2. * (1 + alpha0.first) * pow(3 + alpha0.first, 3) * wsum * sigma0));
        double GMe2 = 1e2
            * sqrt((3
                    * (sqr(werr) * sqr(1 + alpha0.first) * sqr(3 + alpha0.first)
                       + 4 * sqr(alpha0.second.second) * sqr(wsum)))
                   / (2. * (1 + alpha0.first) * pow(3 + alpha0.first, 3) * wsum * sigma0));
        pair<double, pair<double, double>> GM = make_pair(std::move(GMv),
                                                          make_pair(std::move(GMe1), std::move(GMe2)));
        // ratio
        pair<double, pair<double, double>> R;
        R.first = sqrt((1 - alpha0.first) / (tau + alpha0.first * tau));
        R.second.first = R.first * alpha0.second.first / (1. - sqr(alpha0.first));
        R.second.second = R.first * alpha0.second.second / (1. - sqr(alpha0.first));
        for (size_t ix = 1; ix < 6; ++ix) {
          double val;
          pair<double, double> err;
          if (ix == 1) {
            val = wsum;
            err = make_pair(werr, werr);
          }
          else if (ix == 2) {
            val = Geff.first;
            err = make_pair(Geff.second, Geff.second);
          }
          else if (ix == 3) {
            val = alpha0.first;
            err = alpha0.second;
          }
          else if (ix == 4) {
            val = R.first;
            err = R.second;
          }
          else {
            val = GM.first;
            err = GM.second;
          }
          _est[ix - 1]->binAt(en).set(val, err);
        }
      }
    }

    /// @}


    /// @name Histograms
    /// @{
    map<string, Histo1DPtr> _h;
    BinnedEstimatePtr<string> _est[5];
    string _sqs = "";
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2023_I2677290);

}