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 J/psi to p pbar n nbar
  class BESIII_2012_I1113599 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2012_I1113599);


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

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

      // Initialise and register projections
      declare(Beam(), "Beams");
      declare(UnstableParticles(), "UFS");
      declare(FinalState(), "FS");

      // Book histograms
      book(_h_proton, "ctheta_p", 20, -1., 1.);
      book(_h_neutron, "ctheta_n", 20, -1., 1.);
    }


    /// 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);
      Particle outgoing;
      for (const Particle& p : fs.particles()) {
        nCount[p.pid()] += 1;
        if (p.pid() == 2212 || p.pid() == 2112) outgoing = p;
        ++ntotal;
      }
      if (ntotal == 2) {
        if (nCount[2212] == 1 && nCount[-2212] == 1) {
          _h_proton->fill(outgoing.momentum().p3().unit().dot(axis));
        }
        else if (nCount[2112] == 1 && nCount[-2112] == 1) {
          _h_neutron->fill(outgoing.momentum().p3().unit().dot(axis));
        }
      }
    }

    pair<double, pair<double, double>> calcAlpha(Histo1DPtr hist) {
      if (hist->numEntries() == 0.) return make_pair(0., make_pair(0., 0.));
      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 = 1.5 * (bin.xMax() - bin.xMin());
        double b = 0.5 * (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 = (-3 * sum1 + 9 * sum2 + sum3 - 3 * sum5) / (sum1 - 3 * sum2 + 3 * sum4 - sum5);
      // and error
      double cc = -pow((sum3 + 9 * sum4 - 6 * sum5), 3);
      double bb = -2 * sqr(sum3 + 9 * sum4 - 6 * sum5) * (sum1 - 3 * sum2 + 3 * sum4 - sum5);
      double aa = sqr(sum1 - 3 * sum2 + 3 * sum4 - sum5)
          * (-sum3 - 9 * sum4 + sqr(sum1 - 3 * sum2 + 3 * sum4 - sum5) + 6 * 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() {
      // proton
      normalize(_h_proton);
      pair<double, pair<double, double>> alpha = calcAlpha(_h_proton);
      Estimate0DPtr _h_alpha_proton;
      book(_h_alpha_proton, 1, 1, 1);
      _h_alpha_proton->set(alpha.first, alpha.second);
      // neutron
      normalize(_h_neutron);
      alpha = calcAlpha(_h_neutron);
      Estimate0DPtr _h_alpha_neutron;
      book(_h_alpha_neutron, 1, 1, 2);
      _h_alpha_neutron->set(alpha.first, alpha.second);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_proton, _h_neutron;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BESIII_2012_I1113599);


}