Parameterisation.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2013-2024  Laurent Forthomme
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#include <cmath>
 
#include "CepGen/FormFactors/Parameterisation.h"
#include "CepGen/Physics/PDG.h"
 
namespace cepgen {
  namespace formfac {
    Parameterisation::Parameterisation(const ParametersList& params)
        : NamedModule(params),
          pdg_id_(steer<pdgid_t>("pdgId")),
          mass2_(std::pow(PDG::get().mass(pdg_id_), 2)),
          mp_(PDG::get().mass(PDG::proton)),
          mp2_(mp_ * mp_) {}
 
    double Parameterisation::tau(double q2) const { return 0.25 * q2 / mass2_; }
 
    const FormFactors& Parameterisation::operator()(double q2) {
      if (q2 < 0.)
        ff_ = FormFactors{};
      else if (q2 != q2_) {
        q2_ = q2;
        eval();
      }
      return ff_;
    }
 
    ParametersDescription Parameterisation::description() {
      auto desc = ParametersDescription();
      desc.setDescription("Unnamed form factors parameterisation");
      desc.add<pdgid_t>("pdgId", PDG::proton);
      return desc;
    }
 
    void Parameterisation::setFEFM(double fe, double fm) {
      ff_.FE = fe;
      ff_.FM = fm;
      ff_.GM = std::sqrt(ff_.FM);
      const auto ta = tau(q2_);
      ff_.GE = std::sqrt((1. + ta) * ff_.FE - ta * ff_.FM);
    }
 
    void Parameterisation::setGEGM(double ge, double gm) {
      ff_.GE = ge;
      ff_.GM = gm;
      ff_.FM = ff_.GM * ff_.GM;
      const auto ta = tau(q2_);
      ff_.FE = (ff_.GE * ff_.GE + ta * ff_.FM) / (1. + ta);
    }
 
    //------------------------------------------------------------------
 
    std::ostream& operator<<(std::ostream& os, const Parameterisation& ff) {
      os << ff.name();
      if (ff.q2_ >= 0.)
        os << "(Q^2=" << ff.q2_ << " GeV^2): " << ff.ff_;
      return os;
    }
 
    std::ostream& operator<<(std::ostream& os, const FormFactors& ff) {
      return os << "FE=" << ff.FE << ", FM=" << ff.FM << ", GE=" << ff.GE << ", GM=" << ff.GM;
    }
  }  // namespace formfac
}  // namespace cepgen